What are the advantages of a Trimmable Horizontal Stabilizer? What are the advantages of a Trimmable Horizontal Stabilizer (THS)? For example, a THS is used on some Airbus and Embraer aircraft. What is the most important advantage, and why don't some other successful airliners have this type of horizontal stabilizer?

Image used under CC BY-SA 3.0 (Source). Depicting the right side THS on an ERJ-170

Note: A Trimmable Horizontal Stabilizer (THS) differs both from a Stabilator and a horizontal stabilizer with a trimmable Elevator

The main advantage is smaller elevator deflection angles. This comes handy in two cases:

When high-lift devices are deployed, the center of pressure on the wing shifts backwards by up to a third of wing chord. Fowler flaps add wing area aft of the trailing edge, and slotted flaps are able to generate high suction peaks. The result is a massive change in trim, and the empennage now has to generate generous downforce. Changing the lift on the empennage by elevator deflection alone will exceed the maximum practicable deflection angle and leave no margin for control. By adjusting the stabilizer incidence, the elevator can be held near its neutral position and has reserves for control.
In transsonic flight the elevator might not always have a linear characteristic. The contour break due to an elevator deflection induces shocks which in turn lead to flow separation which reduces the control effectivity and can even reverse the control characteristic. Since the transition from subsonic to supersonic flight shifts the center of lift backwards, the empennage needs to add downforce when the aircraft accelerates in the transsonic speed range. An elevator deflection might not be able to produce the desired lift change, and only adjusting the stabilizer such that the elevator can be held neutral can restore trim and control effectiveness.

Older airliners from the propeller age had lower wing loadings and less powerful flaps. The center of lift on the wing changed less with flaps, so a fixed stabilizer was sufficient. But once the wing loading goes up to jet levels, and the wing is fitted with slotted fowler flaps, a moveable stabilizer is unavoidable.


Why does having a geometry-limited aircraft lead to lower VLOFs? For this question, please have in mind these excerpts from the 14 CFR 25.107 - Takeoff speeds.

"(d) VMU is the calibrated airspeed at and above which the airplane can safely lift off the ground, and con- tinue the takeoff. VMU speeds must be selected by the applicant throughout the range of thrust-to-weight ratios to be certificated. These speeds may be established from free air data if these data are verified by ground takeoff tests."

and  VR must be:

"(iv) A speed that, if the airplane is rotated at its maximum practicable rate, will result in a VLOF of not less than —
(A) 110 percent of VMU in the all-engines-operating condition, and 105 percent of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition; or
(B) If the VMU attitude is limited by the geometry of the airplane (i.e., tail contact with the runway), 108 percent of VMU in the all-engines-operating condition, and 104 percent of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition."

My question is: why does having a geometry-limited aircraft lead to lower VLOFs? Is it because, since the aircraft cannot reach the angle of stall because of its limitation, the VMU for such airplanes is greater than the stall speed, and thus the maximum Lift Coefficient has not been reached and therefore it offers a higher safety margin regarding stall?

Unless someone can find an old 14CFR Part 25 ruling that has the explicit reasoning, I believe you are right to consider that the rationale for giving extra performance credit to airplanes that have demonstrated minimum unstick speed ($V_{MU}$) limited by tail contact is that they are not limited by aerodynamics. However, I should point out that the intent is to reduce the rotation speed ($V_R$) and not $V_{LOF}$, which is not a performance V-speed.

For Part 23 aircraft, which does not require $V_{MU}$ testing, 14CFR 23.51(c)(2) (pre Admt 23-64) requires $V_R$ to be no less than 1.10$V_{S1}$, or 110% of the stall speed for takeoff configuration obtained out of ground effect.

This mirrors the Part 25 requirement of 14CFR 25.107(e)(1)(iv)(A), which you have cited in the OP. The difference between the two is that Part 25 allows the applicant to lower the $V_R$ by taking credit for the ground effect and the rate of rotation, but also poses additional onus on the applicant to flight test the $V_{MU}$ (which is involved and high-risk).

Vmu can be limited by either:

Stall in ground effect, or
Stick pusher in the takeoff phase, or
AOA limiter (FBW only) in the takeoff phase, or
Tail contact with the ground (i.e. geometry limited).

In the last case, the aerodynamic capability of the airplane has not been reached. Therefore, 25.107(e)(1)(iv)(B) extends some credit to reduce the rotation speed. However, as specified in AC 25-7C, this credit can only be taken if the airplane is actually shown to be geometry limited from 96% to 100% of the lift-off speed ($V_{LOF}$) with the lowest thrust-to-weight ratio allowed for takeoff.


What can pilots do to spot the onset of hypoxia? If there is a loss of/lack of air pressure at high altitude, without an explosive decompression (for instance as on Helios 522 or N47BA) are there any signs that pilots can look out for that they might not be getting enough oxygen?

Does an audible alarm exist on any commercial jets when cabin pressure is at a dangerous level?

It's my understanding that the brain is impaired, and reaction times are increased with oxygen deprivation. Are there any techniques to combat this, after noticing hypoxia onset? e.g. slowed breathing

Yes, all pressurized aircraft have cabin altitude (= altitude at which the same pressure as currently in the cabin normally occurs) indicator with alert when it exceeds safe value.

On Helios 522 that alarm did sound. The crew however mistook it for configuration alarm which only makes sense during take-off roll and thus considered it spurious. When the technician they called realized it was cabin altitude warning, the pilots were already too confused to carry out his orders.

The symptoms are already listed in the other answer. The problem is that the initial symptoms feel just like fatigue. One starts to feel sleepy and at that moment one might not realize it could be due to hypoxia as mental capacity is already reduced.


Relationship between flapping and lagging frequency In my rotorcraft module at university, we were told that blades leading/lagging is as a result of blade flapping. To conserve angular momentum, an upward flapped blade (CG moves toward rotor hub) will speed up and hence "lead". A downward flapped blade (CG moves outwards) will slow down and hence "lag". I understand this, however if the lead/lag was as a direct consequence of flapping, then shouldn't their frequencies be the same? In the above image (Shown to us in lecture) you can see that the lagging frequency is roughly 1/5 of the flapping frequency. How is this possible? If the blade lag is a direct consequence of flapping surely this is not possible.

Are there more factors that affect lead/lag? Perhaps cyclic pitch control alters the drag of the blades and hence affects the lead/lag? Or could it be to do with the hinge and any associated damping effects.

A comprehensive explanation would be appreciated.

I agree with your assessment. Have you discussed this with your prof/instructor? When performing Rotor Track and Balance, a worn lead/lag damper or dampers will directly affect the 1 per revolution lateral vibration with little to no affect to other rotor rpm orders or suborders. This is most pronounced in helicopter models which utilize hydraulic dampers as opposed to elastomeric or friction type lead/lag dampers. Hope this helps.

Another factor that contributes to Lead/Lag is alternating spanwise distribution of lift as a result of forward flight. As a rotor blade advances, the lift migrates toward the tip. Conversely, as the blade retreats, lift shifts more toward the root of the blade. Still a 1 per revolution effect.

Chordwise blade mass distribution (Chordwise Balance Aka Product Balance) accounts for angle of attack changes with respect to power changes. If one blade climbes or dives a greater amount than the other/s in transition from Ground-Flat-Pitch to Hover, there is a chordwise balance disparity, which, in turn leads to effects in Lead/Lag. Still a 1 per revolution effect.


What are the restrictions on performing intentional spins? I know there is controversy over the safety and utility of performing spins for training, but what are the regulations limiting them for training (solo and dual) or 'just for fun'?

There are at least three FARs that apply (maybe more?). First, "aerobatics" as defined in 91.303 seems to include spinning:

  For the purposes of this section, aerobatic flight means an
  intentional maneuver involving an abrupt change in an aircraft's
  attitude, an abnormal attitude, or abnormal acceleration, not
  necessary for normal flight.

That means the general restrictions in 91.303 would apply:

  No person may operate an aircraft in aerobatic flight—

  (a) Over any congested area of a city, town, or settlement;

  (b) Over an open air assembly of persons;

  (c) Within the lateral boundaries of the surface areas of Class B,
  Class C, Class D, or Class E airspace designated for an airport;

  (d) Within 4 nautical miles of the center line of any Federal airway;

  (e) Below an altitude of 1,500 feet above the surface; or

  (f) When flight visibility is less than 3 statute miles.

Second, spinning an aircraft not approved for spinning in the POH would be a violation of 91.9:

  §91.9   Civil aircraft flight manual, marking, and placard
  requirements.

  (a) Except as provided in paragraph (d) of this section, no person may
  operate a civil aircraft without complying with the operating
  limitations specified in the approved Airplane or Rotorcraft Flight
  Manual, markings, and placards, or as otherwise prescribed by the
  certificating authority of the country of registry.

Finally, 91.13 would also prohibit spinning a non-certificated aircraft because it's a 'catch-all' regulation:

  §91.13   Careless or reckless operation.

  (a) Aircraft operations for the purpose of air navigation. No person
  may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another.

But spinning itself is absolutely legal and is required for a flight instructor rating as described in 61.183.

EDIT: there's also a requirement in 91.307 that if non-crewmembers are on board then parachutes are required for maneuvers with 'extreme' bank or pitch, unless the flight is a test or training flight:

  (c) Unless each occupant of the aircraft is wearing an approved
  parachute, no pilot of a civil aircraft carrying any person (other
  than a crewmember) may execute any intentional maneuver that exceeds—

  A bank of 60 degrees relative to the horizon; or
  A nose-up or nose-down attitude of 30 degrees relative to the
  horizon.

  (d) Paragraph (c) of this section does not apply to—

  Flight tests for pilot certification or rating; or
  Spins and other flight maneuvers required by the regulations for
  any certificate or rating when given by—

  (i) A certificated flight instructor; or

  (ii) An airline transport pilot instructing in accordance with §61.67 of this chapter.


Why don't commercial airplanes carry Earth-observing instruments? Governments and private companies spend hundreds of millions of dollars to send Earth-observing satellites to capture imagery or scan the terrain with lasers or radars.

Meanwhile, commercial planes are constantly flying around the world, and even if they do not offer global coverage, they could still carry Earth-observing instrumentation with almost no additional cost. I'm sure the data they could provide would have many scientific and commercial applications. In some cases they could be even better than satellite observations due to much shorter revisit times, less atmospheric influence and shorter round-trip times for laser and radar signals.

For example, some satellites like IceSat and IceSat-2 were designed to repeat laser altimetry observations over the same ground tracks over and over. With much interest in Greenland, the icefields in Yukon/Alaska and Antarctica. Again, in this case, commercial flight could provide very good coverage of the first two areas of interest at a fraction of the cost.

I'm a glaciologist working on the Patagonian icefields, where the persistent cloud cover renders most of the images captured by Earth-observing satellite useless. I'm evaluating the possibility of contacting authorities and airlines to collaborate in a project to put instrumentation on the planes that overfly the icefields several times a day. However, I'm sure that I'm not the first with this idea of putting instruments on commercial planes. Thus, I'm surprised that no commercial airplanes carry Earth-observing instruments.

Therefore my question:

Is there any insurmountable technical or legal limitation to equipping commercial airplanes with Earth-observing instruments?

They do! Well, at least some of them. There is for example a project called TAMDAR (Tropospheric Airborne Meteorological Data Reporting) that e.g. Icelandair is a part of. There is a document published by the Icelandic Meteorological Office about it.

Another example is MOZAIC/IAGOS: Atmospheric Research Using Commercial Airliners.


Can large airliners such as a 747 maneuver without electricity? Small aircraft have the flight controls physically linked to control surfaces by some kind of cable and pulley system. For a large aircraft such as a 747, are the control systems mechanically aided and if so, can they be used without power?

Flight controls are pretty hard to break

For the 747 (at least through the -400) specifically, the controls are entirely hydromechanical, so yes, they will work without electrical power.  In general, all non-FBW airliners are this way.

FBW airliners vary, though.  All FBW Boeings and the first generation FBW Airbii (A320 family, A330/340) have hydromechanical backups to the FBW system.  The second generation Airbus FBW system, found in the A380 and A350, has abandoned hydromechanical backup controls in favor of independently powered electrical backup systems.

In addition, the chances of total electrical failure are extremely remote -- a failed engine will likely be windmilling (spinning from airflow through it) and thus turning its generator to provide some power, the APU has a generator on it in case all the engines seize up, and there is also a generator that is directly or indirectly powered by the Ram Air Turbine on airliners that use one (all non-reversion twins have one, and so do the A380 and the 747-8 -- earlier 747s don't, relying on windmilling engines for electrical and hydraulic power instead).  Finally, the aircraft has batteries that can provide a limited power reserve if all these other systems fail.  (FBW aircraft add to this with dedicated batteries for at least some parts of the FBW system.)

However, power loss is still a serious emergency

The main hazard posed by a severe electrical power failure, though, is trying to fly around in the clouds with one -- it can render you effectively "no-gyro", especially on older/smaller types that don't have a standby gyroscopic instrument with a dedicated power supply/internal batteries.  There have been cases where small airliners have gone down due to power failures -- while the flight controls were working fine, without power or a visible horizon, the pilots became spatially disoriented, and you can guess the rest.


How are weather-related limits on arrival capacity determined? Most airports in Europe have the following traffic controls imposed during adverse weather conditions. I wonder whether these capacity limits are determined by airport operator's experience, or if there are any special algorithms?

ATC has to keep planes a minimum distance apart, and that distance increases as weather gets worse. You could fairly easily calculate from the required spacing in each case to get an expected arrival rate, but the real world never works exactly as planned. The numbers in that table have almost certainly been "adjusted" to make them agree with experience.


Could a plane be constructed to be fly in fixed-stick roll-stable circles? According to https://aviation.stackexchange.com/a/12511/973 it is not possible for a plane to aerodynamically recover from a roll disturbance.  Is that an inherent limitation of how planes could be constructed, or is that merely a design trait that is accepted in exchange for other considerations like fuel economy and ease of control?  Would it be possible to construct a plane such that if the controls were eased into a certain setting, the plane would settle into a stable spiral climb until it reached an equilibrium altitude set by the thinning air, and such that mild disturbances in the roll axis would settle out [possibly changing the plane's heading]?

I would expect that maintenance of roll stability during straight and level flight would be complicated by the fact that roll won't immediately affect the flow of air over a plane in the same way as would a change in pitch or yaw.  If a plane was flying in deliberate circles, however, I would think that a change in roll would rather quickly change the rate at which the plane would want to rotate about the yaw axis, and that aerodynamic surfaces cause such change to produce torque about the roll axis, thus counteracting the disturbance.

My intuition would suggest that even if roll stability could be achieved, a plane designed for such stability would probably be annoying to fly in normal usage.  If, however, it was possible to activate a control which would induce such stability, it would seem such a thing might be helpful in some situations where a pilot might otherwise be in trouble [e.g. unexpected sudden IMC or other conditions causing loss of visual horizon reference].  Would such a thing be aerodynamically impossible, theoretically possible but practically unworkable, or workable but not insufficiently useful to be worthwhile?

This will become another of my long posts; after all, lateral stability is more complex than longitudinal stability and involves many more factors. Short answer: It is possible, but not by aerodynamic means, and would incur a drag penalty.

Starting condition

Assume we have an aircraft with fixed controls, trimmed for straight flight, and make it a glider to remove propulsion effects. Next, let it fly through an asymmetric gust which lifts one wing. For what follows we assume calm air again.

Initial sideslip

For an outside, earth-fixed observer, a component of lift is pointing sideways and not compensated by weight, so the aircraft will accelerate to that side. From the point of view of the aircraft, lift is still acting in the plane of symmetry, but gravity does not and will cause it to sideslip. This will mainly evoke these reactions:

Directional stability $c_{n\beta}$: The aircraft will yaw into the wind because the vertical tail will create a side force ("weathervane effect").
Dihedral effect $c_{l\beta}$: The additional lift on the windward wing will lift this wing. Also, the additional side force on the vertical tail, which yaws the aircraft, will cause a supporting rolling moment. Roll damping ($c_{lp}$) will limit the amount by which the aircraft rights itself up, however.
Yaw moment due to dihedral and wing sweep: The lift difference due to dihedral and wing sweep will retard the upwind wing, causing a yawing motion into the wind. This effect can be added to the directional stability $c_{n\beta}$.

Eventual (almost) coordinated turn

The aircraft will start to fly a coordinated turn, because all forces try to drive sideslip to almost zero, and on their way to do so start a yawing motion. In contrast to that, the roll angle will not change further once sideslip stops, because the sideslip-induced rolling moments stop as well. However, if a roll angle remains, the aircraft will now begin to turn. This opens up a new bunch of effects, because  now we have asymmetric flow: Due to the yawing motion, airspeed varies over wingspan and sideslip angle varies over length:

Rolling moment due to yawing motion $c_{lr}$: The outer wing (which was the leeward wing during sideslip before) now travels faster, creating more lift. This causes a rolling moment which increases roll angle.
Since the angle of attack would be the same in a coordinated turn over the whole wingspan, drag will behave proportional to lift and increase with the local turn radius. This drag difference will create a small yawing moment which will let the inner wing advance, until the dihedral effect will kick in and create enough of a difference in angle of attack that the drag on the inner and on the outer wing are equal again. Since induced drag is proportional to the square of the lift coefficient, the wing will still create less lift on the inner wing than on the outer wing, but the difference will be reduced. If the airplane climbs or descends during the turn, angle of attack will vary over span and will create an uprighting moment on the descending airplane (and vice versa).
Yawing moment due to yawing motion $c_{nr}$: This is also called yaw damping and creates a yawing moment opposed to the yawing motion. Contributing factors are the vertical tail which sees a sideward flow component due to the yawing motion, and the drag distribution along the wingspan. Aircraft with a long tail have high yaw damping and, in combination with plenty of dihedral, tend to be more roll stable than short-tailed aircraft. Here the long tail forces the wing into a sideslip which uprights the aircraft. This configuration is found on free-flying models which do indeed upright themselves from shallow banks.
Centrifugal forces: The sideward component of lift is now balanced by the centrifugal force due to the turn. However, the centrifugal forces also will create an uprighting rolling moment which depends on the spanwise distribution of masses.

Note that the outer wing has a bigger turn radius $R$, but the same angular velocity $\omega$ as the rest of the airplane. This causes it to experience more centrifugal force $m\cdot \omega^2\cdot R$ than the inner wing (symbolized by the length of the parallel arrows). I used a rather steep attitude to get the point across, but the same holds true at shallower bank angles. This is the uprighting inertial moment.

Depending on the relative size of these effects, the aircraft will either right itself up, stay at this roll angle or dive ever deeper into the turn. Agile aircraft with low roll inertia will have too little uprighting moment and are likely to spiral dive. More stable configurations with big vertical tails and ample of dihedral will either continue to turn gently or right themselves up.

Effects at non-zero roll rates

Now this was all done under the assumption that the roll angle changes very slowly. A too small vertical tail and/or too much dihedral will excite the Dutch roll motion, and now the roll angle oscillates, adding more roll-induced effects. To keep this answer reasonably short, I will not list them here.

Sweptback configurations optimized for fast flight (= with small vertical tails close to the wing) are typical examples where the ratio $\dfrac{c_{l\beta}}{c_{n\beta}}$ is too high and the Dutch roll eigenmode has too little damping. Free flying model aircraft, on the other hand, cannot afford to fall into a spiral dive, and by giving them big vertical tails and long lever arms, which allows them to have substantial dihedral, and a high radius of roll inertia, they can be made to upright themselves.


Why isn't the bell distribution used? Why has the bell distribution not become widely used? Is the bell distribution only ideal in inviscid flow scenarios or is there another reason for the lack of use?

Something like it is widely used, only not by this name.

Although less elegant and general than Prandtl's paper, the same solution was found 17 years later by R.T. Jones when he optimized the ratio of lift to mass of wings in NACA TN 2249. The figure below is from the concluding page of this report.

However, induced drag is not all. By increasing span, wing surface area is also increased which will increase viscous drag. Therefore, the optimum really is a combination of both; having the least amount of wetted surface for the most net lift (total lift minus wing weight). This is somewhere between an elliptical and a triangular load distribution where size also matters. Small aircraft are closer to the elliptical distribution whereas large aircraft tend more to the triangular one. Negative lift at the tips only happens with extreme span to elliptical span ratios (note the curve for S/S$_\text{e}$ = 1.30). Negative lift as in the bell-shaped lift distribution, however, is a prerequisite for the Horten type of flying wings because it allows to create proverse yaw. If you have a multi-engined aircraft and need to size the vertical surface for the engine-out case, covering adverse yaw will be trivial and there is no need for a true bell-shaped lift distribution.


How realistic is Google Earth Flight Simulator? Sometimes I like to fly a bit in flight simulator in Google Earth (not the Pro version), and I was wondering how realistic it is. There are two planes you can fly. I included some stuff I personally tried.

F-16

Max altitude (starting from ground): 46200 ft (14081.8 m)
Max altitude (starting from the highest google earth let me start): 80000 ft (24384 m)
Max speed while falling down: 1450 knots (2685.4 km/h)
Max vertical speed: 980 knots (1814.96 km/h)
Flies rather stable, I can do some aggressive maneuvers without any problems. Difficult to land. When I'm very high, it's difficult to get control of the aileron.

SR22

Max altitude (from ground): 19800 ft (6035.04 m)
Max altitude (from highset starting point): 60000+ ft (18288+ m)
Max speed while falling down: 550 knots (1018.6 km/h)
Max vertical speed: 155 knots (287.06 km/h)
Not very powerful. Quite unstable (aileron always moves by itself). Easy to land.

Interestingly, Google Earth allows you to fly beneath the surface of oceans, and it flies exactly the same as in the air. The support for taking off from a real airport is rather limited, but it's there. It supports a full ROM afaik.

How realistic is Google Earth Flight Simulator?

It is very unrealistic in most respects.

I believe there are a number of important ways you can measure realism from a pilot's perspective:

view out the windows
appearance of instrument panel
mode of operation of pilot aids like autopilot, autothrottle
flying controls

physical character
responsiveness

interaction with ATC
effects of wind, weather, turbulence, rain, ice
aeronautical characteristics of aircraft

It would be easy to focus solely on the last of these, but from my perspective this is not really the most important.

Here's a few dissimilarities that occur to me.

View

The SR22 doesn't have a HUD.

I'm pretty sure the SR22 cockpit windows would take up a much greater area of my vision than my computer screen does. Especially peripheral vision. Also in a real SR22, there is important stuff to look at below the windows.

Instruments

Do these look alike?

Controls

In Google Earth you don't have anything like the range of controls that have to be used, where are all the switches, knobs and other dohickeys you find in the real SR22?

Waving a mouse around is nothing like moving a control column around. With a control column you have a much different sense of the position of the controls. Moving a mouse involves completely different muscles and different motions.

Moving a throttle lever is completely different to pressing PgUp and PgDn buttons to move a throttle position in increments

Feel

In Google Earth you don't hear any engine, you dont feel any vibration, you don't feel any turbulence, you don't feel anything in an uncoordinated turn or when pulling out of a dive, you don't feel the unevenness of a runway.

It's hard to really feel oriented in a simulator when your sense of balance and peripheral vision tell you you are sitting upright at your desk, not inverted and spinning towards the ground.

Disclaimer

I'm not a pilot, I have sat in the pilot's seat of a small Cessna and manipulated the control-column and rudder to make a few turns and attempt to maintain speed and altitude under the direction of a pilot. I've done the same in a glider, and I have done some take-offs and landings in a fairly amazing full (though static) commercial 737 simulator with full cockpit reproduction (all seats, panels, motorised levers, switches etc that you'd find in a real 737 cockpit, wired to a large rack of computers).

 A more realistic simulator

Google Earth is nothing like any of them in almost any respect.

It's a fun game though.


What vintage aircraft was registered as NC7210? Can anyone help identify this vintage aircraft, origin, and possible flight history. Airplane has this number of its wing, NC 7210.

My step grandfather, Roy Danischefsky, purchased the Ryan B-1 Brougham #157 NC 7210 in Sept 1928 from Mahoney-Ryan Aeronautics for Midwest Airlines (Wisconsin). He flew it for Mid-West as a private charter and to promote flight as a new transportation option. Here is a photo of him in front of the plane.
Also - Here a photo of the plane from San Diego Air and Space Museum Archives
https://www.flickr.com/photos/sdasmarchives/5684139991/in/photostream/
Where is this plane now? I would be VERY EXCITED to know that it is still exists. Many of these planes did not survive.


Is there currently any heat shielding and paint for speeds of up to Mach 6? The Blackbird SR71 could travel faster than Mach 3 but its speed was limited to prevent its skin from melting. Its newer variant is said to be named the SR72 and it and other, civilian hypersonic aircraft in development are expected to travel at speeds of up Mach 6.

Is there currently any heat shielding and paint for repeated use at speeds of up to Mach 6, and travelling at the cruising altitude of the Concorde?

I'm reasonably certain the speed limit (so to speak) on the SR-71 wasn't to prevent its skin from melting.

The hottest the skin got during flight was less than 600 C. That's definitely hot--but it's a long ways short of the melting temperature of titanium (1668 C).

Early supersonic aircraft often had control problems, because the leading edge of the air foil would cause a shock wave that separated the air flow. The control surfaces at the trailing edge little enough air flowing smoothly that they lost authority.

In the X-15, they combated this by building a vertical stabilizer that was basically a V-shape--thin at the front, but much wider at the rear:

This would induce a lot of drag at low speed, but for the x-15, low speed wasn't really a major concern.

The SR-71 took a different approach, using a pair of vertical stabilizers.

The leading edge of each stabilizer produces a V-shaped shock wave. At (approximately) the normal cruising speed, that V comes back from the leading edge of one stabilizer, and hits against somewhere close to the trailing edge of the other. Thus, we still have a solid air flow across the trailing edge (and rudder) of each, and maintain nice directional stability.

For that to work, however, the angle from the leading edge of one stabilizer to the trailing edge of the other has to (approximately) match the angle of the shock-wave formed at the leading edge. Outside of the designed speed range, that no longer happens.

On the SR-71, the vertical stabilizers are angled somewhat. This gives a (still fairly narrow) range of speeds at which the aerodynamics "work", rather than having only one specific speed. Nonetheless, the difference in separation between the top and bottom of the stabilizers isn't very large, so the range of speeds at which it works is fairly narrow. If you try to exceed that range, your stabilizers no longer work, and your control over the aircraft quickly deteriorates.

Having said all that, however, the answer is a clear "yes", if heat were to become a problem, there are materials available that can withstand substantially higher heat than titanium. One obvious example would be the Inconel X that was used as the skin for the X-15 (which flew at a bit over mach 7). Another possibility would be carbon or ceramic tiles, like those used in the Space Shuttles (or some of the other heat shielding it used, such as flexible blankets).

Those have some fairly serious shortcomings so they'd probably be avoided unless absolutely necessary. Inconel X is quite a bit heavier than titanium, and while the ceramic tiles were quite heat resistant, they were fragile mechanically, which led to a lot of maintenance work on the shuttles.


Why would only a specific model of drone be allowed to fly over crowds? The FAA allowed CNN to fly a drone over crowds at extremely low altitudes. Its scope is within the UAVs. Only a specific model/type is allowed to operate on these flights.

Why is this model of drone allowed to do such a thing, and others not? CNN claims that it is to minimize the damage to people, if the drone crashes or something.

CNN has applied for and received multiple waivers for 14 CFR § 107.39 Operation over human beings as allowed by § 107.205 List of regulations subject to waiver.

Issued August 29, 2016 — This allows "Small unmanned aircraft system (sUAS) operations over human beings in accordance with the CNN UAS Operations Manual, Fotokite Pro 1 Operator Guide, and Fotokite Pro 1 Specifications Sheet." It allows operation "over private or controlled-access property with permission from a person with legal authority to grant access" and prohibits operations over open-air assemblies of people.
Issued July 20, 2017 — This allows operations "conducted over human beings for the purpose of closed-set motion picture and television filming". Make and model of sUAS not specified, but all operations must comply with the operator's manual.
Issued October 13, 2017 — This waiver is granted for the Vantage Robotics Snap only, and allows "operations over all human beings (including Nonparticipants)". It also limits the altitude when operated over people to 150 feet AGL, as opposed to the normal limit of 400 feet AGL. I assume this is the waiver you are interested in.

The simple reason why only one particular model of UAS is allowed is that CNN only submitted that model for waiver and no others, and the FAA approved that.

This is a somewhat unsatisfying answer, but neither CNN's nor the FAA's reasoning is included in the text of the waiver itself. The rest of this answer is speculation on my part.

As can be seen on Vantage's specifications page for the Snap, the Snap is reasonably lightweight (1.37 lbs / 620 g including battery), has caged props, and has a high-quality camera. (The camera isn't a matter of safety, but it is presumably something a cable news provider is interested in.)

If a prop were to fail catastrophically, the cage would hopefully contain at least some of it, thus eliminating risk of a crowd being hit by sharp objects. The height restriction of 150 feet AGL reduces the potential for injury of a nonparticipant should the sUAS fall on them, and the prop cages prevent contact with moving parts.

I suspect that CNN applied for a waiver specifically for this model and the FAA approved it based on the following:

It's specific. The more general the application, the more likely it is to be rejected. If you know what you're working with, you can evaluate all the parameters and come up with a safe solution. If there are more variables, it's harder to reason about.
It has certain safety features as discussed above, which make it more suitable for flight over nonparticipants than some other models.
It's standard. CNN has a single platform to train their over-crowd operators on. Once again, this also has safety benefits, because the operators are extremely familiar with one model of unit, instead of broadly familiar with many different ones.
This particular model of sUAS had all the features that CNN wanted and that the FAA was interested in for this application. This may include such factors as weight, safety features, well-written operation and maintenance manual, and ease of maintenance. Additionally CNN may have liked the cost (the FAA wouldn't care about that).

The FAA maintains a list of all Part 107 waivers granted.


At what altitude do vacuum powered gyros become ineffective? Obviously, in space a vacuum powered gyro would not work because there is no air to move, but, how high do you have to go before they are unable to spin fast enough to work properly?

would a vacuum gyro work properly in a sr71 at 80,000 feet?

Just like altimeters, it depends on the specifications.

The manual for this random vacuum attitude indicator says up to 40,000 feet.

(Image source)

  In an aircraft, the vacuum source is often used to power gyroscopes in the various flight instruments. To prevent the complete loss of instrumentation in the event of an electrical failure, the instrument panel is deliberately designed with certain instruments powered by electricity and other instruments powered by the vacuum source.— Wikipedia

In a piston engine airplane the vacuum pump is geared to the engine.

Historically electric motors weren't reliable, that has changed now of course.

At 80,000 feet the air density is only 14% compared to 40,000 feet. I highly doubt it would be feasible (size, weight, RPM) to create such a pump. And I'm pretty sure it's of no use to jets.


Could a controller or airport in the US decide to use English-style landing clearances? In the U.S., controllers are allowed to clear multiple planes to land on the same runway, with the assumption that the first plane to land will be out of the way by the time the second plane gets there. However, in England (and I think a lot of other places), you're not allowed to clear a plane for landing until it's the very next plane to land and there are no takeoffs or crossings ahead of it.

Could an American controller, or airport as a whole, just decide not to do sequenced clearances? That is, if they need to cross a plane in front of one that's landing, tell the landing plane "Cancel landing clearance but continue approach", then re-clear them to land once the runway is clear?

The use of FAA's anticipated separation rule is theoretically optional, and there are indeed times when controllers don't use it. While not common, now and then one hears "continue approach" (or similar) instead of an early landing clearance, as is normal under ICAO rules. Ditto for the occasional canceled clearance.

However, anticipated separation exists for a reason, and a trainee who was unwilling or unable to use it when appropriate would not pass. OTOH, they also have to know when it's not appropriate and not use it at those times.


With helicopters, is normal landing or auto-rotation landing any different when landing on water? I noticed something from investigating my other answer to a not-really-related question.

A wiki article here says:

  The "water bird" emergency landing is a technique developed by the Canadian Forces to safely land the Sikorsky CH-124 Sea King helicopter when one engine fails while flying over water. The emergency landing technique allows the boat-hull equipped aircraft to land on the water in a controlled fashion.[37]

Which leads me to the obvious question: Is landing any different for water landings? If so, how?

I don't know much about helicopters. For a normal landing, I assume they just hover down and I thought the same thing would be done for a water landing. But the wiki article I read implies that something is different.

For auto-rotation without any engine power, apparently there is a flare at the end. Is this flare done any differently when landing on water?

Basically, the spirit of the question is, how is landing different (if at all) on the water, whether powered or not?

As a helicopter pilot, I've never made a water based landing, so I'm not speaking directly from experience, only from my training, but there are a couple things that are missed in the discussion here that I would certainly worry about.

If I had a real engine failure, and was forced to perform an auto-rotation, in all likelihood, I would not end the auto-rotation with zero forward velocity, I would probably perform a running landing (where you slide on your skids). This, of course, is assuming I'm landing on a flat surface where I'm not likely to roll over (such as a street, parking lot, or runway). Holding the flare to bring the helicopter to a complete standstill has a few risks for me, the biggest one being that if I flare and stop my descent while I still have forward velocity, holding the flare will start to increase my altitude again. Higher inertia (heavier) rotors in small helicopters will suffer from this problem, for example, in training, a Robinson R-22 will generally not have this issue, but an R-44 has much heavier rotors, and we get trained to anticipate a flare at the end of the auto-rotation. Depending on the helicopter, weather, and how I've performed my auto-rotation, if I'm in a real emergency, I would rather put the helicopter down on the ground and slide a little bit than suffer a rotor stall at 50 feet, but a running landing is clearly not an option in a water based landing.

Once I've performed an auto, and the helicopter is on the ground, I'd apply the rotor brake to get the rotors to stop moving before I exited the aircraft, and generally speaking, won't have to worry about the rotors any longer at that point.

If I am performing a water based auto-rotation, there is quite a bit more to worry about. The first thing I have to worry about is that I won't be able to perform a running landing. This means that I'll have less margin for error in my auto-rotation. The second problem is that as soon as I set the helicopter down into the water, it's going to start sinking, and I'll still be strapped in. If I jump out, I'll have to worry about hitting the rotors (or the rotors hitting me) as the helicopter sinks. This means that as soon as I hit the water, I have to remain in the aircraft, and I'll pitch the cyclic over in the opposite direction from the side I'm sitting on, so that I can stop the rotors and still exit the aircraft on the surface side rather than underneath the sinking helicopter. This gets more complicated in the case of passengers, I would encourage them to jump out before the helicopter hits the water, but after my flare, so that they can clear the aircraft and I don't have to worry about them. I only mention it as an additional complication I'd potentially have to deal with.

All of this complexity is a large reason why most of the helicopter pilots I know really do not like flying very far over water. When I'm flying over the water, I tend to stick to an area near the coastline specifically for this reason.


Why aren’t any JP-1-type (extreme-narrow-cut) fuels in use today? JP-1 was the first U.S. military standard jet fuel (hence the 1 in the designation), with its specifications issued in 1944; it was an extremely narrow-cut, high-alkane kerosene, not far from what would later become RP-1.  As a result, it had both a very low freezing point (-60C maximum) and a very high flash point, a combination that is the Holy Grail of the aviation-fuel industry.  Its only drawback was that it was hard to produce (since, to quote John D. Clark in Ignition!, “not many refineries in the country could produce such a product with their available equipment and crudes”), and therefore expensive and in chronically short supply, which prompted the development of wide-cut fuels (first the extremely-wide-cut JP-3, followed by the somewhat saner JP-4) and the gradual disappearance of JP-1 (although, judging by mentions in various aircraft accident reports, it apparently was still being produced into the 1970s).

On the other hand, those were 1940s/50s refineries that had difficulty producing JP-1 - and what would be tough beans for a 1940s refinery would be no trouble at all for a 21st-century refinery, thanx to the polymerisation/cracking/hydrogenation/witchcraft/etc. techniques originally developed to turn all the heavier, less-profitable petroleum fractions (and the gaseous, hard-to-store ones) into gold, I mean gasoline, which could easily be adapted to turn even the most recalcitrant crudes into extreme-narrow-cut, JP-1-type fuels (especially given that RP-1 is an even tighter kerosene than JP-1, albeit not by much, and yet we haven’t had any trouble getting ahold of that in the vast quantities needed for launching things into space); so why haven’t we returned to JP-1 and similar fuels, which have freezing points rivalling Jet B combined with flash points that blow away (pun intended) even Jet A?

JP-1 is not in use because it's expensive and there's cheaper ways to deal with flash point and freezing point.

The navy developed JP-5 for carrier use where a higher flash point is required, and it is easier and cheaper to produce than jp-1
A higher flash point isn't that big a gain in most use cases, it's not as if the fuel is going to instantly burst into flame if it reaches 40°C - with good equipment and safety practices Jet-A(1) is still pretty safe to handle
Anti-gelling additives, tank heaters and recirculators work very well to keep fuel from gelling or freezing. This has proven very effective, BA-38 is the only case I can think of in recent memory where ice in fuel has caused an issue, and that was due to water ice crystals rather than the fuel itself, which didn't freeze. JP-1 wouldn't have helped in that case


Are canards useful for ultra low flying aircraft? In this question about canards, they are described as being useful mostly/only for supersonic configurations which are unstable at regular? speeds.

But for a low power, ground hugging aircraft, would a canard help create lift with less width?

This question is part of a human powered dream I have.

This highly depends on the overall conceptual design of the aircraft and there is no ideal configuration for any flight mission profile, except if you have very specific design constraints that will favor one configuration over another. In general, the trend when designing a wing of high aerodynamic efficiency, one that you would need for feasible human powered flight, is to have a traditional configuration of wing and empennage. This is for the wing to operate in clean air and not in the wake of the canard plane. Usually you focus on optimizing the wing and thus you want it to operate in its maximum efficiency.

A design example is the MIT Daedalus Human Powered Vehicle (HPV)

For further technical details regarding human powered vehicles and the their feasibility study you can start with this and discover further on your own.


Which moments are acting on a flying airplane? I would like to ask some clarifying questions regarding the moments acting on a flying airplane.

My understanding is that there are following force:

Weight (pointing down and applied to the CG of the plane)
Lift from the wing (pointing up and applied to the aerodynamic center AC)
Lift from the horizontal stabilizer in the tail (this resultant force is pointing down, it is smaller than Lift_wings, and it is applied to the AC of the tail).

The airfoil, on its own, also has an aerodynamic moment (positive)
   which would make the wing rotate nose-up about the CG of the wing
   itself if the wing was placed free in the wind stream. However, the aerodynamic moment about the aerodynamic center is constant negative (nose down). Is that correct? The physical effect is the nose up intrinsic moment of the cambered wing due to the fact that the force on the bottom and the force on the top of the wing do not act along the same line of action.

For longitudinal stability, are the participating moments the moment due to Weight, the moment due to the wing lift and the moment due to the horizontal
 stabilizer that need to add to a zero resultant moment? Can the positive aerodynamic torque of the wing be ignored because much smaller in magnitude than the other torques?

In most planes, starting from the leading edge of the wing, the aerodynamic center AC of the wing is placed behind the plane's CG. How far behind CG should the wing AC be placed? The placement of the tail's AC should easily derives from the moment equilibrium equation (sum of moments equal to zero). I realized that some big airplanes have the CG behind the wing AC. In that case, the lift from the tail is force always directed upward and not downward like when the CG is in front of the wing AC.

The airfoil, on its own, also has an aerodynamic moment (positive) which would make the wing rotate nose-up about the CG of the wing itself if the wing was placed free in the wind stream.

Well, that depends entirely on the location of the wing's cg. The airfoil camber determines its moment, which is negative around its aerodynamic center or neutral point for positive camber. The first derivative of that moment with respect to the angle of attack is positive, so the cambered airfoil is unstable in pitch.

  the aerodynamic moment about the aerodynamic center is constant negative

Yes. That is correct. When hinged at its quarter point, the wing will pitch down, and that moment is independent of the angle of attack if the aspect ratio of that wing is large enough.

  The physical effect is the nose up intrinsic moment of the cambered wing due to the fact that the force on the bottom and the force on the top of the wing do not act along the same line of action.

No, not really. Aerodynamic forces are pressures acting on an area, and the force is the result of the pressure difference between both sides of the wing. There is not an isolated lower and upper side force. Again, which sign the moment of the wing has depends on your choice of reference point. You only will get a nose-up moment if your reference point is aft of the center of pressure.

  For longitudinal stability, are the participating moments the moment due to Weight, the moment due to the wing lift and the moment due to the horizontal stabilizer that need to add to a zero resultant moment?

Longitudinal stability is achieved by a negative gradient of the sum of all pitching moments over angle of attack. If you balance the moments, you trim the aircraft, but that does not say anything about stability. To simplify this task, I recommend to place your point of reference into the point in which one of the forces attacks, so you can neglect this force's moment contribution. The two preferred points are the center of gravity and the neutral point of the whole aircraft, because both don't move in flight.

  Can the positive aerodynamic torque of the wing be ignored because [it is] much smaller in magnitude than the other torques?

Yes, in a first-order approximation. The tail moment is an order of magnitude bigger and dominates the pitching moments. If, however, you need to calculate the required tail size in order to trim the aircraft at all speeds, flap settings and center of gravity locations, of course you need to include the wing moment into the calculation.

  How far behind [the] CG should the wing AC be placed?

It is much easier to shift the cg than to shift the aerodynamic center around - that one comes with the wing planform. Also, for stability you should look at the distance between the whole aircraft aerodynamic center and the cg - it just happens that a cg near the wing's aerodynamic center provides the aircraft with decent stability. But the cg can very well be a bit behind the wing's aerodynamic center.

  I realized that some big airplanes have the CG behind the wing AC. In that case, the lift from the tail is force always directed upward and not downward like when the CG is in front of the wing AC.

Now you mix up center of pressure and aerodynamic center. That is one reason why I prefer to call the aerodynamic center "neutral point". The tail lift is only upwards for a rear cg location and high angle of attack, which is quite different from "always".


Structural design software for airplanes What are the main structural design software packages that are used by the most of the aircraft manufacturers. There are a number of software packages but which one is widely accepted and used for the structural design of the aeroplane.

There are many package available for structural analysis, but I know that NASTRAN/PATRAN are used extensively in aerospace engineering.

The software was originally developed for NASA in the sixties and has been expanded with a suite of problem solvers for various structural problems including fatigue and acoustics.


Can a pilot use their GPS/FMS to fly a VOR or NDB approach? GPS is more accurate than a VOR or NDB signal.  Can we use a GPS to fly a VOR or NDB approach from the initial approach fix through the missed approach and hold?

I know there are two types of approaches that deal with GPS and VOR/NDBs:

NDB RWY 35L (KCOS RWY 35L) without GPS in the title
VOR or GPS-A (KGUC VOR or GPS-A) with "or GPS" in the title

I'm specifically asking about the first type or approach without the wording "or GPS in the title.

In the US they can (Effective: May 26, 2016).
This change allows for the use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure (IAP) based on a VOR, TACAN, or NDB signal. The underlying NAVAID must be operational and monitored for the final segment course alignment.

Still can’t fly an ILS or localizer approaches.

Update: 2017-04-19 Max Trescott has a good explanation of how to use GPS in conjunction with ground-based navaids on ILS and VOR approaches.

  AIM Section 2. Performance−Based Navigation (PBN) and Area Navigation
  (RNAV)

  1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional
  Procedures and Routes

  Use of a suitable RNAV system as an Alternate Means of Navigation when a VOR, DME, VORTAC, VOR/DME, TACAN, NDB, or compass locator
  facility including locator outer marker and locator middle marker is
  operational and the respective aircraft is equipped with operational
  navigation equipment that is compatible with conventional navaids. For
  example, if equipped with a suitable RNAV system, a pilot may fly a
  procedure or route based on operational VOR using that RNAV system
  without monitoring the VOR.

  NOTE−
  4. The navigation database should be current for the duration of the flight.

  c. Uses of Suitable RNAV Systems. Subject to the operating
  requirements, operators may use a suitable RNAV system in the
  following ways.
    1. Determine aircraft position relative to, or distance from a VOR (“VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass
  locator” includes locator outer marker and locator middle marker.),
  TACAN, NDB, compass locator, DME fix; or a named fix defined by a VOR
  radial, TACAN course, NDB bearing, or compass locator bearing
  intersecting a VOR or localizer course.

  Navigate to or from a VOR, TACAN, NDB, or compass locator.
  Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
  Fly an arc based upon DME.

None of the substitutions mentioned above include the localizer portion of an ILS. So you may not substitute GPS/WAAS for and ILS or localizer approach, but you may still use it for situational awareness.

You need to dig into the AIM to find out what they mean by “suitable RNAV system” but basically both certified GPS or WAAS count.


Have there ever been forward-swept horizontal stabilizers, and what would be its advantages? After going over one of my older questions about forward swept wings, I realize I have never seen a forwards-swept horizontal stabilizer. Do they exist? What would be their advantages, if any?

Forward sweep is chosen when the designers value resistance against tip stall and a rear location of the wing spar higher than low mass. None of those advantages of a forward swept wing has any weight in the design of a horizontal tail, so a forward swept tail has never been selected.


Does having lead-in lights in sight allow descent below MDA? Does having lead-in lights in-site during an approach that includes them allow for a pilot to descend to 100 feet above the touchdown zone elevation? 14 CFR 91.175 allows a pilot to make such a descent if the approach light system is in-site, but does a lead-in light system qualify?

The approach also states the following:

  When visual reference established, fly visual to airport via lead-in lights to Rwy 13L or 13R.

Also worth referencing here is this question about what counts as runway in sight, but this question is specific to lead-in lights and is not answered directly by that question.

No, you may not descend below MDA having only the lead-in lights in sight.
Reference FAA Order 6850.2b, "Visual Guidance Lighting Systems", page 1-3 through 1-5.
According to this order, the list of approach lighting systems is as follows:

Medium Intensity Approach Lighting System (MALS).

Medium Intensity Approach Lighting System with Sequenced Flashers (MALSF).

Medium Intensity Approach Lighting System with Runway Alignment Indicator Lights (MALSR).

Simplified Short Approach Lighting System with Runway Alignment Indicator
Lights (SSALR).

High Intensity ALS with Sequenced Flashers (ALSF-2).

Omnidirectional Approach Lighting System (ODALS).

The Lead-In Light System is not in this list.  Instead, it is found further down the page.  Here's the key sentence:

The LDIN may be terminated at any approved approach lighting system, or it may be terminated at a distance from the threshold compatible with authorized visibility minimums permitting visual reference to the runway environment.

The LDIN is not an approach lighting system and so you may not use it to descend below MDA.


How accurate must fuel gauges be for VFR flight? All Private Pilots should know the acronym TOMATO FLAMES for the Minimum Equipment List (MEL).

On that list, F means Fuel Gauges.But what exactly is required of fuel gauges?

Yesterday, my rental plane came with a note that the fuel gauges were swinging wildly, and bouncing to zero occasionally.   I planned a 90 minute flight, starting with full fuel, so there was no real risk of running out of gas.I decided the fuel gauges probably didn't meet the spirit of TOMATO FLAMES, but I admit that I don't know what the exact requirement is.

How accurately do the gauges have to work to be legal?

The indicator must be calibrated to read zero when that tank is actually empty of all usable fuel (in level flight).  Bouncing is fine as long as they do not bounce when the tank is actually empty.

See regs:

23.1337
91.205


Are there any cockpit indications when an engine has separated from the aircraft? On this answer, there is a comment that asks if there is an alerting alarm of some sort when an engine falls off an airplane.

Is there a direct alarm from the engines specifically saying "You lost an engine" to warn that an engine has fallen off from the aircraft?

This is the indicator that shows you when one engine falls off in a multi-engine aircraft:

If the right engine falls, off, the little ball will move left. If the left engine falls off, the little ball moves right.

If you only have one engine, or both engines fall off, you need to use a different indicator that looks like this:

if both engines fall off, this white pointer will start to move towards the "40", and you might hear an alarm, a voice shouting "stall, stall", or feel the stick shake.

if you have one engine and it falls off, the white pointer will move, but in an indeterminate direction.


Could the center of rotation of a spinning aircraft move away from CG? Picture a barbell shape or twirlers baton spinning.  Perfectly symmetrical.  Rotating at its center of gravity.

Now replace one of the ends with fins of equal mass an spin it again, once in a vacuum, then in air.  Will the centers of rotation be different?  Why?  Could this apply to aircraft?   Thanks.

In some sense, rotation is rarely around the centre of mass. It is extremely convenient to consider any motion as rotation around CM plus translation of that CM, but it doesn't have to be so.

Take the most basic control input for example: from level flight, you pull up for a short climb. A typical explanation of what happens says that the tail produces a pitch-up moment, which rotates the airplane around its CM to a higher angle of attack, this increases lift and it starts to climb. However, a more detailed analysis will point to the fact that initially, the tail will produce an extra negative lift. This is not just a pure moment, this is an unbalanced force applied away from CM. As a result, the airplane will lose some of its lift and will start sinking. Only later will lift grow and pull the airplane up.

Now, if you consider it as a single motion (which it is, fundamentally), you can rightfully say that it is a pure rotation about a point ahead of CM. (It would be nice to draw a diagram here, but not now). The pilot will perceive it as such. On some designs, particularly with delta wings, this initial loss of lift from elevons is so strong that the virtual point of rotation can be ahead of the aircraft (and thus the cockpit). Space shuttle reportedly had such configuration. For pilots, this creates a weird sensation that the aircraft behaves contrary to the expectation, particularly on landing where perception of altitude is acute. This can lead to pilot-induced oscillations.

So, for any motion, not only spin, to say that rotation always happens around CM is a simplification. To be able to say that and remain valid, you have to consider all the translation effects as well. In most cases, this is still the easiest way, but if you want to speak of rotation alone, remember that the instantaneous centre of rotation rarely aligns with CM.


Are you mandated to use a Fault isolation manual when clearing a defect on an aircraft? I operate a Bombardier Dash 8 Q400. I want to know if aircraft engineers are mandated to close or rectify a defect by only following the fault isolation manual (FIM). Can an engineer instead use his knowledge and experience of the systems to isolate the problem and resolve the defect?

FIM contains procedures for both A/C systems (E.G. EICAS, ECAM, CPDS) messages and pilots/maitenance observations (e.g. smell of burning oil, windshield delamination).
All kind of reasonably possible faults should be covered by FIM.

Just simply use yor FIM preambule to determine if it specifies what to do when fault is not covered by FIM and to determine either mechanic judgement or OEM (A/C manufacturer)contact is requied in such case.
If such information is not included contact OEM.


How are helicopter cyclic and collective jacks connected? Regardless of the degree of collective pitch applied, the cyclic pitch controls must retain their full range. Both controls are via jacks, so how are the jacks connected?

The helicopter Collective and Cyclic controls are connected to the rotor bladesby swash-plates. The following is a swash-plate :

The following animation shows the connection assembly working :

Swash plate in the resting position.

A raised swash plate causing negative collective blade pitch and thus down force. Note that the control arms are on the trailing sides of the blades, causing raised swash plate to decrease the blade pitch.

A tilted swashplate giving cyclic blade control. Note the change in pitch of the blades during rotation.

Source : Wikipedia


Why does an airplane captain sit in the left seat? For almost every airline, the captain sits in the left seat, and the first officer on the right.

We know that in most countries right-side traffic and some have left-side.
During training, the student sits in the left seat, and the trainer on the right.

Keeping these two points in mind, why does the captain always sits on left? It is not based on culture (see #1 above), and not on who has more experience1 to fly an airplane from both seats.

1: I know that there can be a situation when the first officer is more experienced than the captain, but most of the time, it is not the case.

There are several reasons, mostly historical, or theories behind this:

After World War I, most airplanes had rotary engines with left turning tendencies because it followed the torque of the engine. Therefore, turning left was easier than turning right. Because of this, pilots considered left turns as more convenient maneuver, and thus more experienced pilot started sitting on the left.
Since the early days of aviation, the fact that pilot was sitting on the left, they keep the airplane on the right side along the airway. At that time, pilots often navigate visually following roads and railways. Opposite traffic following the same path then pass each other on the left.
Because captain sits on the left, left-hand traffic patterns are more common.
In order to avoid a head-on collision, each airplane must turn right. Since the captain sits on the left, they will have an unrestricted view of the other airplane, and therefore can assess the situation easily.

In modern times, almost all airplanes (other than single-pilot airplanes or where one pilot sits behind the other), both seats have full controls to fly the airplane.

Regulations do not specify which seat should be occupied by the pilot-in-command (PIC) or the Pilot Flying (PF).

Please note that on many helicopters, captains sit on the right side.

Bibliography:

Why a Captain sits on the left side of a plane?
Why Does The Captain Sit On The Left?
Why Captain sits on the Left?


Do the A380 ailerons act as air brakes on landing? Last month I was on an Emirates A380 to London Heathrow. I noticed on the landing, the right ailerons were fully up along with the spoilers when decelerating on the ground. Are the ailerons configured for acting as air brakes on landings? Or was the pilot perhaps counteracting a crosswind?

Answer is yes, good eye. From the A380 flight crew manual:

You can watch it here.


Why is the weather radar mounted on the left wing of the TBM? Weather radar is mounted starboard on the Pilatus PC12 or in the Piper Meridian/M600/M500.

It adds drag to the right wing, thus a right yawing moment, and helps to counteract the slipstream effect and P-factor that tends to yaw the aircraft to the left.

So why is the wx-radar mounted port on the TBM ? (the propeller rotates in the same direction as the PC12, so it seems like the wx-radar only adds more yaw to the left)

After some detective work I have concluded:

In the TBM planes, the center portion of the right wing leading edge is occupied by the stall warning vane (or an optional AOA sensor in newer models). The two pitot tubes are located under each wing and landing lights are positioned at wingtips. This would leave the current location for the optional weather radar pretty much the only option available.

TBM 700C pilot information manual, stall warning vane description on page 7.15.1

Source for picture


Why does airflow separate from the wing during stall? What causes the airflow separation from the airplane wing, that triggers a stall?

To be more precise, what is the detailed explanation of the physics for why flow separation occurs at certain speeds & angles of attack?

Air flowing underneath the wing at an angle is pushed downwards, regardless of the shape of the lower surface: high pressure has few practical limits.

Air flowing over the upper surface cannot suddenly change direction though because it is driven by an under-pressure gradient. It can follow a slowly curving surface to the limit of the pressure differential between ambient and suction, but too steep a curve and the airflow cannot follow and has to separate: a stall occurs.

Picture source


When would these two fixes at KMEM be used? When flying IFR into a class B airport you usually file a STAR at the end of your plan. After the last fix on the STAR you will then receive vectors from ATC to the approach for the assigned runway. At that time you would use the appropriate approach plate for information to complete the approach.

I was looking at some of the approach plates for KMEM and some of them contain multiple IAF's. For example, if you are coming in on the HOBRK 1 STAR and are assigned 36C it says that you should turn right to 53° and expect vectors after ROCAB.

But when you look at the approach plate for 18C you see two extra IAF's, OWAFO and IHTEM being approached directly from the UJM and HLI VOR's respectively.

If one should expect to be on vectors by that point when would these other two IAF's be used?

Those are part of the Basic T of the Terminal Arrival Area (TAA).

  The basic design of the RNAV procedure
  underlying the TAA is normally the “T” design (also
  called the “Basic T”). The “T” design incorporates
  two IAFs plus a dual purpose IF/IAF that functions as
  both an intermediate fix and an initial approach fix.
  The T configuration continues from the IF/IAF to the
  final approach fix (FAF) and then to the missed
  approach point (MAP). The two base leg IAFs are
  typically aligned in a straight-line perpendicular to
  the intermediate course connecting at the IF/IAF...

  – AIM 5-4-8

They're used in vectoring. Depending on where the aircraft is coming from, it'll be directed to the dual purpose IF/IAF or one of the flanking IAF's. According to the AIM they're not used when traffic levels are high. They can be used with or without a STAR, either way they reduce the workload.

  TAAs are primarily used on RNAV
  approaches but may be used on an ILS approach when
  RNAV is the sole means for navigation to the IF;
  however, they are not normally used in areas of heavy
  concentration of air traffic.

Follows is a brief description of TAA's. (Linked article is also informative.)

  TAA effectively replace feeder routes, and they enable ATC to issue succinct, efficient clearances to RNAV-capable aircraft flying along an airway that passes over or near an airport or proceeding direct toward an airport from any direction. Using TAA eliminates the need for a series of vectors, which also makes them useful in areas where ATC may have poor radar coverage at the altitudes typically needed to guide airplanes onto feeder routes or the final approach course.

  – bruceair.wordpress.com


Why is the temperature of the cabin so low during a flight? While I understand that at very high altitudes the ambient air temperature is extremely cold outside the aircraft (-56 F), it always bothers me that I find myself sneezing and genuinely uncomfortable once I'm about 30 minutes into my flight.

The question I am specifically asking is, why is the chosen temperature inside the cabin low as opposed to high? Surely it could be set a few degrees higher to a "warmer" temperature?

On the 737, I can make the cabin as hot as I want to; the challenge is more often adequate cooling if the aircraft got hot on the ground. With the insulation that the aircraft has, the generated heat of 100+ people in a confined space overwhelms the cooling of the outside air. (Yeah, it's utterly counter-intuitive, and I can't explain the why, but that is the way it goes. With the loss of all bleed air, the cabin will get hot -- although that's at 10,000', where you descended after the depressurization.)

I've never been told to run the temperature hot or cold as a fuel-saving measure. In theory, you save a little gas by not running the AC packs in "High", but again, that's mostly used to get more cooling air. Or to quickly warm a jet that sat unheated all night in freezing temperatures.

If the cabin was unbearably cold on a particular flight, somebody just wasn't attentive to the temperature. On any modern airliner, we have the tools to warm things up. Maybe the flight attendants prefered a cold cabin & had sweaters to wear; maybe "really cold" to the OP is actually pretty comfortable to others; maybe the thermostat was indicating erroneously high temperature in the cabin so the automatic system kept putting out cold air; or, maybe the pilots were being jerks and "freezing out" the flight attendants (and passengers) for some reason. Unlikely, that last, but not impossible or unheard of.

As far as a preference to run slightly warm or cool, cool will generally win, simply because warm tends to make airsickness worse, while cool has the opposite effect. On a bumpy flight, the last thing you want is a warm cabin; the saying goes that "popsicles don't puke," and it is accurate.


How is heading / windspeed written? You have been given wind direction and speed, how would it be expressed?

Thanks

METAR wind is expressed like this:

04011KT indicates the wind is from 040° true (north east) at 11 knots (20 km/h; 13 mph). In the United States, the wind direction must have a 60° or greater variance for variable wind direction to be reported and the wind speed must be greater than 3 knots (5.6 km/h; 3.5 mph).

04011G20KT indicates wind gusts reaching 20 knots.

090V150 indicates the wind direction is varying from 90° true (east) to 150° true (south-southeast).


What is the purpose of fins on the empennage on some helicopters? The S-70 Black Hawk has some kind of movable surfaces on its tail, look:

Source: Airliners.net

They look like "elevators" on airplanes. Does anybody know what are these things for?

It's a stabilator used on the Blackhawk to reduce attitude changes with airspeed and to offset pitch effects from the tail rotor.  It's part of the automatic flight control system.

All "traditional" design helicopters have horizontal stabilisers to push the tail down to reduce the nose down pitch in forward flight.  Fixed ones are normally trimmed to provide a more level deck at typical cruise speed.

On the Blackhawk, it's fully movable. It is automatically adjusted according to collective pitch and airspeed. From here, the benefits are:

Align stabilator and main rotor downwash in low speed flight to minimize nose up attitude resulting from downwash.
Provide collective coupling to minimize pitch attitude excursions due to collective inputs from the pilot.
Decrease angle of incidence with increased airspeed to improve static stability.
Provide sideslip to pitch coupling to reduce susceptibility to gusts.
Provide pitch rate feedback to improve dynamic stability.


Why does the Antonov 225 needs 6 crew members? Why does the Antonov 225 need 6 flight deck crew? What are they called and what do they do?

A quick search points to the following being part of the crew

Pilot
Co-pilot
2x Flight engineers: one on the engine controls; I am not sure of the purpose of the second one but a good guess is hydraulic and electric system monitoring, based on his proximity to the circuit breaker panel (source, scroll down for a 3D photograph of the flight deck).
Navigator: due to the age of the type and the amount of non-standard routes it flies.
Radio operator: same reason as above.

The crew could likely be reduced after a modernization of the cockpit, without upgrading any other hardware like the engines, but the re-certification costs probably outweigh the benefits in this case, for a fleet size of 1.

Mostly sourced from this thread: discussion on Airliners.net about the An-225 crew duties, which also includes good images with annoyingly restrictive copyrights.


What is the air crash featured in Queen's Innuendo official video? There's a short cameo of a total destruction of a plane in Queen's Innuendo official video (2:11).

Here's the video itself:

And here is a screenshot of the most important frame:

Is anyone able to identify this particular air crash and the airplane involved?

It was a test crash of a DC-7 conducted by the Flight Safety Foundation under contract to the FAA and in conjunction with NASA (among other organizations) in 1964.

Here is the FAA film, and a NASA report.


Why isn't the An-72 / Boeing YC-14 configuration used for airliners? Why aren't there a lot of passenger airliners (short flights) using this configuration, since using Coanda effect by blowing wing's extrados is much safer in harsh weather, and would provide extra safety during gusty landings and takeoffs?

I guess extra weight, induced drag, and engine maintenance height may be good reasons, but is it enough to eliminate this solution in the economy/safety equation?

The extra lift provided by the blown flaps and the better protection against FOD is simply not needed in regular airline service, and the higher engine location will make maintenance harder and more expensive. Airport runways are long enough for most purposes already, and only tactical transports could possibly benefit from the enhanced lift and better FOD protection.

At least the An-72 and -74 is operated by a number of civilian and military entities. Note that all of them serve small, local airfields. Note also that passenger service with the An-72 has been discontinued in Finland since this type was not cleared there for passenger services.


Do floatplanes land on rivers? I'm sitting here in a coffee shop on the local river and I noticed it was just wide enough to facilitate a landing. There is no chop on the river, but it does move pretty switfly, and there are bridges and whatnot that would quickly come into play. However, there are some pretty slow-moving, unobstructed rivers out there.

Does anyone land floatplanes on rivers for reasons other than emergencies?

Yes, here is a river landing.

and another, even tighter. This might technically be a beaver dam pond in a river.

and another, prettier. I'd back this one up and watch the approach.


What does the NCrit parameter indicate in a CFD analysis? When analyzing an airfoil in the XFLR5 software I am asked to input some data regarding the free stream. For this, a dialogue box pops up that has the following options:

One field contains the expected Reynolds and Mach numbers for the flow.

The other field says Ncrit. What is this parameter, and what does it have to do with the trip location (top and bottom) which can be inputted together with it?

The whole setting group is called transition settings, and refers to the boundary layer transition point (from laminar to turbulent).
The forced transition trip locations are exactly that, expressed as a fraction of the chord, for the top and bottom surfaces. They are akin to placing a trip strip at those locations.
The Ncrit value is a measure of free flow turbulence and is used to simulate the transition location when no forced trip location is given. XFLR5 uses the same method as XFOIL, which is an adaptation of the $e^N$ transition theory. Note this linear theory loses its validity at a fluctuation level of 1 to 1.5 percent, which corresponds to an N-factor of 6.9 to 7.3, meaning it is not suitable for highly turbulent flows.

As to what this is important for, the turbulent or laminar nature of the boundary layer affects flow separation and viscous drag. A turbulent layer will generate more drag, but separate at higher airfoil angles of attack.
Where this gets really interesting is at low Reynolds numbers, where a laminar layer will detach, but become reattached if turbulence is introduced into the airflow ahead of the airfoil, such as by a turbulator. It will then remain attached if the turbulator is removed, displaying a hysteresis loop. The same effect will of course occur in the opposite direction, with a barely attached turbulent layer detaching and staying that way if the free flow turbulence decreases enough to cause it to turn laminar.
I must note that this effect is confined to low Reynolds numbers, on the order of $10^4$.


Can an aircraft on loan in the US be based outside the US? I'm interested if there is any possible way to get an aircraft (Piper Navajo Chieftain) on loan for a US citizen and then base the aircraft outside the USA?

Any US-registered aircraft can be based in any country subject to that country's rules. Just check with your local aviation authorities in the country where you want the aircraft to be based before you reposition the aircraft. Some may require you to depart the country every so many days/weeks/months, sometimes in as simple a way as a short hop across a border, but to my knowledge most don't have any restrictions.


How can I see the revision history of a particular FAR? For instance, I don't remember paragraph 61.51(e)(iii) at all from way back when I studied it to determine when I could log PIC time.  I'm fairly certain that it has been added since then (actually, there was no sport pilot license back then so I know that it has at least been changed), but I would like to know when it was changed and what the changes were.
At the bottom of the reg, it includes this:

[Doc. No. 25910, 62 FR 16298, Apr. 4, 1997; Amdt. 61-103, 62 FR 40897,
July 30, 1997; Amdt. 61-104, 63 FR 20286, Apr. 23, 1998; Amdt. 61-110,
69 FR 44865, July 27, 2004; Amdt. 61-124, 74 FR 42549, Aug. 21, 2009;
Amdt. 61-128, 76 FR 54105, Aug. 31, 2011]

This tells me when it was amended, but not what was amended.
Does the FAA make that information available?

Changes to federal aviation regulations are listed on the FAA’s Regulatory & Guidance Library (RGL) at http://rgl.faa.gov/:

From the home page, click on the database titled "Code of Federal Regulations"
On the left, in the navigation tree, select one of the branches under "Historical CFR" (for example, "By Part")
On the right, expand the items leading to the regulation that you are looking for (in this case Part 61)
Scroll down to the regulation that you are looking for (61.51) and it will list the original version and all of the revisions:

Each revision is a link which takes you to the regulation as of that version, with links to the NPRM's and dockets so that you can see what has been changed!


What's the aisle width of long range business jet? What's the average aisle width for long range business jet such as Gulfstream G500?

Based on this cabin layout of a Gulfstream G500 found on flexjet.com, I measured the aisle width at various places:

The total width is given as 7' 11'' = 241.3cm. The aisle width at the marked positions is then:

63cm (2' 1'')
68cm (2' 3'')
44cm (1' 5'')
51cm (1' 8'')
46cm (1' 6'')
57cm (1' 10'')
50cm (1' 8'')

This would give an average of 54cm (1' 9'') over the measured positions.

Interestingly, the narrowest part (44cm) is less than the FAA minimum for transport category aircraft with more than 10 seats (Is there any limit on how narrow aisles can be?), which would be 20in = 50.8cm.


How strong must wind gusts be to prevent landings and takeoffs? I hope this is not as trivial as it sounds. Is there a wind gusts threshold based on which, at a given airport, incoming flights are diverted and outgoing ones are delayed?

I assume that aircraft are designed to handle a specific maximum wind gust, so if there is any such threshold, I expect it to be less than the design threshold. Is this correct?

You have two questions here,

  I hope this is not as trivial as it sounds. Is there a wind gusts
  threshold based on which, at a given airport, incoming flights are
  diverted and outgoing ones are delayed?

As far as I know there is no specific threshold for which they will divert. You may want to read up on wind shear here as an airport may advise sever or unsafe wind shear and urge pilots to divert. There will be a point (often at single runway airports) where a landing in a particular type of aircraft becomes unsafe or even impossible as a result of wind but this will vary by plane.

  Also, I assume that aircraft are designed according to a specific max
  wind gust, so if there is any wind gusts threshold, I expect it to be
  less than the design threshold. Is this correct?

Yes, aircraft are designed with whats called a maximum demonstrated crosswind component. The key here is that this is a demonstrated limit not necessarily an airframe limitation. The number is published in the POH for the aircraft made after 1975 and provides a general guideline of the maximum cross wind the plane can be landed safely in however its more than possible to land it in higher wind situations.

Also it should be noted that some of this is up to the pilot in command. Just because a towered airport clears you to land does not mean it is safe for you to do so likewise just because a pilot diverts does not mean they were instructed to do so by the tower they may self elect based on their personal abilities.


Does the Piper PA-28 Cherokee wing have laminar flow behavior? I've heard (and read) that the PA-28 series has a laminar flow wing:

  The Piper had a very thick airfoil that used laminar flow to assure that lift would be lost very slowly up to the stall.

People seem to pitch the Cherokee's wing as some sort of amazing safety and performance improvement. I've always understood that laminar flow wings had an abrupt, harsh stall, but offered reduced drag in cruise. I have a ton of early-Cherokee time on the 'Hershey Bar' wing, and its stall behavior is gentle and predictable, while its cruise speed is almost knot-for-knot on par with a Cessna 172 of comparable power.

Is the PA-28 laminar flow wing just a marketing gimmick? If one were to put a Cherokee wing in a wind tunnel, would we see laminar flow?

Short answer: Yes. You would see laminar flow.

Longer answer: It depends. In case of the wind tunnel it depends on the laminarity of the wind tunnel: Older designs had too much turbulence to allow for much laminarity. It was precisely for that reason that laminar flow was not well understood in the first decades of aviation.

The airfoil on the Cherokee is the NACA $65_2415$, and its Reynolds number in normal operation is in the single digit millions. The laminar bucket is ideally between the lift coefficients from 0.2 to 0.6, and the Reynolds number is low enough to allow for laminar flow. When you approach stall, however, the upper side has a pronounced suction peak near the nose which lets the flow rapidly transition to turbulent flow right after the peak. Laminar flow can only be found on the lower side (if surface roughness is low enough).

The pressure distribution over the rear 50% of the airfoil shows a constant pressure rise, so the onset of stall is marked by flow separation which starts at the trailing edge and slowly marches forward with increasing angle of attack. This leaves the suction area in the forward part unperturbed and keeps lift nearly constant around stall.

Contrast this with the stall behavior of the 5-digit NACA series, which can only be described as nasty. Stall is marked by flow separation right past the suction peak at the nose, and the lift coefficient suddenly drops by at least 0.2 counts when the airfoil stalls. Only some laminar airfoils have a similarly harsh stall behavior; this happens mostly when their designers were too ambitious and achieved the pressure rise with a Stratford pressure distribution. This is a distribution which keeps the separation margin constant along its length, and at stall it is exceeded everywhere at once, creating a sudden, large separation.

A well-designed laminar airfoil has a gentle stall behavior. Its lift coefficients with laminar flow are normally quite a bit away from stall, and its stall characteristics can be tailored independently from the region with laminar flow. Using a NACA $65_2415$ instead of a NACA 23015 is not a marketing gimmick, but a sound choice.

Regarding the performance: As long as the plane has a fixed landing gear, the laminarity of the wing will have little effect on top speed - the parasitic drag of all those things sticking out from the airframe will dominate drag. In addition, the higher the Reynolds number in flight is, the shorter the laminar flow will be before it transitions to turbulent flow, so the effect is much reduced at higher speed. Also, the way the wing is built makes a lot of difference, because every rivet head and every gap will create a wedge of turbulent flow behind it.


Why does enabling engine sync on the MD-80 series trigger the "Master Caution" light? I fly in a consumer PC flight simulator. Among my virtual fleet is a fairly faithfully-reproduced MD-83.

One fairly strange detail I noticed is that enabling engine sync (either on N1 or N2) in the overhead, while on the ramp, triggers not just the alert on the annunciator panel, but also the master caution light -- why is this? Engine sync isn't a particularly abnormal condition. Is this just an inaccuracy in the simulator?

AFAIK The reason you aren't supposed to take off/land with prop or engine sync on is because if the leader engine fails, the follower engine will reduce power to match it, and that is really not ideal, especially low and slow, a la V1 cut or failure on short final. Try it out- fail the leader engine just after V1 and see what happens!


Which kind of identification and authentication is used to secure an airplane? What does the pilot use? a password? my wife identifies and authenticates herself to her car every morning using her car keys. I don't see the pilot in movies use a similar key?! I don't expect the designers to leave an airplane open for any attacker to go to the cabinet and fly away.

Most smaller propeller planes have nothing. When you're in you can fly away. This seems to be the same for those private jets and older Boeings. I don't fly planes, but I've seen that Airplane Repo program on Discovery, and they just get in and fly away. How this works with the newest planes - no idea.


What is the difference between Level Change and ALT INTV during VNAV descent? I'm confused between the two. During descent for 737 PMDG, I usually start a descent via ALT INTV, but sometimes the plane doesn't response to ALT INTV but responds to LVL Change instead.
I need help to understand the difference.

Altitude Intervention will start a descent only if VNAV is the active mode (and the MCP altitude is below the current cruise altitude). If you're in Altitude Hold, then VNAV isn't active, and the Altitude Intervention button won't start a descent.
On the other hand, Level Change is its own mode, and selecting it (with a different altitude set in the MCP than what you're at) will always produce a change - either climbing at climb thrust, or descending at idle.  In both cases, flying at the speed set in the speed window.
VNAV is complicated, because while modes like Level Change, Vertical Speed, Altitude Hold, and GS Capture do "this," where "this" is a single easily understood thing, engaging VNAV means that we've given control to the computer logic, which can do lots of things. And we have to have an understanding of that logic to understand what VNAV will do. The Speed Intervention button and the Altitude Intervention button are inputs to the VNAV system only; VNAV also considers the altitude window and, when it's open, the speed selection. And of course VNAV uses what we've entered into the FMC as well.
For all its complexity, VNAV is one mode; Level Change is another.  To start the descent, Level Change will always do it, but then you aren't in VNAV with its various features (such as meeting the speed & altitude constraints in the LEGS page). If you're level in Altitude Hold and wish to start a descent in VNAV, then you should select VNAV & observe that the FMA changes to show VNAV as the active mode. Now, Altitude Intervention will work, as well as "Descend Now" on the CDU's Descent page.
The key is knowing what mode is active - and that's on the FMA. If Altitude Hold is active, you'll have to change the mode to start descending. You might change to Vertical Speed or Level Change to start down right now.  If VNAV is active, you have the tools which that mode gives you to control your path, and you can also select a different mode too (useful if VNAV isn't doing what you need the airplane to be doing right now).
In line flying, we'll generally prefer to stay in VNAV as much as possible, so if I'm at cruise altitude in Altitude Hold, I'll generally engage VNAV. Thus, I can control our cruise speed via the FMC, and when I'm given "descend at pilot discretion..." I can set the MCP altitude to the new clearance, and let VNAV manage the descent - starting it right at the calculated Top of Descent, managing the speed thru the transition from Mach to IAS, meeting step-down constraints on an arrival, etc.
There are times that the other modes are useful; for a 1,000' climb or descent, Vertical Speed at 1,000 FPM is a more gentle ride than going to full climb or all the way back to idle, and for "descend now to FL xxx and give me a good rate" then Level Change fits the bill.
Mostly, though, VNAV is what's active.


Do aircraft engineers take into consideration engine placement to avoid ground strikes in a cross-wind landing? On a multi-engine airplane with wing-mounted engines, do aircraft designers have a standard way of ensuring that the banked landings associated with cross-wind's do not cause a prop or engine-housing strike on the ground if the pilot were to over-bank?

Is there some rule such as, "the wing tip would need to be the first part to strike if the wheel is in contact with the ground", or similar standard of practice in design?

For jet aircraft, there is no rule about preventing a nacelle from contacting the ground before the wingtip. There will be a design requirement that the airplane can touch down at a certain angle of bank and have the landing gear touch down but not strike a nacelle or wing, but not whether nacelle or wing must strike first. The nacelle is already designed for a wheels-up landing, so it's possible it could survive better than a wing tip. While it's certainly more of a risk on 4-engine aircraft, it can also happen on twin jets.

Propeller aircraft are typically designed such that gear and wingtip will make contact without the propeller making contact, either through having gear directly below props on a low-wing configuration, or through a high-wing configuration. While a jet engine nacelle can sustain some structural damage in a strike, it will typically not affect the engine's operation. If a propeller gets damaged it could cause that engine to become unusable, making it much more difficult for the crew to successfully perform a go-around.


What's the difference between a balked landing and a go-around? Is there a difference between a balked landing and a go-around?

If there is, what exactly is a balked landing?

The only term in the Pilot Controller Glossary is Go Around, so should technically be the only term used, at least when communicating with ATC:

  GO AROUND- Instructions for a pilot to abandon his/her approach to landing. Additional instructions may follow. Unless otherwise advised
  by ATC, a VFR aircraft or an aircraft conducting visual approach
  should overfly the runway while climbing to traffic pattern altitude
  and enter the traffic pattern via the crosswind leg. A pilot on an IFR
  flight plan making an instrument approach should execute the published
  missed approach procedure or proceed as instructed by ATC; e.g., "Go
  around" (additional instructions if required).

  (See LOW APPROACH.)

  (See MISSED APPROACH.)

The ATP PTS also includes a Rejected Landing, which is what our simulator instructors refer to as a Balked Landing, and must be at or below 50 feet AGL.  It has the following note describing it in the PTS:

  NOTE: The maneuver may be combined with instrument, circling,  or missed approach procedures, but instrument conditions  need not be
  simulated below 100 feet above the runway.  This maneuver should be
  initiated approximately 50 feet  above the runway or landing area and
  approximately over  the runway threshold or as recommended by the FSB
  Report.

The pilot follows the same procedures for both of them by aborting the landing, cleaning up the airplane, and flying the appropriate procedure afterwards.


What is this nose cowl mounted display? (YouTube) A crop duster's nose mounted display.

It looks like a poor man's HUD to keep the display and the outside view at roughly the same focus distance, but what is its function?

This is a GPS guidance indicator from AG-NAV, with the main unit visible onboard. Snapshot from your linked video:

Main unit

Using the main unit, different information can be displayed on the lightbar:

GPS cross-track
Angle of Intercept
Overlap in sprayed area
Area already treated
Spray height
Aircraft inside/outside area indicator
...

Basic data can also be shown (e.g. airspeed) See this video. The indicator itself:

Lightbar

More:

Lightbar brochure.
Main unit brochure.


What are the advantages of the Airbus New Engine Option (NEO) program? Airbus is planning to roll out an Airbus A320neo to replace its current A320 model, and they will offer this option on other models too.

How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer? Why wouldn't the manufacturer optimize its plane for one engine specifically, to give best performance and roll out the plane like that?

How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer?

There are a couple of ways of looking at this

Why new options?
Why multiple options

Change

The A320 has been around a while, engines have improved a lot and newer types of engine exist now that were not available before, even though the A320 type airframe is still competetive.

According to Airbus, new choice of engines allow for

Lower fuel costs
Lower maintenance costs

  Airbus is working on offering a new engine for the A320 known as the New Engine Option (NEO). The choice for new engines include the CFM International LEAP-X and the Pratt & Whitney PW1400G/PW1500G. The new engines will burn 16% less fuel: the actual fuel gain on an A320 installation will be slightly less, since 1-2% is typically lost upon installation on an existing aircraft. Airbus CEO said to be "comfortable" with the projections of 20% lower maintenance cost for the Pratt & Whitney's PW1000G family, compared with today's engines.

From Airbus

Choice

Speculating a little:

The people who buy and operate aircraft like having a choice of engine manufacturers. It helps keep the engine market competitive and it means they can, if they want, operate a range of aircraft all using engines from one maker - potentially allowing for savings in engine maintenance infrastructure.

Aircraft operators sometimes don't actually buy engines with their aircraft, they buy thrust from an engine maker and the engine maker (effectively) install their engines on the operators aircraft. The engines might remain the property of the maker.

It may also be that aircraft makers don't wish the future of their aircraft to be tied to a single supplier of engines. It makes negotiating harder and it makes their future prosperity dependent on the continued existence of one manufacturer.