How do I calculate new minimal speed for a glider when I put water into the tank? Lets say a glider weights 400kg and has a minimal speed of 70km/h.

When it has additionally 100l water, so in total 500kg, what is the new minimal speed?

I would argue, that in order to reach the same point in the polar diagram, we have to scale according to

$v_2 = v_1 \cdot \sqrt{m_2/m_1}$

because drag and lift forces scale quadratic with speed.

The result would be 78km/h.

However, in the theory we learn

$v_2 = v_1 \cdot m_2/m_1 = 87km/h$

Q: Isn't that wrong?

$v_2 = v_1 \cdot \sqrt{m_2/m_1}$ is the correct formula.

$v_2 = v_1 \cdot m_2/m_1 = 87km/h$ is simply wrong, possible that it is only a typing mistake, but it is wrong just the same.


Are wings any more efficient at creating lift, versus orienting the engine's thrust downwards? As I understood from this article, wings on an airplane basically create lift by pushing air down.

Maintaining the lift force expends energy because air needs to be continually accelerated downwards. In the "energy budget" of the airplane, this energy cost appears in the form of extra drag due to the wing, which must be compensated by extra engine thrust.

It seems to me then that the power required to keep an airplane at a constant altitude using a wing is exactly equivalent to the power that would be needed if we would "simply" use a downwards-pointing fan instead of the wing.

Now, some posts on this website compare VTOL aircraft to airliners of similar size, and state that VTOL requires much more thrust (e.g. here - I would post more links but I'm not allowed to). But according to the reasoning above, shouldn't it require exactly the same amount of power to put a plane in the air, whether using a wing or directing the engine thrust downwards?

Please disregard, for a moment, the technical difficulties and safety issues. They are discussed in other questions related to gimbal thrust and VTOL aircraft. My question has to do with energy expenditure and power.

Interesting question. Purely empirically, it is the lift-to-drag ratio you are looking for. If you take this value as given for any particular aircraft, you have a direct answer for how much more effective wings are. It is the ratio of the lift to the total drag. The engine only needs to overcome the drag.

With L/D equal to unity you would need the same thrust as for the vertical take off. But even quite "bad" fixed wing aircraft would have L/D about 5. Gliders or similar aircraft built with a strong focus on aerodynamics can have an L/D of 50 or more (at least in some narrow range of airspeeds).

So yes, wings are more efficient. About one order of magnitude as a rule of thumb for common aircraft and optimal airspeed.

Why your reasoning with air pushed down is incorrect is more tricky to explain. I'll start with the assumption that, as air passes an airfoil, its speed relative to the airfoil is unaltered, and only the direction changes. (I know air slows down at least because of friction etc., but these are, at least theoretically, avoidable things not related directly to creating the lift. If there is something intrinsically related to lift which makes airflow change not only direction but speed too, then someone will hopefully correct me here.)

See the image. Airmass moving initially towards the airfoil with velocity $\vec{v_0}$ is deflected down by angle $\alpha$. Therefore the change in the velocity is $\vec{\Delta v}$. This change can be divided into horizontal and vertical components. To hold the aircraft in the air, the vertical component has to be equivalent to the aircraft's weight divided by the mass flow rate over the wing. The vertical component is related to the horizontal by
$$
\Delta v_{\rm horiz}=\Delta v_{\rm vert} \cdot \tan{\alpha\over 2}.
$$

So, from this simplistic view, the drag would be $\tan\alpha/2$ times the lift. Higher mass flow rate over the wing (longer wings, higher airspeed) makes it possible to keep the same lift with lower deflection ($\alpha$), thus less drag due to the generated lift.

Additional comment: how is it related to power and energy

The answer above is focused on how wings decrease necessary engine thrust, but original question could be interpreted in terms of energy efficiency too. I'll try to add some comments on this part.

simple example – rocket engine: not too typical for aircraft, but simple one. Rocket consumes same amount of fuel per second in order to generate a unit of thurst regardless of its size and regardless if it is pointing upward (and is static with respect to air) or forward (and moving through the air). You need to burn proportionally more fuel per second to generate higher thrust. So, for the rocket propulsion, you will save fuel in the same ratio as the necessary thrust decreases.
Propeller or jet engines are more complicate as their thrust and fuel consumption depends on engine movement through the air too. As David K pointed out in his answer, we can use momentum and kinetic energy of accelerated air to get power needed for unit of thrust.

With some simplifications, thrust is mass flow rate through the engine/prop multiplied by change of flow speed it causes. $T = \dot m \cdot (v_{\rm out} - v_{\rm in}) = \dot m \Delta v$. Power needed for this is $P = \dot m \cdot {1\over2}(v_{\rm out}^2 - v_{\rm in}^2)=\dot m\Delta v\cdot(v_{\rm in} + {\Delta v\over 2})$. Thus
$$
{P\over T}=v_{\rm in} + {\Delta v\over 2}\,.
$$

For stationary engine holding against gravity, higher thrust compared to flying fixed wing aircraft is needed as shown above. If we do not "cheat" by increasing mass flow rate through engine (like making it helicopter rotor or using multiple engines), $\Delta v$ has to be increased in  order to achieve the necessary thrust. So you not only need more power because of increased thrust but also more power because of increased Watts per unit of thrust. Note that even "helicopter cheat" does not work too well. To match power consumption of engine generating less thrust thanks to the wing's L/D you need to improve P/T too – by decreasing $\Delta v$, thus increasing mass flow (rotor/propeller radius even more than proportionally to increased thrust).

What about decrease of P/T due to the movement through air? Well, it depends on particular engine and its $\Delta v$. It will be typically in similar order of magnitude as airspeed (or even less), so we can not neglect $v_{\rm in}$ in watts-per-thrust equation above. There is a efficiency penalty when engine works on moving aircraft. But it should be still worth of it as the gain provided by lift is greater.

A simplified example: We have an engine capable of producing enough thrust to lift aircraft vertically. It can be throttled by changing $\Delta v$ without any practical problems or change in its internal efficiency. And let's assume that mass flow rate through it is fixed area $S$ multiplied by air density and multiplied by arithmetical average of speeds of entering and exiting air. For hovering aircraft and stationary engine producing thrust equal to aircraft's weight, $w$ it is
$$w=\dot m \Delta v_{\rm hover} = \rho S \Delta v_{\rm hover}^2 / 2\,;\quad \Delta v_{\rm hover}=\sqrt{2 w \over \rho S}$$
and thus
$$P_{\rm hover}=w\cdot \Delta v_{\rm hover}/2=\sqrt{w^3\over 2\rho S}\,.$$

The same aircraft flying on its wings does only need $w\over L/D$ of thrust. Airspeed is $v_{\rm air}$. Equation for the thrust: ${w\over L/D} = \dot m \Delta v_{\rm flight} = \rho S \cdot (v_{\rm air}+{\Delta v_{\rm flight} \over 2}) \cdot \Delta v_{\rm flight}$. Thus
$$\Delta v_{\rm flight}=\sqrt{{2w\over(L/D)\rho S}+v_{\rm air}^2}-v_{\rm air}$$
and
$$
P_{\rm flight}={w\over L/D}\cdot(\sqrt{{w\over 2(L/D)\rho S}+{v_{\rm air}^2\over 4}}+{v_{\rm air}\over 2})\,.
$$

Unfortunately, I do not see any way how to simplify and compare $P_{\rm hover}$ and $P_{\rm flight}$ so some concrete numbers:

Light aircraft, 1 ton, 100 knots, $S = 5\,\rm m^2$, $L/D = 15$: $P_{\rm hover} = 290\,\rm kW$, $P_{\rm flight} = 35\,\rm kW$.
Heavy aircraft, 100 ton, 200 knots, $S = 50\,\rm m^2$, $L/D = 15$: $P_{\rm hover} = 90\,\rm MW$, $P_{\rm flight} = 7\,\rm MW$.

So, based on these simplifications flying with wings with similar kind of engine should be significantly more efficient in energy terms too. And, additionally, you are already moving forward using power $P_{\rm flight}$. For vertical engine extra power would be necessary for overcoming air drag due to the movement.


Why do the Diamond DA-42 and DA-62 have a minimum fuel temperature requirement? On the turbo diesel DA-42 and DA-62 aircraft there exists a minimum fuel temperature limitation where you may not start or operate the aircraft below that.  Does anybody know why this is?  It seems strange as the turbo Diesel engines themselves have their own dedicated engine fuel pumps which raise the pressure of the fuel to 19,500 psi prior to infection in the cylinders.  This should make the fuel plenty hot for combustion.

Is this minimum fuel temperature requirement typical for all reciprocating Diesel engines?

From the DA-42 TDI Approved Flight Manual:

Fuel filter is not heated and can clog if the Diesel fuel flocculates. Jet fuel does not pose same limitations.


Why there is no online radar coverage of Madagascar? I noticed that no planes heading to FMMI are plotted on all the prominent online radars despite the fact that most of them are likely equipped with ADS-B.

What does it take to remedy that lack of coverage?

For services like FlightRadar24 they typically get data in two ways: via volunteers hosting ADS-B receiver stations on the ground, and directly from the ATC radar system in places where that data is readily available.

For the ADS-B equipped aircraft these services would need volunteers in the area of Ivato Airport (FMMI) to host ADS-B receivers. They would almost certainly be happy to have more receivers, and they even offer free "premium" services (and occasionally free receivers) to people who help provide data. If it's legal in your area, and you are willing to run a receiver station you may want to look into it, and this will get you aircraft which are broadcasting ADS-B data (Mode S+ES with GPS position information).

For the aircraft which are not broadcasting ADS-B information (because they lack an appropriate transponder or GPS system) the problem is a little more complicated - the organization that runs ATC in Madagascar would need to set up radar sites (which presumably exist), and make that information publicly available (which may or may not be the case). A system would then need to be developed to allow the online radar services to process that data and make the flight tracks available (subject to whatever delays may be imposed in that process).

As I don't know much (anything really) about the ATC system in Madagascar, so I'm not sure how robust the radar infrastructure is or how open they might be with their data, so I can't really comment on that half of the system.


What is this thing between the engines on the F-14 Tomcat? On the F-14 as you can see from a picture there is some exhaust like thing in between engines. What is this?

This is the line for dumping fuel in an emergency. If the F-14 needs to land quickly but is too heavy for landing, fuel can be pumped overboard through this pipe. To the left of it is an antenna, and the rod below the fuselage is the arrestor hook.

The Australian F-111s use the same arrangement to great effect during flight displays.

From the F-14 Walk-Around Photo Index Page:

  Back end of the "beaver tail".  The red hole is the fuel dump point.
  The beige item is the rear ALQ-126 antenna, paired to the ones on the
  wing glove area.  This antenna, like the forward ones, was introduced
  with block 110 F-14As and all subsequent aircraft.  Also visible is
  the hook portion of the arresting hook.


What's are the advantages of having the pilot in the cockpit? Instead of a pilot in the cockpit, how far are we from having commercial airplanes with remote pilots (like drones)?

The remote pilot won't have any kind of space disorientation
Each of its actions would be scrutinized in real-time
pilot could be swapped every t time, so you'd always have a rested pilot
less costs, all remote pilots could have a 9-17 type of work. The airlines would just pick one in the matching time zone
the cockpit could be converted into a 1st A class space

Is there any situation where the pilot needs to feel the airplane, maybe not to override the information from instruments, but to complement it?

I'm aware that this might be a big no-no for some passengers, but how far are we of implementing this in a cargo airplane?

The concept would be a hard sell for both passengers and non-passengers alike. You not only risk lives in the air, but on the ground as well. I would not want a fully laden B747 flying above me. The biggest reasons are:

The human pilot in the cockpit has a vested interest in performing a safe flight. The vast majority of pilots are not suicidal (99.999%). The remote pilot on the ground does not have the same incentive.

Any computer system can be hacked. This would turn said B747 into a weapon.

A remote pilot does not significantly lessen the chances of human error. The remote pilots are still human. And, the programmers of the software needed to make remote piloting possible are equally human.

Satellite communication is not entirely fool-proof. It would have to be perfect to be considered safe enough to warrant that hazard. The current GPS interference is an example of that. That is one of the reasons why GPS is still considered non-precision for flight. You have to have the capability to use traditional NavAids even when you are using GPS.

When (not if) crap hits the fan (or fan disk), and there is a systems failure, the best chances to preserve life is to have the human pilot in direct control. Think of what happens in an electrically power failure, a systems shutdown, a loss of communications capabilities, etc.


Is it legal for a crop duster to fly right over my house while spraying? A crop duster is flying directly over my house while spraying. I can see the mist coming out while I am standing in the middle of my yard (6 acres). Is this legal?

No. 14 CFR 137.37 - Manner of dispensing is pretty clear:

  No persons may dispense, or cause to be dispensed, from an aircraft, any material or substance in a manner that creates a hazard to persons or property on the surface.

Flying directly over you or your house while spraying is illegal. There are factors like altitude etc, but spraying pesticides over people is rarely legal.


What instruments were "standard" in WWI aircraft? What instruments made up a typical panel of a WWI airplane?

Did any country regulate the required cockpit instruments prior to the United States "Air Commerce Act" of 1926?

If you are asking about military aircraft there would often have been a compass, oil pressure gauge, a not very sensitive altimeter, a tachometer and some sort of airspeed indicator. Not sure about fuel gauges. I have also seen bank indicators. There was not the standards for instruments as in later days and some items were specific to individual aircraft types. Some of these guys could fly a sort of instrument flight but had not much in the way of navigation aids.


Can airlines choose whether or not to enable MCAS on their fleets, and if so, have US airlines chosen to do so? I've been told that, although all 737 Max aircraft are equipped with MCAS, the system can be deactivated by the individual airline.  I've also heard that US airlines never activated MCAS in their 737 Max aircraft.  Any truth to either?

No, MCAS was required for the 737 MAX to be certified by the FAA. It cannot be deactivated by any airline, including airlines in the U.S.

Source:

  "M.C.A.S. was necessary then for the airplane [737 MAX] to be
  certified by the F.A.A. to have met all of the regulatory design
  requirements for stability and control."

  https://www.nytimes.com/2019/02/03/world/asia/lion-air-plane-crash-pilots.html


Why does a banked aircraft create sideslip? Below is an excerpt from Raymer (Aircraft Design) where he explains the dihedral effect. He states that a banked aircraft (due to say a roll disturbance) will have a sideslip which leads to a restoring moment back to equilibrium. What is the physical explanation for why there is a sideslip?

Incorrect explanations abound for why an aircraft tends to sideslip when banked, in the absence of any "coordinating" rudder input.  Often the fact that the weight or gravity vector has a component pointing towards one wingtip is invoked.  See for example the diagram in Martin Simons' "Model Airplane Design".
The truth is that this situation exists in a coordinated turn as well as in a sideslip, and so this cannot be the reason for the sideslip. According to Newton's principles, a steady sideways force tends to drive a curvature in a trajectory, not a linear sideways motion.  It is not correct to say that the weight or gravity vector tends to cause the aircraft to slide toward the low wingtip whenever the aircraft is banked, just because the weight or gravity vector contains a component that points toward the low wingtip.
Here is one reason that an aircraft tends to sideslip when banked-- banking tends to cause a turn (a curvature in the flight path), and the resulting curvature in the flight path "bends" the airflow or relative wind.  In other words, as the aircraft starts to yaw in response to the changing direction of travel, different parts of the aircraft are moving through the airmass in different directions at different speeds at any given instant in time, and so different parts of the aircraft experience different velocities (speed and direction) in the local relative wind, which can be described as a "curve" or "bend" in the direction of the relative wind.  In the absence of any rudder input, the vertical tail experiences a relative wind component that tends to oppose the turn and slow the yaw rotation rate.  This tends to cause a sideslip.
If this is unclear, consider a pinwheel, or an aircraft in a flat spin.  As the body rotates, clearly, different parts of the body are moving through the airmass in different directions at any instant in time.
This phenomenon can also be described as "yaw damping".
This is also the phenomenon that is described as the "long-tail slip" effect in Section 8.10 of John S. Denker's "See How It Flies" website.
If the aircraft's fuselage could bend like a banana to conform to the curving path of the turn, then every part of the aircraft from the nose to the tail could be streamlined to the local direction of the curving relative wind, and this phenomenon would not occur.
Rotational inertia in the yaw axis also plays some small role in driving sideslip as the bank angle and turn rate increase.
Explanations that attempt to account for the relationship between banking and and sideslip, without discussing the curvature in the flight path that generally results from banking, are invariably incorrect.


What is the process to get a simulator FAA certified for IFR training? Assuming I've got the hardware and software that meets the FAA requirements for a BATD (Basic Aviation Training Device), what is the process I would go through to get the simulator certified?

The FAA Advisory Circular 61-136 contains the guidance for Aviation Training Devices, including the BATD's.

The AC states that as long as the system has been FAA approved by the manufacturer (and in your case you say that it "meets the FAA requirements"), that no specific approval is required for training conducted under 14 CFR 61.  For use under 14 CFR 141 however, you would need specific FAA approval (see below).

14 CFR 61 Approval

You must simply follow the conditions specified in the AC:

  5. APPROVAL OF BATD FOR USE UNDER PART 61. To be approved for use for pilot   training and certification under part 61, a BATD should:

  a. Be capable of providing procedural training in all elements for
  which it is to be used.  Those elements should be specified in an
  acceptable training curriculum or as specifically  authorized by the
  FAA and meet the description and suggested criteria outlined in this
  appendix.

  b. Have the following documents available for inspection by the
  student and instructor:

  The list of ATD components as required by paragraph 3c,
  A copy of AFS-800’s letter of approval and authorized use of the ATD, and
  A copy of the QAG for the ATD being used.

  c. Successfully pass the start-up self-test described in paragraph
  8c(2). If the device is being used in a course of training for a pilot
  certificate or rating, a person authorized by the FAA to provide
  ground or flight instruction should observe this test. After the ATD
  self-test is complete, no other software of any kind may be started on
  the computer running the ATD software.

  d. Remain in the approved configuration during the training session.
  Authorized ATD  instruction may not proceed after a malfunction of the
  ATD system has occurred. The operator  must correct the ATD
  malfunction and repeat the start-up test described in paragraph c of
  this section before resuming authorized instruction. In addition, a
  BATD may be used in a training curriculum that provides for:

  A similar scope and content of a syllabus that is used under part 141;
  Instructional materials for flight events;
  A means of showing progress, such as an outline of stage (phase) checks and criterion  levels of performance; and
  The authorized use as approved in the AFS-800 letter of approval.

14 CFR 141 Approval

141 Approval is almost the same, but must be approved by the school's POI (principle operations inspector):

  6. APPROVAL OF BATD FOR USE UNDER PART 141. Local FSDO may approve  BATDs as part of an overall part 141 curriculum approval and
  certification process. Pilot schools that want to use a BATD as part
  of their training curriculum must notify their principal operations
  inspector (POI). The POI is responsible for approving how the BATD is
  to be used in the certificate holder’s part 141 curriculum. To be
  approved for use under part 141, a BATD should:

  a. Be capable of providing training in all elements in which it will
  be used, as specified in the syllabus, and meet the description and
  suggested criteria outlined in this appendix.     b. Have the
  following documents available for inspection by the student and
  instructor:

  The list of ATD components required in paragraph 3c,
  A copy of AFS-800’s letter of approval and authorized use of the ATD, and
  A copy of the QAG for the ATD being used.

  c. Successfully pass the start-up self-test described in paragraph 8c
  of this appendix. A  person authorized by the FAA to provide ground or
  flight instruction should observe this test. After the ATD self-test
  is complete, no other software of any kind may be started on the
  computer running the ATD software.

  d. Remain in the approved configuration during the training session.
  Authorized ATD  instruction may not proceed after a malfunction of the
  ATD system has occurred. The operator  must correct the ATD
  malfunction and repeat the start-up test described in paragraph c of
  this section before resuming authorized instruction.

  e. Be used in an integrated ground and flight training curriculum
  which provides for:

  The scope and content of a curriculum in compliance with part 141,
  The authorized use as approved in the AFS-800 letter of approval,
  Instructional materials for flight events, and
  An outline of stage (phase) checks and criterion levels of performance.


What is the difference between flow separation and turbulent flow? For air flowing over a wing, both flow separation and turbulent flow involve disturbed flow next to the surface and smooth flow further away. At what point does one say "oh, this flow has changed from turbulence to separation"  (or vice versa) and why?
I am wondering if separation involves only a disturbed boundary layer, while turbulence can involve a wider disturbance such as in a stall?
For example, is it correct to say that in the stall, an already-turbulent flow (sometimes experienced as burbling) becomes detached?
Or that vortex generators, designed to re-energize a stagnating boundary layer, do so by creating turbulence in order to prevent separation?

At what point does one say "oh, this flow has changed from turbulence to separation"

At the point where the flow reverses direction.

Flow separation. The bold curve is the surface/wing.
Yes, that can happen.
Both turbulent and laminar flow can separate. Turbulent flow is in fact less likely to separate than laminar flow. This is why aircraft wings often have devices that deliberately create turbulence on the wing.
(Yes, separated flow produces negative skin friction, but at the price of huge pressure drag)
Here's a (badly hand-drawn) diagram showing the difference between laminar, turbulent and separated flows.

Just a clarification about stall. The stall is when the lift reduction caused by flow separation overwhelms the lift increase caused by flying at an increased angle of attack. Flow separation can happen without stall, and it will reduce the benefit gained from a higher angle of attack in proportion to the extent to which the flow is separated, but stall cannot happen without flow separation.
Indeed, many wings have separated flow at the trailing edge sometime before "stall" is reached. As one draws closer to "stall", the region of separated flow expands forward. The turbulence created by the wake of this separated flow hits the tail, causing "buffeting", which gives the pilot a warning that he is approaching stall. Airfoils that lack this feature, such as supercritical airfoils or sharp supersonic ones, tend to be dangerous to fly at slow speed with it's inherent high angles of attack.
And as you can see from the diagram, the flow separation at a given angle of attack is much worse for laminar flow than for turbulent flow. So the laminar separated case is more likely to be a stall than the turbulent separated case.

Lift vs angle of attack for thin, sharp wings vs thick ones. Airfoils designed for laminar flow fall in the thin category. And as above, just having or not having laminar flow on a wing can make a similar difference.
And yes, vortex generators prevent separation by creating turbulence, which causes high speed freestream air to get mixed with low-speed boundary layer, speeding up the boundary layer. It's a tradeoff between the drag of a turbulent boundary layer and the even greater drag and lift loss from flow separation.


Why are on-board computers allowed to change controls without notifying the pilots? Recently we've had the 737-MAX debacle, where it increasingly appears that the MAX on-board MCAS system repeatedly would issue nose-down events, leading to the Lion Air crash, and possibly another

  On the Lion Air flight, when the MCAS pushed the jet’s nose down, the captain pulled it back up, using thumb switches on the control column. Still operating under the false angle-of-attack reading, MCAS kicked in each time to swivel the horizontal tail and push the nose down again.

Apparently there is a notification light for MCAS that was an optional package but will now be required.

It reminds me of two other incidents that lead to crashes

Air France 447 A330 - A frozen pitot tube caused the computer to switch to Alternate Law 2, which meant that the computer would no longer prevent dangerous flight inputs. While not the same as the MCAS issue, it lead to confusion of the cockpit crew (who had been making maximum nose-up inputs while at TOGA power), and thus the plane crashed due to stall. It was noted that there was nothing direct to alert the pilots that the switch had occurred.
Scandanavian Air 751 MD81 - Ice accumulated on the wings. When the plane took off, the ice fell off, striking the engines. The pilots noted the engines were surging and reduced the throttle, but an auto-throttle system not disclosed to the pilots kept increasing power back to full, thus causing engine failure and a crash landing.

Has anyone ever stated why aircraft systems are allowed to make changes without notifying the pilots? Is it something that just hasn't been a large enough problem to address until now?

There is a general design principle, that some, but not all, of the behavior of the flight guidance system or autopilot should be visible to the pilot.  Usually automatic engagement or disengagement of a control system is indicated, but sub-modes of these controls or manual changes might be unindicated. That sounds simple and logical, but in reality it's a complex and tricky human factors issue. Displaying too much information to the pilots is just as bad as displaying too little, as too much information can cause them to ignore the most important indicators, like a stick shaker warning about a impending stall.

I know this isn't a very satisfying answer, but it's hard for anyone other than a human factors expert to answer a very general question like this with hard-and-fast rules.

For example, many autopilots will skip the aural warning about disengagement if the disengagement was directly caused by a pilot action. The pilot in this situation only gets a visual indication that AP has changed behavior. This sounds perfect, but on Aeroflot Flight 593 a pilot let his child into the cockpit, who then applied enough pressure on the yoke to disengage the automatic aileron control. There was no warning chime because the disengagement was considered intentional by the autopilot. This was unlike previous planes the pilot had flown, which contributed to the pilot's confusion when the plane began to turn and then go into a sharp bank.

The indicator light you mention for the 737 Max Lion Air crash is for "AoA disagreement", not MCAS engagment as you suggest.  While these kinds of AoA indicators are generally considered good ideas, it's still an area of debate how much they'd help confused pilots like in the Lion Air crash or the Air France 447 crash.

Similarly, the quality of indication was not the biggest cause to the accidents you've listed. For example, the Scandinavian Airlines Flight 751 could probably have returned to the airport successfully, but a compressor stall due to ice striking both engines at low altitudes is a bad situation no matter the auto-thrust behavior. Bad sensors and confused pilots can crash a plane with perfectly designed indicators.

Edit:  since this answer is getting a lot of attention, I think it's important to note that I don't really cover any special considerations for the edge of the flight envelope, auto-trim vs auto-disengagement vs changing sub-modes, or idiot-proofing of flight systems. I'd love to see this covered in more detail by people with experience.


When a plane flies faster than the speed of sound, does the distance between plane and sound increase? I had this discussion with my dad about a question he had asked. Basically he said:

  If a plane flies faster than the speed of sound, will he be able to overtake his own sound and hear itself?

(let's leave behind the fact that he can run out of fuel or that the sound finishes before he hears it)

I disagreed because it seemed illogical to me, but we never actually found hard evidence to determine if the statement is true or false.

Can a plane actually do this?

I think you are missing some physics knowledge, so let's start from here:

  If a plane flies faster than the speed of sound

Sound is a compression wave that travels through air. Given a certain air temperature, the sound will travel at different speeds through the air.

If a plane flies faster than the speed of sound means that the compression waves it is generating will remain behind the aircraft:

image from here

  will he be able to overtake his own sound and hear itself?

As you can see on the right picture, if the aircraft is travelling faster that the speed of sound, the compression waves it generated in the past will remain behind. If it would stop/slow down, the waves would then catch up with the plane and a person on board would be theoretically capable of hearing them.


What does SVC TAR/SSR U/S mean? Looking through NOTAMs. What does SVC TAR/SSR U/S mean? Does that mean I cannot shoot practice approaches in the area?

Here is the NOTAM text:

  07/052 (A0474/17) - SVC TAR/SSR U/S. 21 JUL 12:00 2017 UNTIL 21 JUL
  17:00 2017. CREATED: 20 JUL 12:08 2017

What does svc tar/ssr u/s mean?

Terminal area surveillance radar and secondary surveillance radar (raw radar return and the secondary return from your transponder) is unserviceable.

If you were VFR you could practice the approaches without ATC radar vectoring, or ATC could give you a non-radar routing via an airway, then transition route, then the instrument approach procedure.

In other words, the area for the time being (NOTAM period) has no radar.  Non-radar procedures would be utilized.  If the local ATC area was normally a radar environment and the radar equipment was taken down for preventive maintenance (for example), providing separation for practice approaches may be more than they are willing to do.

In the US ATC system, controllers are initially trained, tested, and qualified using a non-radar environment at the FAA Academy. Non-radar approach control is infrequently necessary in actual operation (unless it is in an area where radar coverage is not available full time, which is becoming rare in the US).

Call the local ATC Approach Control in the area you are referring to and see what services they can provide.


Why do many large turbofans have a gap (of turbine blades) between the LP turbine and HP turbine? After browsing around a while, I noticed that many...I think almost all...large turbofans have this gap between the LP turbine and HP turbine. I took some screenshots and put a red circle around the gap.

Why do they have this gap? It's certainly not for the combustion chamber. That part comes before the LP turbine. The only thing I can think of is the LP and HP are on different spools so maybe they need some space for the bearings to brace the two shafts. But there should be bearings bracing the shaft all along its length so idk.

I'm guessing it would be more efficient to eliminate this gap and should allow a smooth transition from high pressure to low pressure. At the very least it would save some room at the end of the engine, shortening its length and maybe saving some weight.

EDIT: At first I thought it was totally empty. But it appears that space has stator vanes not shown in the pictures. I'm editing the title with the phrase "gap of turbine blades" because the original spirit of the question was about way there's a big space between the two sets of turbine blades. The space happens to have something else in it, but not turbine blades and I was wondering why those sets are separated.

(Source) Cutaway for the GE9X.

The gap you see is not empty. Marked above is one of the nozzle guide vanes (they have removed most of them for illustration purposes). Those vanes are analogous to the compressor stator stages.*

Labels 91 and 99 below are those vanes—sadly the stators and vanes are almost always omitted from 'drawings', yet they are as important.

(Source) Turbine stages of the RR Trent 500.

The reason why the diameter difference is steeper in the GE compared to the RR, is because the RR Trent 500 is a 3-spool engine.

In short, the vanes in both engines when omitted from drawings, give the illusion of there being a gap (wasted space).

* Nozzle guide vanes have nothing to do with the propelling nozzle at the rear. In turbine lingo, nozzle means the stator stage for the turbine.

On the funny side, here's a thumbnail from RR's own YouTube channel, absolutely bonkers exhaust. Don't trust illustrations.


How far can a 777 fly with just one engine at altitude? If a 777 pilot wanted to fly further than with both engines running, how many more miles could the plane go with just one engine running, at high altitude?

Flying with only one of two engines will not increase range, but reduce it. A gas turbine consumes fuel in proportion to the mass of air moving through it, so with increasing altitude consumption decreases proportional with air density. Thrust goes down with density as well, so thrust-specific fuel consumption is roughly constant (actually, the engine becomes more efficient with altitude due to the lower air temperature, see Carnot cycle for more on that).

Generally, a jet transport wants to fly as high as possible for maximum range. Its cruise altitude is limited mainly by the thrust available from all engines. If the pilot shuts one down, the aircraft needs to fly much lower until thrust has increased sufficiently to overcome drag. Since friction drag is also proportional to air density (and the square of flight speed), the aircraft has to fly much slower at that lower altitude, which cuts range since the fuel consumption of the single engine is now roughly what both consumed at altitude, but now the aircraft covers less distance in the same time. In addition, the pilot has to compensate asymmetric thrust with rudder deflection, which again increases drag.

Short answer: The aircraft will not go MORE miles, but FEWER.


What counts as "runway in sight" when flying an IFR approach to minimums? I've watched a few videos on youtube where the pilot flies an Instrument Landing System (ILS) approach to the minimum allowed altitude before establishing visibility of the runway (MDA or minimums).

What I'm wondering though is what constitutes "runway in sight"?  I mean, in some cases that's going to be obvious (ie., you can see the runway clear as day long before minimums.)  But what if the cloud base is right at minimum and you can kind of see outside, but not in every direction (ie. maybe it's slightly clearer just a bit to your left than to your right.)

What do you have to see to consider the runway in sight?  The whole runway?  Just the lights? Just the approach lights?  Just the VASI/PAPI?  Would it be considered enough if, perhaps, you could see a windsock that you know is close to the runway?  Or perhaps there's a cross street right before the runway that you're very familiar with, would that work?  What would be considered "runway in sight" so that you can continue the approach rather than going missed?

Note/Clarification: The general rule, of course, is "when in doubt, go missed".  So in a lot of ways this question is academic.  What I would love, though, is a two part answer, with one part that has the exact rules (per the FAA) and the other with some common sense guidance on when to continue and when to go missed.

From FAR 91.175:

  (3) Except for a Category II or Category III approach where any necessary visual reference requirements are specified by the Administrator, at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

  (i) The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.

  (ii) The threshold.

  (iii) The threshold markings.

  (iv) The threshold lights.

  (v) The runway end identifier lights.

  (vi) The visual approach slope indicator.

  (vii) The touchdown zone or touchdown zone markings.

  (viii) The touchdown zone lights.

  (ix) The runway or runway markings.

  (x) The runway lights


Do instrument approach procedures sometimes have a minimum temperature? Non-precision instrument approaches generally have altitude restrictions which get lower when you get closer to the airport. I always figured these restrictions were AMSL using the current altimeter setting, not compensating for temperature.

Some have heard the mnemonic that mountains are higher come wintertime, which basically means that colder weather make your altimeter read higher than you actually are (or, as most pilots prefer to think, you're lower than what your altimeter reads)

Have a look at this VOR approach into Newark

Most altitude restrictions are a minimum level, so you're free to fly higher if it's a particularly cold day. But have a look at LOCKI intersection, the final approach fix. That's at 1500 ft, not at-or-above. At -40, this will put you around 1100 ft above ground level. Although I don't see any obstructions that high during this segment of the approach, as far as I know instrument approaches are supposed to guarantee a 500 ft obstacle clearance, do procedures using this kind of restrictions have a minimum temperature?

There are procedures with temperature restrictions, related to altitude constraints. An example is Innsbruck:

The text in the red box says:

  Procedure N/A below AD temp -7°

For effect of temperature on altimeter: How will the altimeter read in air colder than ISA?


Is all jet fuel the same? I'm a Chemistry student preparing an in-class lab demonstration that involves the combustion properties of jet fuel used in typical US based airliners.

Is all jet fuel the same, or do I need to consider specific blends when designing my experiment? Is there a standard chemical formula for jet fuel?

And for bonus points, are there any critical handling considerations?

The fuel used by civilian airlines, fueled in the USA, is Jet A. To acquire some, go to a nearby airport which can handle operations by Turboprop or Turbofan aircraft. Some of the smaller airports, only supporting piston aircraft operations, may not be able to help you, but anything which can handle a Lear Jet should be able to supply you with examples of it.

There's no specific chemical formula for this. It is a mixture of a variety of compounds. And no, you can't isolate just one particular compound and use it as a representative sample. There are reasons for the various additives, and all of them will have SOME effect on the burn characteristics.

Try not to breathe the fumes, before or after combustion. Do not operate any kind of radio transmitter (including a cellphone) within 50 feet of the vapors of it (when the fuel is exposed to air, before or during combustion). Make sure that whatever vessel you're going to burn it in is metal (plastic doesn't respond well to burning petrochemicals :-) and make sure there is good electrical conductivity between the vessel and the ground. Have a large, CO2- or Halon-based (NOT water or dry chemical) fire extinguisher nearby.

When I was refueling F-16s in the Air Force:

no radio transmitters (walkie talkies, at the time) were allowed with 50 feet of refueling operation
the aircraft and the refueling vehicle BOTH had to be grounded and connected to each other BEFORE connecting the refueling line
no refueling ops were permitted if there was lightning within 5 miles
no refuelings ops were permitted without a 50-pound halon-based fire extinguisher and a dedicated person (fire guard) to man it

This should give you some idea of proper safety around this stuff.


Should DUAL flight be logged as SINGLE PILOT TIME? Should DUAL flight be logged as SINGLE PILOT TIME?
Should total SINGLE PILOT TIME + total MULTI PILOT TIME = TOTAL TIME?
In what cases SELF could be specified in PIC NAME on DUAL flight?

EASA regulations applied! (Hungary)

Many thanks

1) If you are under training in a single pilot aircraft (e.g. C172), it is logged as dual in single pilot. If you are under training in a multi pilot aircraft (e.g. A320), it is logged as dual in multi pilot.

2) Correct.

3) If dual (under training), the PIC name should always be the name of your instructor. The only time you use SELF is if you were PIC, and therefore the flight cannot be dual.

See EASA Part FCL; FCL.050: https://www.easa.europa.eu/sites/default/files/dfu/Part-FCL.pdf


Why were the specifications of the Pipistrel Panthera downgraded? I'm curious if this was a failure of the simulations or a result of the engine swap they did at the last moment. Originally, they said they could cruise at 200kts @ 10g/h fully loaded. Now its down to 155kts.

http://www.panthera-aircraft.com/technical-data

EDIT:
Just wanted to add, for reference, a page which gave their original estimates.
http://www.pipistrel-usa.com/models/panthera.html

You will also notice at the top of the page for the first link where they reference the 200kts with 4 people for 1000nm.

This sounds like an overly optimistic marketing department which wanted to make the aircraft look better. Engineering would allow to predict top speed within a few percent, so I would blame neither the simulations nor the engine swap.

The web page for the Panthera lists 155 kts only for the maximum range condition, which makes sense. Propeller aircraft need to fly rather slowly to maximize range. The cruise speed with maximum power is still close to 200 kts.

Please consider that the required engine power of propeller aircraft grows with the third power of flight speed. To give you the 200 kts top speed, the plane would need to run the engine with 215% of the power setting (and fuel flow) needed for 155 kts.

Once you know the installed power, you get reasonably good results just with the zero-lift drag and the propeller efficiency when you use this formula:
$$v = \left(\frac{2\cdot P\cdot \eta_{Prop}}{\rho\cdot F_{ref}\cdot c_{D0}}\right)^{1/3}$$
Nomenclature:
$v\:\:\:\:\:\:\:\:\:\:$ speed in [m/s]
$P\:\:\:\:\:\:\:\:\:\:$installed Power [W]
$\eta_{Prop}\:\:$ Propeller efficiency; use 0.85 if not known
$\rho\:\:\:\:\:\:\:\:\:$ density of air [kg/m³]
$F_{ref}\:\:\:\:$ Reference area for coefficients, usually wing area [m²]
$c_{D0}\:\:\:\:\:$ zero-lift drag coefficient

The zero-lift drag coefficient for small propeller airplanes is around 0.02 to 0.025 if they have a retractable gear. A very well designed airplane might come down to 0.016, but that is already pushing it. To fly fast, a high wing loading is essential, and then the influence of all non-lifting components on the total drag is rather high. A well-designed airfoil might achieve a $c_{D0}$ of 0.0035, but the whole airplane will be closer to 0.02.

Things get even worse when you aim for higher flight Mach numbers. The Dornier 328 turboprop was initially calculated for 320 kts cruise speed, but Marketing felt the need to claim 340 kts. The difference does not sound substantial, but all engineers thought they must be crazy over in Marketing. In the end, the Do-328  needed 30% more installed power to fulfill the promises made by the (rather irresponsible) Marketing department. The increased engine power indirectly caused an accident in flight test when a propeller (with an unchanged, but now too weak propeller mount) broke off in sideslip and cut through 1/3 of the fuselage. Luckily, the aircraft landed without further incident after that ...

(Disclaimer: This is from the back of my head, and I think I remember the facts and figures correctly. If any of you know more, please edit the last paragraph!)


Gliding range if engine is off The engine was turned off at 1000m above sea level. If you ignore the landing pattern, how long can the runway be within a few kilometers of the landing to safely land? However, the wind is blowing at 72 km / h from behind.

Empty weight = 90kg, fuel = 30kg
zero lift drag coefficient = 0.05
Oswald = 0.75
Span 4.5 m
Wing w Area 3.7m^2
Cl =0.1(alpha + 1)

I am really confused if the wind is blew from head or tail how it influence on range.

I'm going to assume that when you say 1000 meters above sea level, what you really mean is 1000 meters above ground level. 1000 m AMSL can be underground in many locations.

First, you need to understand that the airplane, once in the air, always moves together with the surrounding air. The surrounding air can be moving relative to the ground – what's usually referred to as wind – but assuming sufficient altitude above local ground, the airplane is largely unconcerned with what the ground is doing under it.

Second, every airplane has some glide ratio. The glide ratio tells you, for each amount of distance covered, how much altitude you will lose. Typical values for light motor fixed-wing airplanes might be around 10:1 to 12:1, while commercial airliners might get 15:1 to 20:1, and high-performance gliders can exceed 50:1. For simplicity, I'll assume a glide ratio of 10:1, which is pessimistic, but which should be achievable by most light motor airplanes. (User XRF pointed out in a comment that these values might actually be on the optimistic side, but I'm sticking with them because the point is to illustrate the principle, not to give an exact calculation.)

Typically, the glide ratio is quoted at speed and configuration of best glide, so your first course of action when losing the engine should be to configure the airplane for best glide, and re-trim to match.

What the 10:1 tells us is that, for each 1*X meters (or feet) of altitude that you lose in a glide, you will cover 10*X meters (or feet) of distance. Since you start out at 1000 meters above ground, this means that you can cover 10*1000 meters or 10 km of distance before your flight path intersects ground.

However, again, that's distance covered through the surrounding air mass.

To figure out how far you can travel along your ground track, you need to figure out the amount of time for which you can remain in the air. Let's say that the speed of best glide in your airplane is 100 km/h indicated airspeed. Since you can travel 10 km through the surrounding air mass, this means that your travel time after losing the engine is 10/100 of an hour, or six minutes.

In six minutes, the 72 km/h wind will push your airplane 7.2 km along the ground, because six minutes is 1/10 of an hour, and 1/10 * 72 = 7.2.

Therefore, if you maintain that tailwind, with this highly simplified example, the point where you reach ground level will be 7.2 + 6 = 13.2 km from where you lost engine power.

If you instead make a 180° turn to head straight into the wind, you now have a 72 km/h headwind and a 100 km/h  indicated airspeed, for a ground speed of 100-72 = 28 km/h. You can still travel 10 km through the airmass before you reach the ground, and this still takes 6 minutes, but in those 6 minutes, your ground track distance is only 2.8 km.

The major advantage of turning into the wind is that when you reach ground, you will do so at a relative speed of 28 km/h, rather than 172 km/h, assuming an unchanged 100 km/h indicated airspeed. (This is why you always land into the wind.)

Therefore, with that wind, depending on which direction you turn into, your point of contact with ground, with these assumptions, will be somewhere between 2.8 km from where you lose engine power, and 13.2 km from where you lose engine power, expressed in terms of ground track distance.

In practice, however, if you lose the engine while in the air, this is all awfully academic. Rather, you'd pick out a nearby spot on the ground that looks reasonably even, level, and large enough to set down on; and head that way. There are virtually always ways to lose altitude, but without engines in an airplane not designed as a glider, very few ways to gain altitude. With experience, you might be able to quickly judge if you'll make it to the runway or not; but any controlled landing, even an outfield landing, is always better than a crash. You can always worry about getting the airplane back to the airport once you're safely on the ground.


Why can't planes use only flaps in the tail section instead of stabilizers? I imagine plane having only four flaps in the back (one on top, bottom, left and right), similar to ailerons but longer and narrower. They can open from the fuselage to correct the plane's yaw or angle. I predict that this would increase efficiency of the plane since there's no drag from the horizontal and vertical stabilizers.

Can this design work?

Your question is partly covered here regarding why the vertical stabilizer is important. The empennage (tail surfaces) of conventional planes provides both passive and active stability. Like the feathers on an arrow (which is where the word empennage comes from), they help the airplane to remain pointed in the direction of flight as well as providing active control surfaces. The cost of providing stability is added weight and drag, but if the surfaces are removed, the stability aspect will need to be provided manually, or alternately. This is how unconventional aircraft like the B-2 flying wing, or even modern fighter aircraft, are able to fly.

So, yes, we can fly airplanes without conventional tail surfaces. Some designers like Burt Rutan have created unconventional designs for general aviation. However, design changes will also have drawbacks. At the very least these designs behave differently than conventional planes, which requires special attention. The strange appearance is also reason for some pilots to distrust them.

If the plane is going to be unstable without computer control, that introduces safety problems. In military aircraft the pilots can eject when issues arise. But unlike modern commercial planes, the pilots cannot simply take over control if the computers fail.

There is also an aerodynamic issue with what you propose. Conventional control surfaces deflect the entire surface, so air continues to flow past both sides. However, if you only deflect one side of the surface, air is only flowing past the outer face. This acts like a spoiler and creates extra drag. Though this drag will be the force controlling the plane, it is not as efficient as moving a control surface. As Peter Kämpf pointed out, it is much more feasible to increase the effectiveness of the existing surfaces than to eliminate them.

Modern airliners represent a compromise between efficiency and safety based on currently available technology. Although alternate designs have been proposed, these configurations create their own unique problems.


How does the A350 variable camber system decrease drag? Just curious how the A350 reduces drag/fuel burn by extending the flaps ever so slightly during cruise. Everything I've read about extending flaps says that extending them pushes the centre of pressure rearwards. This causes a pitching down moment, which means you need to increase the AoA, but this affects drag and fuel burn negatively (a pitch up command increases the down force from the tailplane which has the result of increasing the effective weight of the plane, which requires an even higher AoA to compensate, which also increases the induced drag).

Here's how it works as per the (A350 Flight Deck and Systems Briefing for Pilots)

  Differential Flap Setting and Variable Camber

  The Differential Flap Setting and Variable Camber enable to optimize the loads and drag on the wings.
  Small flaps deflections (4° maximum) either symmetrically or asymmetrically, enable to automatically:

  Optimize the wing camber to reduce wing loads and drag
  Perform an optimized Lateral Trim function.

Henning Strüber, one of the Airbus engineers behind this system, has written a paper on it.

In cruise:

  This can be applied in early cruise phases to shift the center of lift more inboard and by that reducing the wing root bending moment, which can be transferred into a structural weight saving.

A plane that can be built lighter will have lower drag [for the same payload].

For a heavy and/or a hot and high takeoff:

  In high-lift configuration a more outboard loaded lift distribution can be achieved to reduce induced drag during take-off.

For how that works, see here. The answer there by @PeterKämpf confirms that outboard loading requires a heavy wing relative to the whole mass (scaling laws work for an insect, but not for an albatross or an airplane). So those two regimes may appear to be contradictory: if the cruise system allows a lighter wing, then how can that lighter wing achieve more outboard lift for a heavy takeoff.

Accounting for the gust loads in cruise is the key here, which are smaller for takeoffs. (Thanks to @PeterKämpf for this insight; see comment posted below.)


Was the Horten Ho 229 flying wing a three-point-landing aircraft? The Ho 229's front wheel is visibly bigger than the main landing gear; during landing, did it touch down on all wheels simultaneously, on the front wheel first, or in the normal manner?

The term "3 point landing" is derived from old taildragger (3rd wheel in back) designs.

In reality a 3 pointer is not desirable, as the wheel, front or back, being away from the center of gravity, can produce a nasty pitch if it bounces. At landing speeds this can be uncontrollable.  The term "3 pointer" does imply good landing speed management, as the plane, slightly nose up, has been flared properly, and will "settle" gently to the ground.  Most of the time the pilot will strive to land on the main gear, bleed off more speed, then allow the 3rd wheel down before taxiing off the runway.

The Horton Ho229, if it ever flew, was designed to land under wartime conditions.  Even jets were very vulnerable when slowing to land, and many times paved smooth runways simply were not available (survivable) options.

So the huge front wheel was basically overdesigned for less than perfect landings.  However, landing technique, even for this fantastically futuristic aircraft, would be similar to other planes.


Could a quadrotor use a main gas powered rotor with small electric rotors for attitude control? Here is my idea of heavy lifting quadcopter. (Although it is not a quad copter anymore because of the center rotor)

Have a big coaxial fixed pitch rotor at the center. The rotor is gasoline fueled.
Have the other rotors at extended length with electric motors to control pitch yaw and roll.
The center coaxial rotor will do the heavy lifting.

Can it work? What are the difficulties?

Early versions of Sikorsky's VS-300 helicopter prototype used something similar to what you're describing, as his first attempts had three rotors on the tail boom, one vertical rotor for anti-torque and yaw, and two horizontal rotors for pitch and roll.

Of course, if you're already using large main rotors, you might find, as Sikorsky found, that cyclic pitch control works even better and eliminates the need for the two horizontal rotors.


What stops planes from being stolen? Related question: do planes have keys?

Let's say I happen to spot a fine-looking plane at my local airport and decide the owner probably wouldn't mind if I borrowed her for a little while - what stops me from stealing this plane? On commercial jets, the cockpit door is locked, but as I understand smaller planes don't have cockpit doors. The linked question suggests that single-engine planes have ignition keys, but twin-engine planes don't. Are the outer doors (cabin doors) locked? What else restricts me from walking up, climbing in, and flying away? (Other than the fact that I don't have the least clue how to fly a plane.)

(This is about all types of planes.)

There are a several theft-deterrent mechanisms in place.

On small airplanes, most commonly, the door to the airplane itself is usually
locked with a key. (Notice the key cylinder in this picture below the silver door handle:
Prop locks:
Throttle locks:
A lot of airplanes do have an ignition key which you turn to engage the starter.  Of course, as jaytre mentioned, this can be circumvented by hand-propping (which is a very dangerous endeavor for unskilled people who haven't done it before)
You would have to know how to operate the basic systems of the airplane as well as how to fly.
AOPA's Airport Watch Program
Fences and other physical security measures around airports.


Can a plane with a thrust-to-weight ratio less than one fly vertically and perform loops? In the game Battlefield 1 you can fly a Sopwith Camel vertically and perform different kinds of aerobatic maneuvers like loops. According to its specifications, its thrust-to-weight ratio should be about 0.28 and hence it should not be able to fly vertically. Can it?

With thrust/weight ratio above one, you can use the thrust of the engine to accelerate upward (at least at some point in the flight envelope).  As long as the engines can deliver full thrust, you can maintain that position.
Below one, vertical flight is still possible, but you will be losing speed in the maneuver.
Gliders are capable of loops without any thrust at all. The only energy in the maneuver is the speed of the glider on entry.


Is the aircraft used by NOAA specially modified? I have just seen this short video showing "NOAA hurricane hunters" flying through current hurricane "Irma".

My guess is that normally an aircraft would not be able/allowed to fly in winds sustained wind speed of 160kts, gusting to 180+kts, with the associated turbulence.

Is this aircraft modified in any special way? For example, reinforced structure, or specially designed engines?

Except for modifications related to instrumentation, not much. The WP-3D was developed from the Lockheed P-3 Orion, which was  a navalized L-188 Electra.

Orion is a sturdy platform. Used during the Cold War as an antisubmarine patrol aircraft, the plane is built to take punishment.

As the NOAA noted in a Reddit AMA

The WP-3D is built like a tough old pickup truck. It can handle a lot of abuse in the storm environment and come back and fly the next day. This is mainly because of the extremely talented maintenance team at NOAA’s Aircraft Operations Center (AOC)

Simply put, it is a normal aircraft with extensive instrumentation, albeit maintained well. The same applies for the WC-130J used by the 53rd WRS:

The 53rd WRS uses the WC-130J to penetrate tropical storms. These aircraft are not reinforced in any way in fact the only differences between a WC-130J and a C-130J is the addition of two external fuel tanks (giving them longer range), a radiometer pod on the left wing and the two addition crew pallets in the cargo bay...

and of course, the instruments. Actually, the NOAA has replied to your question in their FAQs:

Why aren't NOAA's Hurricane Hunter planes torn apart in storm?
Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control. That's why NOAA's Hurricane Hunter aircraft don't fly through tornadoes. In a like manner, NOAA pilots and crew routinely (but never casually) fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for "hot spots" of severe weather and shear that they can often identify on radar and avoid if it's too severe.


What is this aircraft (Le Bourget airshow 1965) Approximately 31 seconds into this video of a walkaround of the 1965 Paris airshow, there's a curious aircraft shown next to a Navy F-4.

The question is - what is it?

It's a Nord 500 Cadet. From Wikipedia:

  A model kit presenting the concept was first shown at the Paris Air
  Show at Le Bourget in 1965. The aircraft was driven by two ducted
  fans, with three blades per fan, mounted on short wings that were able
  to pivot between providing vertical and horizontal thrust.

  Two prototypes were constructed, one making its first (tethered)
  flight in July 1968.

This early VTOL concept aircraft was designed by Nord Aviation (formely Potez), the French manufacturer. They were known for their military Transall and Noratlas seen everywhere in Western Europe in the 60s.

This aircraft is similar to CL-84 and XC-142 designed at the same period. The only one to have flown:

(Source)

(Source)

Powered by Allison T63 (aka Model 250) turboshaft, 316 hp.
Maximum speed: 350 km/h.
Ceiling 4,101 ft.

The prototype used for static demonstration:

(Source)

Bonus: Trying to fly the prototype on Youtube.

A quite funny video I may say. With nearly all the attributes of my fellow countrymen of this era: beret, cigarette... just missing a baguette under the arm.


Why are vertical stabilizers always at the rear of an airplane? Sharks and whales have them in the middle of their body, so why not planes?

With sea creatures the dorsal fin acts as a keel providing the actual sideways force during the turn. They remain level while turning.

Planes on the other hand bank to turn where the sideways force is provided by the lift of the wings.

To provide yaw stability the vertical stabilizer should be as far from the center of mass as possible. They are not in front because that would be unstable as the minor deviation from straight ahead would increase the force into the deviation rather than provide a force to counteract it.


Why would a pilot ask the tower to turn the VASI off? This past Saturday I was taking off from runway 22 at Catalina Airport (IATA: AVX) when I heard on radio a GA pilot requesting the tower to turn off VASI after requesting weather advisory.  I did not know if that airplane was on ground or in air.

As the clouds were gathering over and I am not an IFR pilot, I was concentrating on the fast moving clouds. After a while he asked for VASI to be turned back on.

I wonder why did that GA pilot ask to turn off VASI and get them turned on shortly afterwards?

On a side note, I got lucky and saw the giant Shuttle fuel tank being towed down below near LA Forum to LA Science museum; took some pictures!

It could have been a flight instructor with a student that was becoming too reliant on the VASI's slope information for visual approaches to landing, and he wanted to force the student to do without.


Where can I do an FAA IPC in Canada? I have an FAA Commercial License and my instrument checkride was completed 8 months ago. Now I am in Canada and trying to convert my license. Problem is the currency of the instrument rating. Transport Canada said they require an IPC check and I want to know if there are any FAA CFIIs here in Canada who I can do the IPC with. I don't want to go back to the US just to do that.

I was reading the other similar post by a user who says he signed an IPC for FAA pilots in Canada. If you are that user and are reading this post please let me know where I can do my FAA IPC in Canada.

You will need to find a US CFI-AI who can perform a competency check for you.  It can be done in the US or in Canada (in a US registered aircraft unless that CFI holds TC and FAA credentials).

If you are within 6 months of being current, you can do approaches with a safety pilot in Canada. If you are flying a US registered plane, then you will most likely need a US certificated safety pilot.  If you are flying a Canadian plane, then your safety pilot should be a Canadian pilot, acting as PIC for the flight.  You could still log the approaches and simulated instrument time.

In my experience, it will be far easier to find a US CFI in Canada than it is to find a Canadian CFI in the US.  Ask around.  Consider contacting an organization like AOPA if you are a member, or COPA, and asking them for help locating US CFIs.  I would also ask if they have additional guidance because they may have letters from Transport Canada and the FAA which may have policy considerations which might be good to know.


How to calculate the power required during transition from VTOL to fixed wing? In a VTOL, I have calculated the power required for take off and cruise which was not difficult at all. Between take off and cruise, there is a phase where the VTOL has to transition by tilting the rotors from 0 to 90 deg. I am finding it hard to calculate the power required for this transition. I feel this is very important because it consumes more power than take off since it is creating lift (hasn't reached the minimum velocity for the lift requirement) for balancing weight and also needs to provide thrust for forward motion.
Given to me are:

Rotor parameters

number of rotors
Rotor diameter
rpm
number of blades

Fixed wing parameters

span and chord (rectangular wing)
lift and drag coefficients

What are the factors considered to calculate the power required for transition?
Please note: Make as many assumptions/approximation as you like. I just want a simple formula for a basic understanding.

The VTOL is a tilt rotor type with ducted fans

Like Hobbes said. For instance, after take-off, tilt the thrust 10° forward. The cosine is 0.98 so you only lose 2% of propulsion lift. The sine is 0.17, so forward thrust is 0.17 * total thrust.

The craft will speed up until aerodynamic drag = forward thrust:

$$ T = D = C_D \cdot \frac{1}{2} \cdot \rho \cdot V^2 \cdot S$$
$$ V^2 = \frac{2 \cdot T}{C_D \cdot \rho \cdot S}$$

Then tilt to 20° where you have sin(20) = 34% of the propulsive thrust pointing FWD and 94% pointing UP, etc.

Note that it is usual to express the drag equation relative to wing area S.

A simple real-time approximation in a spreadsheet would look like underneath. 9810 N thrust pointed back 10 deg results in sin10*9810 = 1703 N forward thrust, which takes about 300 seconds to speed up the craft to 97 m/s. Of course, because of the aerodynamic lift the craft climbs, changing AoA and creating induced drag etc. The simulation should be extended with reduction in rotor thrust proportional to aerodynamic lift. But I hope the gist of things is clear enough.

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Are there any differences between turbofan and turbojet engines besides bypass air? I think I understand both engine types, but I am just wondering am I right in saying that the only difference between the two types of engines is the bypass air (i.e. a turbofan is just a turbojet engine with bypassed air).

If not, what are the other differences between the engines apart from the bypassed air?

Any help on this matter would be much appreciated.

You are right about the bypass being the same result as a tubofan.
However the word turbofan refers to an extra turning bladed wheel in front of the compressor and larger in diameter it turns slower and moves air past the engine on the outside.


Is it possible to fly a Straight and Level flight at high speed? The question arise while discussing with some of my student pilot friends that as we have been performing Slow Flight we have to keep increasing the AOA of the aircraft to compensate for every lose in airspeed. What about "FAST Flight", decreasing the AOA of an aircraft for every gain in airspeed. Maybe Negative AOA.
I'm aware that lift could be generated with negative AOA from the graph of AOA vs CL. And if you could explain to me why there is a specified cruise setting?

Yes, with good engines and low drag you can fly straight and level at really high speeds.

What about the AOA? It gets smaller, but only down to a certain limit. At level flight, the weight of the aircraft is equal to the aerodynamic lift:

$$ W = L =C_L\cdot\frac{\rho}{2}V^2\cdot S $$

The lift coefficient depends on the AOA ($\alpha$):

$$C_L=C_{L0}+C_{L\alpha}\cdot \alpha$$

Therefore,

$$\alpha = \frac{1}{C_{L\alpha}}\left(\frac{2W}{\rho V^2 S}-C_{L0}\right)$$

For very high speeds the first term of the parenthesis will get close to zero, so that the desired AOA setting becomes

$$\alpha_{V\rightarrow\infty} = -\frac{C_{L0}}{C_{L\alpha}}$$

This AOA is indeed negative for most aircraft. If your AOA gets smaller you won't be able to sustain level flight.

The plot below shows the AOA needed for straight and level flight on a Cessna 172 and compares the two terms in the parenthesis of the equation for $\alpha$ (multiplied by 10 for better readability). You would need to fly much faster than $V_{NE}$ to get the first term significantly smaller than the second one.

Source: own work with data from Wikipedia and polar.


What are these sensors on the Night Stalkers' Chinook MH-47G? What are these sensors (or lights) on this heavy assault Chinook MH-47G of the US 160th Special Operations Aviation Regiment, "Night Stalkers"?

Source, picture by Russell Hill

This regiment was the subject of Ridley Scott's Black Hawk Down.

Those sensors are part of the AN/AVR-2A Laser Warning System. Here is a closeup of the system components.

The AN/AVR-2A is a passive laser warning system which receives, processes and displays threat information resulting from aircraft illumination by laser designators, range finders and beam riding missiles. ... The AN/AVR-2A LDS consists of one interface unit comparator and four identical sensor units.; image from thaimilitaryandasianregion.wordpress.com
These sensors are installed on the MH-47G Chinook, as given in americanspecialops.com

Defensive systems
Common Missile Warning System (CMWS)
Suite Of Integrated Radio Frequency Countermeasures (SIRFC)
AN/AVR-2b Laser Warning System
XM216 Dark Flares (invisible to naked eye)


What is the go-around pattern and procedure for Heathrow? An hypothetical situation in which an airplane taking-off at Heathrow must go-around / land immediately after taking-off. What will happen during this scenario:

Is there any specific action to be done in this scenario, i.e. setting transponder to some code?

What is the go-around / return pattern for Heathrow? Over which villages will the plane fly and at which altitude?

Needing to return shortly after taking off is not very common, and as such there is no specific established procedure or route to be followed. The pilot will inform air traffic control of their intentions, and ATC will route the flight back for an approach. Depending on the situation, the flight might require priority, in which case other approaching flights will get bumped back in the sequence.

Transponder codes are used to identify flights on ATC radar. Simply speaking, one specific flight will have one specific transponder code during the whole flight - it is not changed because the flights needs to do something unexpected. Transponders are not used to transmit intentions (although there are a few special codes that can be used in case normal means of communication are unavailable).

For a flight needing to return to Heathrow, the return path will be similar to that of any other aircraft approaching the airport.


How do pilots make rudder inputs during negative G flight? During negative 'G' flight, the forces on the pilot (or pilots) will tend to push their legs up and away from the rudder pedals on your airplane.  Needless to say, this is not conducive to making rudder pedal inputs!

Given that, if you are in negative 'G' flight in a suitable aircraft, and wish/need to make a rudder input during that negative 'G' maneuver, how do you do it?  Do military and acro aircraft use any sort of special mechanism (such as foot pedal straps) to help the pilots keep their feet on the pedals during negative-'G' regimes?

The short answer is that at negative 2 G, the airframe limit for the A7-E, it took little effort to keep your feet on the rudder pedals.

In controlled flight there are aircraft limitations on both positive and negative G. I flew the A7-E for the US Navy and have some operational experience that might offer some context to the question.

I believe the positive G was limited to somewhere between 7-9 G. Can't quite remember the exact number, and it did depend on the external store configuration as well. Newer aircraft have higher G loading capabilities, although the human body has its own limit. Fitness and training can extend the pilots envelope to high G.

As mentioned negative G in the A7-E was limited to -2. There was no issue at these sorts of values keeping your feet on the rudder pedals. It is worth mentioning that the rudder is controlled by the AFCS (Automatic Flight Control System) system and so there is no real need to keep your feet on the pedals, although I did use them at times. Negative 2 G's was hard on the body, and a pilot would come back to the ship with bloodshot eyes after undergoing maneuvers that required this sort of thing.

I can think of 3 times that a pilot would find themselves in a negative G environment. I am not including standard acrobatic maneuvers, but more tactical situations. One, and this wasn't typical, was during training. At a high altitude you unloaded the aircraft to -2 G's and set a 45 degree dive. As the airspeed increased it entered an area on the IAS instrument that was no longer valid, and this was close to Mach. The goal was to observe the characteristics of the aircraft when it went supersonic. There was a gentle pitch down as you broke the sound barrier. I did this maneuver once, although I commonly practiced 45 degree dive bombing deliveries.

The second, and quite common, situation was a maneuver called a bug out. A bug out was used by a flight to engage other aircraft in air-to-air combat. It was at once a defensive and offensive maneuver. The lead called "Bug out. Bug out," and the flight went to -2 G's, unloading the aircraft, and giving up altitude for airspeed. As your adversary turned towards your 6-o'clock the flight would "Pitch back," and pulling positive G's, gaining altitude. Trading airspeed for altitude got the aircraft into slower flight where its turn rate could be maximized.

The bug out placed the aircraft immediately in free fall with military thrust applied. Rolling inverted and applying positive G's is like being in a turn in the sense that the positive G bleeds energy.

The third situation was a defensive maneuver. At low airspeed and with someone directly on your 6 o'clock the pilot might try to "jink." Jinking was a last ditch maneuver where the pilot randomly pulled maximum G, and then went full negative G. I used this against an F-14 in a fight, and afterwards the pilot came and told me he couldn't get a firing solution.

As for uncontrolled flight there is the flat spin which can exert substantial negative G on the pilot. If I remember correctly a flat spin had never been positively observed in an A7-E. It was postulated, however, that given asymmetric wing loading it might be possible. On the other hand the F-14 could enter a flat spin. Talking with pilots who had knowledge about this sort of flight I was told that the negative G could be large enough where it would not be possible to get your hands on the ejection handle, either low or high. Not sure about the feet :)

An interesting side note. At times a pilot will find themselves in a high g turn, grunting against the inflated g-suit, and fighting the encroaching tunnel vision. When I found myself here I would pull on the stick a bit more and watch my peripheral vision go dark (tunnel vision). A bit more g and the tunnel got even narrower, until it was like I was looking through a hole. Then I would ease the stick and watch the hole get bigger. Pull back and smaller. Kind of interesting.


Are propellers obsolete? Now that we have jet engines and all, is there still a use for propeller based flight (besides helicopters)?

Most airlines nowadays don't use propellers, right?

Propellers are not obsolete by any measure.

The jet engine offers two important advantages over propeller aircraft- higher speed and better fuel economy over long range. However, for low speeds and shorter routes, propeller (tuboprop) aircraft are better.

Also, for light GA aircraft, jet aircraft is prohibitively expensive to purchase and maintain. Among other things, the higher speeds associated with jet aircraft make them unsuitable for use in GA. Also, every jet pilot is first trained in propeller aircraft; in fact, airline pilots should have a minimum prior experience before piloting jet aircraft.


How does an RMI respond when tuned to a VOT station? I know that a VOT station for testing VORs broadcasts a 360 radial signal, so on a VOR, the indication would be 360 with a 'FROM' or 180 with a 'TO' indication, but how does an RMI respond if tuned to a VOT station?

Would the RMI simply home to the station because it points to the direction signal is coming from, or would it point at the 180 position because it detects that it is on a 360 radial 'FROM' the station?

Yes, it will point at the 180 position

VOR Test

  A VOT differs from a VOR in that it replaces the variable directional signal with another omnidirectional signal, in a sense transmitting a 360° radial in all direction

VOR Receiver Check

  With the Course Deviation Indicator (CDI) centered, the omni−bear- ing selector should read 0 degrees with the to/from indication showing “from” or the omni−bearing selector should read 180 degrees with the to/from indication showing “to.” Should the VOR receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180 degrees on any omni−bearing selector (OBS) setting.


What are "secondary precessional forces" that apply to boomerangs in flight? note: This question is about the aerodynamics of boomerang flight and how it differs in microgravity environments. Some background information available here in Aviation SE: Are frisbees and boomerangs technically gliders? and in Space SE: How does one throw a boomerang in space? Does it return?

Professor and former astronaut Takao Doi is apparently the fourth person or the third person to throw a boomerang in space and the first to throw one specifically designed to work in microgravity.
The Space-Toys.com article says:

Finally, on March 18 2008, Japanese astronaut Takao Doi took a three bladed boomerang (supplied by Yasuhiro Togai) aboard the Space Shuttle Endeavor on a mission to install equipment on the International Space Station. The boomerang that he threw was the only space boomerang that was specifically designed for space flight. The photo of the boomerang that Maurizio supplied to me show that the blades are recessed to the rear, as you would find in a Fast Catch boomerang. This is the optimal design for a space boomerang because when the blades are shifted to the rear, the secondary precessional forces that cause a boomerang to lay over in flight are retarded. The Japanese team of Yasuhiro Togai and Takao Doi deserve credit for making and throwing the first [multibladed] boomerang that was optimized for use in microgravity conditions.

What exactly are the "secondary precessional forces" referred to in the article?

Takao Doi in a space three-point stance throwing a returning boomerang aboard the ISS:

The aerodynamic forces on the boomerang are roughly like the springs that limit the (primary) precession of a rate gyroscope.  These forces introduce a secondary torque, which in turn produces the secondary precession in question, which is needed to tilt aka "lay over" the boomerang.  (My physics is rusty; the physics SE may be more fruitful.)


Can a pilot work flight engineer's station? In kinda reverse of Did Flight Engineers progress to be pilots?

In three-crew cockpit can a person qualified as pilot work in the position of flight engineer? Does a flight enginner have different qualifications that are not covered by an ATPL, thus rendering a pilot unqualified for the position?

The pilot cannot just sit down in the flight engineer station.  First there is a different FAA flight engineer certificate that the pilot needs to obtain.  Secondly, the pilot would need to go through the airline's training program for that station.

Granted, a pilot who has flown that airplane before should quickly get those done.


What were the make and model of the engine that failed in flight Southwest 1380? Regarding the tragedy of Southwest 1380 flight on 2018-04-17 involving an engine failure, what company manufactured that engine, and what engine model/type was it?

According to this article published on news.com.au:

Engine model is CFM56-7B, debuted in 1997.
Manufacturer is CFM International, jointly owned by General Electric of the United States and Safran Aircraft Engines, a division of Safran of France.

More specifically, AirFleets.net reports:

Engine model: CFM56-7B24
Plane: Boeing 737 Next Gen, variant group 737-700


Is lower wing loading safer? Is lower wing loading safer?

Looking at the ultralight specs in Canada, they call for a max wing loading of 12psf.  In fact, amateur built aircraft in Canada cannot have a wing loading over 20psf,I'm told.

While in the US, we see aircraft like Rutan's Boomerang at about 42psf.

Seems many GA aircraft seem to be stuck at 14-17psf range

With VG's or slats and flaperons  (full length flaps) and then slatted
(junkers flap(?)), Zenith is getting a nice cl of about 3.0, about double a typical ultralight

Why not higher wing loading?  This would lead to smaller wings, better aspect ratios, less drag, which means smaller engines, which means more affordable GA aircraft.

In fact, Rutan's motorglider (solitaire(?)), had a top speed of about 200(?) mph, with just a 4 cycle 18hp industrial engine ( rated for continuous duty), the kind you can buy at a big box store for $500.

I understand M. Coulumban, the designer of the cri-cri, just switched to low cost industrial honda engines as well.

On a saturday morning 1,000 mile cross-country trip ( pretty routine for the long-eze crowd)and have engine problems? No problem. Just go to a big box store and buy a new one and install it in an hour. Might as well make it an inline twin for that low of an engine price. Rutan's twin Defiant could take off from either engine alone, or both.  Seem's pretty foolproof to me.

Seems like pretty good economics for GA aircraft in my mind.

So..... why limit ultralights to 12psf.  I assume lower wing loading means it's safer.  Lot's of wing means it's harder to stall.

Is this correct?

Yes in general it's accurate to say that lower wing loading is safer, especially in the hands of a pilot with low experience flying in calm weather with an unreliable engine.  Flying in gusty winds and strong turbulence, there would be safety advantages to a higher wing loading.

The basic idea of limiting the wing loading is to limit the stall speed and thus the destructive energy involved in a forced landing.  As the question suggests, perhaps it would be wiser to limit the stall speed, or to allow deviations from the prescribed minimum wing loading in the case of a demonstrated low stall speed.

However the Canadian regs may also be written with the idea of limiting the top speed too.  Exactly why that's a good thing is not completely clear, but in the US at least an ultralight must not be able to exceed 55 knots in sustained level flight.


What is the effect of wingspan on wing stiffness? In an article in aviationweek about flight testing the GE9X, this statement is made.

  To reduce aeroelastic interaction between the testbed and the big
  engine, GE has also stiffened the wing by removing the 6-ft. tip
  extension developed for the standard -400 series. The reduced span of
  just over 195 ft. will reduce potential exposure to limit cycle
  oscillation and maintain an unrestricted test envelope.

How does removing the wingtip extension make the wing stiffer?

What is "limit cycle oscillation"?

Limit cycle oscillation is a fancy term for flutter with constant amplitude. The limit cycle is an attractor, meaning that the frequencies and amplitudes of a range of oscillations will converge to the limit cycle oscillation as time progresses. One example can be witnessed in this video.

By removing the wingtip, GE could raise the bending eigenfrequency of the wing, like you make a pendulum swing faster if you shorten it. Adding a heavier engine before certainly reduced the eigenfrequency, so the shortening was a way of compensating for the changed dynamic characteristics of the modified wing-engine combination.

Wingtip removal does not make the wing stiffer, only larger spar caps, a thicker spar or employing a stiffer spar material would. Stiffening the wing spar would raise its bending eigenfrequencies, and removal of the wing tip would do the same by different means. I guess the author wanted to express that bending frequencies were raised but picked the wrong explanation.


Is it ok to lift locked armrests on aisle or window seats? While reading this question, I came across a little trick, of how to lift up the locked arm-rests in window and aisle seats. Though I always wondered why they were locked, knowing that little trick now makes me wonder is it legal to lift locked arm-rests. Would it get me into any trouble with the cabin crews or the airline?

I'll have a go at this, but this answer applies only to the US.

First, is it legal? As far as I can see, yes. There are various regulations (e.g. 14 CFR 121.317) about specific items that passengers have to be briefed on and comply with instructions on. They include smoking, seatbelts, emergency exits, screens and tables etc. But there's no explicit mention of armrests and I didn't see any generic clause like "any other equipment" or whatever. So in the absence of any explicit or general regulation that says you can't, I would say you can.

Second, will it get you "into trouble"? Maybe. As the answer to a related question shows, the FAA doesn't want armrests to become an obstacle to evacuation and they advise crew members to make sure that armrests are locked down for takeoff and landing. Many airlines I've traveled on have an announcement about the armrests, even if there's no regulation that requires it. If the crew ask you to put your armrest down and you refuse then the 'best' thing that can happen is that they decide you're rude but not dangerous and let it go. But if you refuse repeatedly and they decide you're being confrontational or unlikely to follow other instructions then 14 CFR 91.11 would apply:

  No person may assault, threaten, intimidate, or interfere with a
  crewmember in the performance of the crewmember's duties aboard an
  aircraft being operated.

Under that regulation, if the crew decides you're "interfering" with their duties (which include keeping you and everyone else safe), you'll probably get a police escort off the aircraft, especially if alcohol is involved.

But having said all that, I don't think this is a big deal at all. The FAA's concern seems to be about armrests that get stuck at an angle, not about ones that can be fully lifted. I've often seen passengers put up their armrests and I've never heard a crew member comment on it, but if they do ask you to lower them, arguing about regulations would probably be a bad idea.


Is a VOR check required to legally fly an ILS approach under IFR? The FARs require a check of the VOR receiver before IFR flight:

§91.171   VOR equipment check for IFR operations.
(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft

...has been checked using a procedure described immediately following.
Does this apply to an ILS approach? Specifically, consider a flight conducted entirely using GPS, except that it terminates in an ILS approach. In most (or all?) aircraft, the ILS and VOR receivers are one and the same. However, the mechanism of how an ILS and VOR work are very different.
The FARs speak of "the VOR system of radio naviation". Is that specifically for actual VORs, or does it encompass the ILS by virtue of being the same receiver and CDI?

This regulation does not apply to an ILS approach.

You are right is saying that "the VOR system of radio navigation" refers to actual VORs.

If the flight is conducted using GPS to an ILS approach then you aren't using VOR for navigation. It does not matter that ILS uses the same instrument display.

Intuitively, though ILS and VOR use the same display and frequency band, they are functionally different. The VOR beacon sends out a omnidirectional signal, from which a bearing (0-359 degrees) is calculated. The ILS sends out a focused, directional signal, which the receiver centers on.


What is the relation between the Lift Coefficient and the Angle of Attack? I'm trying to understand the relation between the Lift Coefficient and the Angle of Attack using PEP software for the A320 IAE. However, I notice that often for a given Angle of Attack, there could be more than one Lift Coefficient ("slightly different" but different).

Is it an error or a real issue? If yes, what are the standard conditions assumed for the Cl vs AOA graph?

Normally the relation is linear within the practical range of angles of attack used in flight. However, the Mach number and the Reynolds number will cause shifts. For the Mach effect, use the Prandtl-Glauert rule in subsonic flow up to Mach 0.7: $$(\alpha - \alpha_0)_{Mach} = (\alpha - \alpha_0)_{incompressible}\cdot\sqrt{1-Mach^2}$$

A higher Reynolds number means a relatively thinner boundary layer, and since the boundary layer on the suction side of the airfoil grows in thickness with increasing angle of attack, the same airfoil will have less boundary layer growth at a higher Reynolds number. This translates into a higher effective angle of attack at the same geometrical angle, so the lift curve slope becomes steeper and will reach stall at a higher angle.

There are no standard conditions, but a proper lift curve graph should note both the Mach and Reynolds number for which it is valid.


In turboprops, is there a mechanism to prevent the blades running in opposite direction? Because, it might lead to reverse flow of air and could stall the plane. Is there some mechanism to prevent this?

There was a crash in Ireland a few years ago, when the pilot of a small charter plane accidentally put one of the turboprop's in reverse.

Most manufacturers realise this is a serious risk so most are 'gated' in that you have to do something to get them into this range. This is like pulling up the reverse tabs on the throttle levers on the B737 or A320 that is only possible when they are in the idle position.

ATR 42/75: Has something called Idle Gate that prevents throttles from going back too far when engaged.
Dash 8: Has gated throttles that requires pulling up a pin to get them into reverse.

There is to the best of my knowledge nothing (in general at least) to stop to the thrust from being 'reversed', or more specifically to angle to blades to what is known as the 'beta' range, if you are intent on doing it. Unlike jets where doors open for instance and the process is a little more complex, this is just rotating the blade below a certain angle.

I found the accident report [PDF] for the Manx2 flight. There were multiple things going on here rather than a single cause:

Generally bad piloting for putting them in a late go-around situation to start with.
There were errors in the engine, causing a thrust mismatch.

This meant that one engine proceeded to continue into negative thrust.

It does then not come as a complete surprise that you manage to flip the plane over from considerable unsymmetric thrust when you are close to stall speed.


What's the appropriate course of action if an airliner bounces 5-10 feet on touchdown? I regularly fly in a PC flight simulator, and I'm currently trying to refine landings in medium-to-large airliners in various weather conditions.

A problem I've faced in some conditions while flying the 737-800 has been bouncing on touchdown, where I lose just enough lift in the last few feet of my approach to touch down firmly on the main gear only, before bouncing back up 5-10 feet.

While I'm already looking into avoiding this kind of bounce whenever possible, I'd like to know what common procedures are in this situation assuming the gear is left intact and there's no urgent failures or factors (such as a fuel emergency) which would time-constrain a landing. Specifically; would a real-world flight in this situation go around, adjust thrust, adjust flare, or do something else entirely?

Well, "5 to 10 feet" is not "slightly."  One or two feet is "slightly," and in that case you can maintain your landing attitude or pitch very slightly nose-down -- really a matter of the slightest change in pressure, not really a perceptible change in pitch -- and you'll land soon after that.

However, for a larger bounce, the concern, especially on the longer 737's, the -800 and -900 and -900ER, is a tail-strike. The tail skid protects the tail from a strike on takeoff, but on landing the geometry is different and the skid won't be what contacts the runway; it will be the fuselage.  And while tail strikes on takeoff are more common, it's the tail strikes on landing that are far more damaging & expensive. So avoiding one of those is a major concern.  Thus, the usual guidance in the event of a bounced landing in the 737-800 is not to attempt to salvage the landing, but rather to go around.  You may touch down again during the go-around, and that's okay, but you're better off aborting the landing & taking it around to try again, than to risk damaging the aircraft by trying to recover from a high bounce.

One thing that complicates recovering from a bounce is the fact that your spoilers may have extended when you touched down the first time -- and the last ten feet to the runway with spoilers fully extended & power already at idle, won't look nearly the same as the usual last ten feet to the runway, with power coming off & no spoilers. You'll settle fast, and hard, and you won't have the elevator authority you're used to for adjusting the sink rate.  You can pull back, but the change in attitude probably won't arrest the sink rate the way you think it will -- you'll just hit harder & in a more nose-high attitude.  Thus the increased risk of hitting the tail.

On a shorter aircraft without the tail-strike concerns (say a 737-200,  -300, -500, -600, or -700), it might be possible to salvage the bounced landing, although 10 feet up is a pretty high bounce -- even if you had a lot of airspeed to play with and runway ahead of you, I'm not sure it would be wise trying to sort that situation out -- go around & try again.