Is airflow ever laminar in the boundary layer, near the wing leading edge? I often see images of the boundary layer over a wing indicating the airflow is laminar over the first portion of the wing:

Transition from laminar to turbulent flow over the wing. Left: source, right: source

I was also told this cannot happen in a full-scale aircraft, as the Reynolds number involved are not compatible with laminar flow.

Would these pictures be misleading?

Is the flow laminar or not before the boundary layer is separated?
If so, what are the conditions that make a flow transitionning from laminar to turbulent?

In still air every boundary layer starts laminar. How soon it transitions to a turbulent boundary layer depends on:

the local Reynolds number,
the pressure gradient,
wing sweep and
disturbances like bugs, rivet heads or turbulators.

Flat plate flow (without pressure changes) normally transitions at a Reynolds number of around 400,000. If the flow is accelerated, all speeds in flow direction increase while cross flow will not be affected, so a laminar boundary layer in accelerating flow is stabilized. On modern gliders the lower surface is laminar in excess of 80% chord at higher angles of attack, which can correspond to a Reynolds number of 5,000,000 or more when transition eventually occurs.
On the other hand, a pressure rise in flow direction corresponds to a deceleration in flow direction, so any movements perpendicular to the flow direction will grow relative to the flow speed, and as a consequence the turbulent transition occurs rather quickly. Upper side flow past the suction peak near the leading edge is a prime candidate for transition, and that is what causes flow around the "traditional airfoil" to become turbulent earlier. The graph in your question is misleading because the lower side flow of the traditional airfoil should be as laminar as that of the P-51 airfoil if the surface smoothness of both is comparable.
How laminar was the P-51 wing?
At the flight speed of the P-51 very little laminar flow was left; the full effect of laminar airfoils can only be exploited at Reynolds numbers below 5,000,000. At higher Reynolds numbers it needs progressively steeper gradients to keep the boundary layer laminar, such that the range of angles of attack where a long laminar boundary layer is possible on both sides of an airfoil (the laminar bucket) gets smaller and smaller.
The "rooftop" distribution of the 6-digit NACA airfoils did help, though, because it gives them a higher critical Mach number than the "peaky" distributions of earlier airfoils. The suction peak near the nose of older airfoils will lead to local supersonic flow at a lower flight Mach number, and increased drag from the shocks which would follow. Most important for its low drag, however, was the very smooth wing surface of the P-51 with no gaps ahead of the spar. See this rec.aviation.military post for details.
Influence of Sweep
Wing sweep will also make it hard to maintain laminar flow. As explained here, on a swept wing only the speed component perpendicular to the wing will be affected by it, so the accelerating flow past the stagnation point will curve inwards on a sweptback wing. At the same time, viscosity will slow down the flow near the wing skin. The consequence is a twist in the speed distribution over the boundary layer, which destabilizes the laminar flow and leads to early transition.
A C-172 with its four-digit NACA airfoil has the peaky upper surface which will trip the boundary layer very early on the upper surface. On the lower surface laminar flow will last a little longer but will be destabilised by gaps in the surface, so most of the flow on the C-172 is turbulent. On an airliner the Reynolds number is in the tens of millions, so transition will be very early and very little laminar fraction remains. This is mostly found near unswept leading edges like the engine nacelles. Only with advanced technologies like boundary layer suction will it be conceivable that a larger part of an airliner wing can be kept laminar.
Transition and Separation
Laminar separation sometimes happens when the flow separates shortly after negotiating the nose, like on a five-digit NACA airfoil or an undimpled golf ball. This leads to an abrupt stall and should best be avoided. Normally, the boundary layer transitions into the turbulent state and stays attached until the turbulent boundary layer separates, either at the trailing edge, or progressively further ahead of it when the airfoil stalls.
Sometimes, transition happens in a laminar separation bubble. The decelerating flow past the suction peak is slowed down by friction near the surface, and both effects combine to bring the flow to a standstill at some point. The boundary layer thickens, such that the pressure rise is momentarily suspended, and speed oscillations in the boundary layer get amplified such that cross flows become more intense, mixing outer and inner parts of the boundary layer. The speed profile becomes fuller and the flow near the wall picks up speed again, such that the separation disappears and pressure rise resumes.
Below I plotted the XFOIL results for the pressure distribution around the HQ-17 at Re = 1 Mio (the HQ-17 is used on the ASW-22 Open Class glider, for example). The dashed, black lines show the inviscid pressure while the colored lines show the viscous flow results. On both sides you will see a kink in the colored lines - this is where the laminar separation bubble is.

When the flow separates, the pressure line becomes horizontal. After transition it jumps back down near the inviscid line, which shows how much steeper the pressure gradient is that a turbulent boundary layer will tolerate. Reattachment is complete when the steep pressure rise has brought the local pressure back near the inviscid level. Note in the airfoil plot at the bottom that the boundary layer thickness peaks where the separation bubbles are.
Yes, the boundary layer is laminar before and into the separation here. This phenomenon occurs at the scale of model aircraft, gliders and small GA aircraft (100,000 < Re < 5,000,000) but is absent at higher Reynolds numbers because then the transition happens before the laminar flow separates.


Why are TFRs issued around stadiums during games in the U.S.? I want to know the FAA's reasoning behind issuing TFRs whenever a game is going on in a stadium. Is it just because of a large crowd of people? Do engine noises just bother people? Could an aircraft's wake blow a ball off course?

The 'stadium TFRs' were originally required by Congress after 9/11 to reassure people and encourage them to continue their normal activities without worrying about aircraft flying low overhead. And since then, Congress has repeatedly voted to maintain them; the FAA is just implementing them as directed.

There's more information in FAA Notice 8900.370:

  Following the events of September 11, 2001, Congress enacted
  legislation that restricted civil aircraft operations over sporting
  events such as NASCAR, Major League Baseball (MLB), the National
  Football League (NFL), National Collegiate Athletic Association (NCAA)
  Division I Football, and other open-air events containing more than
  30,000 people. These restrictions are implemented via a TFR. In 2004,
  Public Law (PL) 108-199, Consolidated Appropriations Act, Section 521
  was modified to allow civil aircraft limited use of designated
  sporting event TFR airspace.

The TFRs are generally very unpopular with pilots and perhaps as a result, there have always been rumours about other reasons why they were created, or - even if you accept the original reasons - why Congress insists on maintaining them today. For example, the NFL benefits from not having 'unauthorized' aircraft taking pictures or towing banners over their games, when they could be selling those rights exclusively for a lot of money. So perhaps the NFL has pressured Congress to maintain the TFRs for 'security' reasons, while the real motive is making more money. The same comments and thinking are often applied to the 'Disney TFRs', by the way.

Having said all that, there are some arguments that 'security theater' - i.e. feel-good measures that provide no actual security - can have a positive impact on people's comfort and confidence, so it's not a black and white issue. But that whole topic belongs on another site.


What are the names of the forces acting upon the side of a vessel? People often refer to the four forces in navigation as weight, lift, thrust and drag. Are there separate names for the forces acting upon the side of a craft?

As we engineers are extremely boring people, and given the fact that a name seems not to be available, we generally refer to them simply as "side forces".

Or, at least, I never encountered a different term on any of the textbooks that I have read.


Are run-ups required for multiengine piston aircraft? Are run-ups required or advisable on multiengine piston aircraft?

Yes, From the FAA handbook

  ...be operated from its crossfeed position during the run-up. The
  engines should be checked individually and allowed to run at moderate
  power (1,500 rpm minimum) for at least 1 minute to ensure that fuel
  flow can be established from the crossfeed source. Upon completion of
  the check, each engine should be operated for at least 1 minute at
  moderate power from the main (takeoff) fuel tanks to reconfirm fuel
  flow prior to takeoff.

And also

  Some multiengine airplanes have a ramp weight, which is in excess of
  the maximum takeoff weight. The ramp weight is an allowance for fuel
  that would be burned during taxi and run-up, permitting a takeoff at
  full maximum takeoff weight.

For what its worth a multi-engine piston plane is mechanically double what a single engine plane is, and often carries a similar if not the same engine as a single. The things you are checking in run-up, primarily the ignition system holds true for both single and multi-engine aircraft.


Why are visual approaches (in the US) preferred by US pilots, but not European pilots? I listen to ATC a lot and often hear airliners cleared for visual approaches in the USA. Interestingly, if it's European airliner pilots they deny the visual approach and request ILS even if they are in an emergency and probably should land ASAP. Why is that? Are European pilots not used to hand flying? I assume pilots flying, for example, A380, should have extremely good piloting skills. What is the problem with doing a simple visual approach?

I also haven't heard yet from ATC to clear an airliner for a visual approach in Europe.

Here is the audio source (via YouTube): 30% of time is spent dealing with turning ILS on for 31L.

Capacity
If a visual approach is offered, and it gets accepted by the pilot, the airport can declare higher capacity. Whereas IFR procedures reduce the airport's capacity.
US and European airports handle their slots differently. Europe plans for the worst, which can on good days limit the full potential. The US plans for the best, which can on bad days end up in huge delays.[1] I've also explained it here. In short, if in the US they started offering more IFR procedures during good weather (VMC), they won't be able to back up the declared capacity.
Generally, airports are closer to the cities in Europe than in the US (apart from coastal cities) due to the different city planning (old vs. new). Airports in close proximity to cities have approach procedures that are designed for noise abatement, and are best flown as precisely as published.
Familiarity
So in general, the pilots are used to different systems. Also pilots that don't fly a lot in the US—like foreign carriers—are not as familiar with the surroundings as local pilots are. A Flight Safety Foundation briefing[2] discusses the risks associated with visual approaches for airplanes heavier than 12,500 lb:

Accepting an air traffic control (ATC) clearance for a visual approach or requesting a visual approach should be balanced carefully against the following:

(...)
Crew experience with airport and airport environment.

It's about knowing what to look for, especially when a particular visual approach does not have a visual approach chart.

An example of a visual approach chart showing landmarks; airnav.com

Use of ILS in emergnecy
Regarding the specific video about the bomb threat and the pilot's request to have the ILS turned on, for that scenario in particular it's because an ILS approach is less demanding,[3] especially in such high workload environment. (Also the flight was diverting mid-route.)

Poor professional judgment/airmanship (...) Decision to execute a nonprecision approach, instead of an ILS approach, in demanding conditions to
expedite arrival (...)[3]

1: Odoni, A., and T. Morisset. "Performance comparisons between US and European airports." Proceedings of the 12th world conference on transport research (WCTR), July 11–15. 2010. (paper; presentation)
2: Flight Safety Foundation ALAR Toolkit. ALAR Briefing Note 7.4 — Visual Approach. (PDF)
3: Khatwa, Ratan, and Robert L. Helmreich. "Analysis of critical factors during approach and landing in accidents and normal flight." Flight Safety Digest 17.11-12 (1999): 1-2. https://www.jstor.org/stable/44729511


Does the FAA advise or require pilots to wear eyewear designed to protect them from lasers? With several instances of pilots being subjected to laser lights being shone on them, I wonder if the FAA is actually doing something about it.

It's easier said than done to stop people on the ground from shining their laser lights. Maybe a preventive strategy is better. With a host of eyewear available which minimise the risk of laser light, I am curious as to what the FAA and other similar authorities are doing about it.

I'm not aware of FAA (or any other regulatory body) recommending protective eyewear for lasers. FAA safety Advisory doesn't include them. There are a couple of problems with using protective eyewear for lasers:

The eyewear is usually wavelength sensitive; The pilot has to wear the correct eyewear that blocks the incoming laser beam; he/she should know the wavelength of incoming laser.
The protective eyewear filters (atleast most) of the incoming light in that wavelength. This may affect the normal sight of the flight crew.

Due to these reasons, the regulatory authorities are more concerned with procedural changes that protective wear. From Boeing's protection against lasers:

  ... however, airlines should consider the drawbacks that are associated with them. Filtering light reduces the total amount of light entering the eye, which can adversely affect normal viewing, ... . In addition, filtering green light can remove some green flight symbology on flight deck displays and change the appearance of some of the other colors used. As a result, protective glasses should be used with care.


How does the nozzle diameter affect the thrust of a ducted propeller? I currently do some experiments with ducted propellers in which I try to figure out which effect a nozzle has on the thrust produced.
My theory goes like this: If I reduce the exit diameter of the duct the pressure is going to decrease and the air velocity and thus the thrust is going to increase. Consequently, you would want a small exit diameter.

However, in my experiments I measured the thrust of a 12x12 inch propeller at around 5500 rpm and got 20 N without a nozzle (just a constant diameter duct) and only 4 N with a nozzle reducing the exit diameter to 50 percent of the prop diameter.

These results are contrary to my theory! Does anybody have an explanation for this ? And what should I change to actually increase the thrust compared to the prop without a nozzle ?

Here are some pictures:

Welcome. I'm afraid your theory wasn't actually working. Reducing the duct's exit diameter led to an increase in its internal pressure, increasing load on the propeller, and likely even causing some reverse flow.

Nozzle design is a complex subject. I can't think of a way to condense it, even limited to one specific case, into a suitable answer; maybe I don't get it enough myself. Keep that in mind; the below is just one small shard of the whole and by no means the full picture.

In general, the job of a nozzle is to match the pressure at jet engine exit to that outside it. If the pressure is different, it gets matched outside the engine, where it doesn't produce thrust.

When the engine is a rocket, which creates high pressure, the nozzle needs to expand the gas, converting pressure to thrust through acting on the nozzle. When the engine is a cold fan, which accelerates air, it's the opposite - the nozzle needs to compensate for the loss of pressure with a bit of compression, so that the air stream can exit without fighting the pressure of outside air at the back.

It's important for a convergent nozzle not to compress the exhaust to a higher pressure than the outside air, else it will destroy thrust. That was your case, the nozzle was too narrow, so it compressed the air to above ambient - which caused it to try and blow back through the fan.

To give a practical answer, an optimal nozzle at these velocities would be very similar to a simple duct, narrowing just a percent or two at the end, with a smooth exit shape.


What are these spinning discs next to the throttle of the Boeing P-8 Poseidon? The source video can be found here and the GIF is from 0:21 onwards.

What are those 2 spinning things next to the throttle ?

Those spinning discs on either side of the throttle quadrant are the stabilizer (stab) trim wheels.

The stab trim system can be actuated by electric trim switches on either the left or right control yoke, the autopilot (AP), or by manually turning the stab trim wheels. The yoke mounted electric trim switches "spin" the trim wheels at the high speed seen in the GIF above, while the AP turns the wheels at a relatively slower rate. A stowable handle in the trim wheel can be pulled out to allow for faster manual turning.

I've heard that this handle can be very painful to the knees if left unstowed and either the AP or especially the electric trim turns the wheels.

See "Why I hate the Boeing 737" for more info on the system, including photos of the trim wheel and stowable handle:


Are planes allowed to fly through a planned professional fireworks display? I was watching 4th of July fireworks tonight, and I noticed a very low plane circling the fireworks display.

I'm wasn't sure of its altitude, but I would estimate around 1000 ft or no higher than 1500 ft.

I'm thinking, "that's pretty low around here", and "it would be cool to be in that plane, with that view."

The plane then turns sharper toward the fireworks, then flies right through the middle of it, as fireworks are going off around the plane.

The plane then circles around again and flies just above display this time.

Is this even allowed?

If not, how would this pilot be held accountable/caught?

If so, how is this safe?

Most fireworks go no higher than 1200 ft and consumer sized stuff is more around 500ft and below. which is fairly low for most regimes of flight for all but general aviation planes and planes taking off/landing. With this in mind your main point of concern would be shooting them off near an airport, or people flying low and slow near your display.

I cant find any regulation explicitly forbidding it although its unwise and it most likely would be considered "careless or reckless operation as per §91.31" if you flew directly (and deliberately) into a display at low altitude, Fireworks are noted in the NOTAM's and it is the pilots responsibility to make themselves aware of all flight risks and en-route information as per § 91.103 Preflight action. You can see a good example here in todays NOTAMs listing various firework displays and their locations.

(source)

Its also the responsibility of the launcher to file the NOTAM in the first place.


What does "stuff it in the back right corner" mean? I'm still catching up with Flightchops, and came across this episode about his first tail wheel solo.  As he touches down for his first landing (around 5:03 in the video) he says "Get ready to stuff it in the back right corner".  He says it again a few seconds later.

I am low time in tail draggers (about 3 hours post endorsement) and I don't remember ever hearing my instructor use this phraseology.

Question:

What does he mean by this?  Is this common aviation terminology or just something he picked up from his instructor?

I can make an educated guess that he means to deflect the stick to the rear and right to allow for a three point landing with a right x-wind, but I'd love to get confirmation of this from the experts.

Note: I did google this, and came up blank.

As far as I know, it isn't a standard aviation term, it's just a casual phrase that he's using to emphasize or remind himself about the stick movement he needs. It's like your instructor telling you to "get dancing" on the rudder pedals: it's not an aviation term as such, but it's a very clear, descriptive reminder.

In a three-point landing you pull the stick all the way back after touchdown to keep the tail down. That helps to prevent unwanted pitch movements and gets weight on the tailwheel to help with directional control on the ground (as airspeed decreases and the rudder loses authority). Deflecting the stick to the right would be because of a crosswind from the right. Put those two things together, and the required stick movement is back and to the right, which is one "corner" of the range of motion it has.


Is a 5 hour 3 session process normal for a plane checkout you're already trained in? I've joined a flying club and am in the checkout process for their 152. I've trained in it, was endorsed by a 20,000+ hour instructor (who felt comfortable enough renting his personally owned 152 to me after the checkride), and my DPE (who a commercial pilot tried to steer me away from due to her supposed high fail rate, i.e. she's demanding) thought I flew the plane well. I'm a very new VFR pilot with about 70 hours total, and find this checkout process is taking longer than usual under these circumstances. Has anyone gone through a similar situation? How have they dealt with the matter? The instructor is also making me feel less safe as a pilot doing things like banking with full flaps, and pushing "flap religion" on me (i.e. 10 on DW, 20 on base, 30 on final without really considering the approach environment). Due to W/B restrictions on the 152 he's the club's only viable checkout instructor.

It is normal for flying clubs and flight schools to require a checkout before allowing a renter to solo, which may take several hours, regardless of their license level or previous experience. They often require additional training if you dont have a certain minimum time in type.

Your club might be slightly more paranoid than average, but that probably translates to cheaper club insurance and thus lower rental rates, which means you will probably save money if you stick around.

Also, it should be noted that 70-200 hours is the period with the highest accident rate, so some paranoia on their part is probably justified. Even if it's "just a 152."


Does the person directing the plane to parking not risk getting ingested by a jet engine? According to answers in this post, a jet engine can suck a human into its engines, even on idle power. Aircraft use their engines post landing taxi and into their parking space, and I notice people directing the plane to and from the parking space while engines are on.  Do they not risk getting ingested into the plane's engines?

A marshal should always ensure that he is in the line-of-sight of the pilot, failing to do so will effectively mean that the pilot is losing its extra sets of eyes. It is unable to see the hand gestures of the marshal guiding him into a parking spot.
To be visible for a modern jet liner pilot, you have to be meters in front of the nose of the aircraft. In the picture below, you have to be roughly on the right side of the orange line to be visible to the pilot. Modern airports use a digital marshal on a screen mounted to the terminal building.

Incidents of people getting sucked into the engine mostly happen due to miscommunication. Someone already walking up to the rear wheels with the blocks while the pilot spins up the engines to move the plane a little bit forward.


Airbus towing: is it necessary to switch on the Y HYD pump? So, somehow our operations came up with a new requirement: if one of our A320 or A340 is being towed, we are supposed to switch on the Y ELEC pump (if the engines are off). This shall prevent inadvertent retraction of the nose gear caused by the towing force.

Is this even mechanically possible for A320/A340 airplanes? I highly doubt it...

Please read fully the answer to understand the matter.

Please be careful, with respect to the 320 it depends on the aircraft generation, as I remember if not all, some A320 landing gears are green operated. Nevertheless, by switching on the yellow electrical pump, the green circuit will be pressurized via the PTU, thus preventing the towing force to cause inadvertent retraction of the nose gear.

This is just an explanation, not a procedure to be followed.


Can a landing aircraft snag multiple wires when landing on an aircraft carrier? There are 4 arresting wires on an aircraft carrier separated by about 50 feet and usually the pilot tries to snag the 3rd one but he does have the option to snag the first or second one.

What stops the tailhook from engaging the wires in front of the airfraft as well?   Is it assumed that they will also get engaged and the regulator on each arresting engine is tuned to accept it?

The answer is quite simple:

1. When touching down the nose is in a pulled up attitude, therefore the tail is farther down then normal. As soon as the airplane touches down (on all wheels), the tail is higher, thus the hook is higher.
You can see how the aircraft is in a nose up attitude, but gear and hook are on the same level. If the aircraft was sitting on all its tires, the hook would actually touch the ground. (This might not even be the case on all aircraft, though!)

2. This is the much more important reason: The hook is pneumatically pushed onto the deck (Google for the "F-14 Tomcat Pilot's Flight Operating Manual Vol. 1, Volume 1". Interestingly Wikipedia (is it a source?) says there are hydraulic and pneumatic hooks) but as soon as the hook hooks to the cable, and pulls on it, that pneumatic pressure is overcome and the vectors of the forces go from where the hook is attached to the plane to where the wire comes out of the ground (and vice versa).

Note: I guess its a combination of both, hydraulic and pneumatic. As hydraulics are not compressible, a valve would be needed to release the pressure built by the yank of the cable. So air pressure (pneumatics) would probably do a better job on keeping the hook down as long as no cable is attached. The retraction of the hook is done hydraulically though, and the way it is written in the F-14 Tomcat Pilot's Flight Operating Manual Vol. 1, Volume 1 it seems like the pneumatic pressure is kept even with the hook retracted...

As you can see here, the hook is actually almost parallel to the deck.

3. Last but not least: There is no space for two wires on one hook. Let's say the airplane would catch the first rope, and for some reason (it should definitely not happen, but let's just pretend it does) the hook is still on the ground where it would hook up to the second wire when crossing it, then it will maybe drag it along for a short time, but it can't really hook on as the hook is made to fit exactly one wire. This can be seen on this picture:


Can I count PIC time when regaining night currency with an instructor? Let's say I want to get night current again, but I haven't flown at night recently so I hire an instructor to fly with me. If I don't count PIC time I don't see a problem, I would just log it as dual instruction. But if I log PIC time, is the instructor considered a passenger? If so I wouldn't be able to have the instructor with me since I am not night current.

Is there an exception for flying with an instructor?

You can log PIC time because you are "the sole manipulator of the controls of an aircraft for which the pilot is rated" (14 CFR 61.51).  That's all that is required in this case for the pilot to log PIC time.

So since a pilot must be night current in order to fly with passengers, (61.57(b)) the question becomes, "is a flight instructor considered a passenger if they are providing instruction"?

AOPA has a flight briefing about this specific scenario where they quote FAA legal interpretations as saying:

  "We agree that, for purposes of section 61.57(b), an authorized
  instructor providing instruction in an aircraft is not considered a
  passenger with respect to the person receiving instruction, even where
  the person receiving the instruction is acting as [pilot in command].
  (The instructor must be current, qualified to instruct, and hold a
  category, class, and type rating in the aircraft, if a class and type
  rating is required.) The instructor is not a passenger because he is
  present specifically to train the person receiving instruction.
  Neither is the person receiving instruction a passenger with respect
  to the instructor. This training may take place, even though neither
  pilot has met the 61.57(b) requirements."

Note that they say in this case, since neither of them is a passenger, that technically neither the pilot or the flight instructor has to be night current.


Why don't the magnetic compass and direction indicator spin the same direction? You set a direction indicator (DI) based on the magnetic compass in certain GA aircraft, e.g. Cessna 172. During training you're taught how to align the DI with the compass, the DI being the opposite side to the compass. Why haven't they been designed in a way to make them visually match to make it easier to set the DI?
This illustrates what I mean:
Compass
|||330|||
 ^

DI
|||330|||
       ^

I think I understand your question, and I know what you mean - it's something I have to sometimes think about when setting the DI.
However, I think your premise is slightly flawed. The issue isn't "compass vs DI", it's the difference between viewing a compass from the top down, vs head on.
If you look at a magnetic compass compass which can be used both ways (Such as this one you'll see that sideways on, it looks the same as the aircraft compass as in it increments right to left. But from the top, it's identical to any DI and increments clockwise - however, it reads "upside down" compared to a normal top-down map reading compass.
I'm not even sure if this would work, but I think what you're asking is for the DI to be upside down. This would go against most concepts that "up" is equal to forward.


Is this photo showing dozens of airliners in close proximity real, or even plausible? CNN has an article about Heathrow that's accompanied by the following photo:

Is this photo depicting an actual event?  If not, would it even be possible/plausible to fly this many airliners this closely together or would wake turbulence and similar factors make this basically impossible?

It looks like several of the planes may be in the process of either stowing or deploying their landing gear, and also have their flaps deployed.  Does this indicate that maybe someone has photoshopped together a bunch of photos of planes landing to produce this image?

The image is a composite image of 42 planes taking off from Heathrow over a one-hour period on November 2, 2016.  Here it is on Getty Images if you want to license it for your own purposes;  it is credited to Dan Kitwood.


Can a drone be seen on TCAS? On condition that the pilot reports to ATC, an aircraft's TCAS sees a drone in the vicinity.

How can the pilot be sure that it was a drone on TCAS? What kind of symbol is shown on TCAS that can be identified as a drone? Or is it just a primary radar target pick up by TCAS?

No, a drone will normally not be visible on an aircraft TCAS. The TCAS system is based entirely on direct communication between aircraft transponders, so both parties (aircraft and drone) must have a transponder in order for TCAS to work. Normal drones do not have transponders, so they will not be visible on TCAS.

If a pilot reports a drone near the aircraft, what he means is that he (visually) has seen a drone out the window, not that it is indicated on an instrument anywhere. Besides, TCAS does not know different aircraft types, so even if a drone had a transponder and it was presented on a TCAS, the pilot would have no way of knowing that it was a drone (as opposed to any other aircraft).

  Or just primary radar target pick up by TCAS.

TCAS is NOT a radar. It is not able to display anything without a transponder. And even ground based primary radars will not necessarily pick up a small drone.


What is the maximum slope for taxiing aircraft? Every airport I have encountered has flat runways and taxiways.

Of course this is not exactly true, as some other questions picked up, and it is also possible that I just happened to visit mostly flat airports.

Is there any specification for maximum slope gradient an aircraft can safely taxi on?

A slope could be useful if an airport wanted to build an airplane bridge for example.

Footnote: I hope this is the correct/ current regulation. Published date is 25 January 2015.

Your question is covered (for EASA regulations) in the document Certification Specifications and Guidance Material for Aerodromes Design (link) in two sections:

Taxiway Slope (CS ADR-DSN.D.265)
Taxiway Slope changes (CS ADR-DSN.D.270)

  CS ADR-DSN.D.265 Longitudinal slopes on taxiways

  (a) The safety objective of limiting the longitudinal taxiway slope
  is to enable stabilised safe use of taxiway by an aircraft.

  (b) The longitudinal slope of a taxiway should not exceed:

  1.5% where the code letter is C, D, E, or F; and
  3% where the code letter is A or B.

  CS ADR-DSN.D.270 Longitudinal slope changes on taxiways

  (a) The safety objective of limiting the longitudinal taxiway slope
  changes is to avoid damage of aircraft and to enable safe use of
  taxiway by an aircraft.

  (b) Where slope changes on a taxiway cannot be avoided, the transition
  from one slope to another slope should be accomplished by a curved
  surface with a rate of change not exceeding:

  1% per 30m (minimum radius of curvature of 3000m) where the code letter is C, D, E,or F; and
  1% per 25m (minimum radius of curvature of 2500m) where the code letter is A or B.

  (c) Where slope changes in (b)(1) and (2)  are not  achieved and
  slopes on a taxiway cannot  be avoided, the  transition  from  one
  slope  to  another  slope  should  be  accomplished  by  a  curved
  surface  which  should allow the safe operation of all aircraft in all
  weather conditions.

The letters refer to the airport categorization.


Why don't helicopter blades look like other propellers? While doing an image search, I found many different shaped propellers and asked about them in this question.

In it, Peter Kampf answered my question with his usual thoroughness. However, as I looked at more images of various shaped blades, I saw images of the helicopters of my youth, Bells. The Bell Huey and the Cobra both have very long, very squared off primary blades. They don't have an aerodynamic looking shape and both the leading and trailing edges are straight.

Why would Bell have chosen this shape and not the shape more like a 'normal' plane propeller?

Helicopter blades need to be long and thin. The diameter of the rotor disc determines the efficiency of the rotor at low speeds and can be compared to the wing span in fixed-wing airplanes. The rotation creates strong centrifugal loads at the blade roots which grow with the square of the tip radius at a given rotation speed, so they cannot be tapered much. Adding chord to the middle of the blade would increase its area and add more friction drag, increasing the torque needed to keep the rotor spinning. If the helicopter is only designed for hover, the blade tips could be tapered, but the added complications of forward flight make a rectangular blade the better choice.

In forward flight, the speeds due to the blade's rotation and the flight speed add up, increasing local airspeed at the advancing blade and reducing it at the receding blade. Since the center of lift is trimmed to be at the rotor hub using the swash plate, the advancing blade has a smaller angle of attack and the receding blade a higher angle of attack. On the receding blade at high flight speed, the root will be in reverse flow since here the rotation speed will not be high enough to compensate for the flight speed. Dynamic pressure is around zero at the middle radius, and only the tip area sees sufficient dynamic pressure to create the lift which is demanded from the blade. Reducing this tip area would reduce the lift which can be produced there, so designers kept the blade chord constant.

Two effects limit the maximum forward speed: The remaining dynamic pressure at the blade tip of the receding blade, and the maximum Mach number at the advancing blade's tip. In order to increase this maximum Mach number, modern rotor tips are swept, but simple sweep will lead to torsion moments which will twist the blade. The pitching moment along the blade needs to be finely balanced so no torsion loads result. To achieve this, helicopter blade airfoils are either symmetric or reflex, and the tip is first swept forward and then backward, so the resulting lift acts exactly at the elastic axis of the blade. See the picture of such a blade tip below. The small trim tab you can see at the trailing edge is used for fine-tuning the pitching moment of the blade.

Eurocopter Blue Edge rotor blade (picture source)

I would not be surprised if an optimized rotor shape has non-rectangular blades, but my impression is that the gains of optimization are small, especially with the wide spread of operation conditions of all rotor blade sections. The rectangular blade is easy to manufacture and good enough for most cases. However, I have not designed a rotor myself and do not know enough about the details to be more specific.


What is that protruding fuselage part? Circled in red above, I suspect that's a high-lift device meant to improve handling at takeoffs and landings, but I don't know for sure really. What is its name and uses?

The strakes function as giant vortex generators.  They generate vortices at high AOAs to improve airflow over the rudder at low speeds.  During the Q400 test program it was found that there was some local flow separation at the lower area of the rudder (reducing rudder power and possibly creating unpleasant vibration) and this is a classic, relatively hassle-free, aerodynamic band-aid kind of add-on to address the problem without major redesign.  You don't see them on the shorter -8s.

Strakes and Vortex Generators work by creating a horizontal tornado so to speak, with the outer half of the tornado in the free stream.  This has the effect of pulling smooth free stream air into the turbulent air close to the surface, which, even though it's spiraling, is better than the random turbulent flow that is there otherwise.

You sometimes see similar strakes on the upper aft fuselage on light aircraft just ahead of the fin, like on a Chipmunk.  These are to improve spin characteristics by generating a vortice over the rudder when AOA is high as in a stall/spin.  They call them "spin strakes" in that case.


What is the meaning of a flashing white signalling light in Aldis Lamp? In my source, I couldn't understand the flashing white description.

I translate for you; in the last row: 'çakarlı beyaz: flashing white' says that: Leave the manoeuvering area in accordance with local instructions.

My question is: What is the meaning of local 'in accordance with local instructions' Is it meant that there were some instructions before electric power was gone and the previous instructions must be followed before leaving the manoeuvering area and returning to the apron?

The signal you are talking about relates to vehicles (flashing whites can also apply to aircraft, but then it has a different meaning).

Local instructions refers to the local written rules/procedures applicable at the specific airport. In order to be allowed to drive on the maneouvering area, the vehicle driver will have undergone training and have read the local procedures. Maybe the local instructions requires the driver to leave the maneouvering area via a specific route or in a certain direction. In such cases, that would be written in the local instructions. They are called local because they are only valid at that specific airport.


How do carbureted and fuel injected engines compare in high altitude? I couldn't find enough information on the internet for this topic. What I found can be paraphrased as:

  Carburetor means easy engine start while fuel injected means manual fuel start up and carburetors are cheaper.

So how do carbureted planes compare against the fuel injected ones for GA purposes especially in long distance cold high altitude flights?

I looked for the M-9 Series and the specs doesn't reveal much.

Disadvantages of fuel injection:

You need to do a business case.  It's possibly not worth the price.  The Bendix fuel injection system used on the Lyc is a fairly crude (by car standards) mechanical constant flow system, and the specific fuel consumption is perhaps 5-10% lower, maybe, at best, .4 lb/hp/hr vs .44 or so for a carb (cars are in the high .3s, diesels in the low .3s).  Lets be generous and say the fuel burn is 9 gal/hr instead of 10. At 100 hrs a year that's a $500-550 saving.  If you spend 5000 extra for the injected engine, it's 10 years just to break even.  Hopefully the fuel injected option is a lot cheaper than that, or you do enough flying to recoup the extra price much faster.

You prime by pressurizing the system with the fuel pump (using the mixture) to inject fuel pretty much where the primer would be on a carbureted engine; so far so good, but it's a somewhat more finicky procedure and easy to flood.

There's a little bit of a pucker factor issue because you have a high pressure fuel distribution system snaking around the engine (yes there are primer lines on a carb engine but they aren't pressurized) so there is a higher fire risk due to a fuel leak (it's the main reason carbureted engines are always updraft - nobody puts the carb on top).

When the engine is hot and sitting on hot day, you can get vapor lock in the distribution manifold and lines, sizzling away like a pan of bacon in the plenum of the cowling, making the engine really hard to start.  If you're on floats drifting away from a dock after pushing off and frantically trying to start the dang thing before you drift into the rocks, it becomes a big deal.

My personal opinion is the fuel burn reduction is not worth the costs, extra failure modes, and loss of simplicity and I prefer carburetors, for simple, low speed airplanes at least.


How do helicopters avoid detection from enemies by using IR suppressors? no IR suppressor

 IR suppressor

All I know is that it blends the hot air before releasing it How does an IR suppressor help an aircraft avoid detection from a missile? Wouldn’t the entire helicopter show up on the IR?

An IR sensor detects objects that are at a higher (or lower but that's not how missiles work) temperature than the surrounding environment, most of the helicopter won't be that much hotter or cooler than the air around it, so wouldn't distinguish it much unless you get very close. Jet engine exhaust is much hotter than the surrounding environment, so it shows up very clearly to an IR sensor from far away.

A suppressor diffuses hot exhaust into a larger air mass, lowering the temperature of the exhaust compared to the surrounding air before the sensor can see it. Lower temperature means less heat for a sensor to see.  A suppressor also makes the infra-red picture fuzzier, i.e. less of a distinct point. The lower temperature and fuzzier image decreases the effective range for a sensor, meaning it would have to be closer to detect it. Suppressors often deflect exhaust, pointing it away from where a sensor would be expected to read, or shield the exhaust.

Suppressors don't prevent an IR detection in the same way that stealth doesn't prevent radar returns, but a suppressor means you need to get closer.


Which aircraft manufacturers still include Vertical Situation Displays (VSD) feature in their cockpits? I'm curious about the aircraft manufacturer names who still interested in including the VSD on lower part of Nav Display (I read about Boeing 737 only). Are there any other guys on this filed still? I know this would be useful during the landing and takeoff.

VSD

Boeing have a patent on a VSD (Vertical Situation Display), which is part of the Honeywell EGPWS (Enhanced Ground Proximity Warning System) which first appeared as an option on 737s about twelve years ago I think.

  The system will be offered by early 2003 as a customer option on in-production 737s and by retrofit on 737-600/-700/-800/-900 airplanes already in service. Implementation of the system on other Boeing models is under consideration.

- Boeing "Aero" magazine

I don't know if Boeing offer licences at acceptable terms to their competitors, or if Honeywell make available, to other aircraft makers, products that incorporate features licensed from those patents.

CFIT

It is possible that other manufacturers have equivalent options for alerting pilots of potential CFIT (Controlled Flight Into Terrain).

TAWS & EGPWS

The FAA, for example, requires aircraft be fitted with systems that comply with TAWS (Terrain Awareness Warning Systems) requirements. EGPWS systems satisfy that requirement.

TAWS requires a terrain awareness display but I dont think it requires something specifically like Boeing's VSD.

Airbus for example, on at least their A350 and A380 models, have a navigation display that incorporates "vertical flightpath data". I guess they don't call it a VSD. They also have what I think is a fairly common type of terrain display showing "peaks" ahead of the aircraft.

- Sources: FlightGlobal and Airbus

RNP

It may be that some of the issues addressed by the VSD are, for many airports in mountainous terrain, taken care of by RNP (Required Navigation Performance) which prescribes the required navigational accuracy of the aircraft's navigation systems. Generally the aircraft is required to be able to follow an approach path that consists of a sequence of curved three-dimensional paths to the runway that avoids terrain. I imagine this path is pre-programmed into the navigation systems along with the go-around paths.


How do we choose a frequency range of interest for flutter prediction? I'm trying to perform some flutter and dynamical (gust etc.) calculations with Nastran. It has been told to use a range up to 30 Hz for an aircraft I'm working with, but why the upper limit is so choosen 30 Hz? Why for some aircraft the range is up 60 Hz and for another one even up to 100 Hz? I suppose it depends on matirials used in a particular structure and/or a frequency range of presumed loads. I would be grateful if someone can explain how the frequency range of interest is exactly chosen. Thanks!

You might want to try reviewing MIL-A-8870C  AIRPLANE STRENGTH AND RIGIDITY
VIBRATION, FLUTTER, AND DIVERGENCE.  The CRJs were certified to this standard.  Available here: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-A/MIL-A-8870C_6746/  Just click the Download File button.


Is overall pressure ratio same as compressor pressure ratio for a gas turbine engine? For a typical gas turbine engine, turbofan in particular, is the compressor pressure ratio same as the overall pressure ratio?
For modelling purposes can we use the same value of overall pressure ratio for compressor pressure ratio too?
If we do so, will we get accurate results for performance?

Modern turbofan engines typically have several stages of compression, the overall pressure ratio is the ratio of the highest pressure to the intake pressure. But the pressure ratios of each compressor can be a different.

The fan and booster add a bit of compression, followed by an optional intermediate pressure compressor and finally the high pressure compressor.

Because of these multiple shafts and therefor having some parts of the compressors not working on their design point you probably won't get accurate results by simulating a single stage compressor.


How can the CVR from a Cessna 560XL only contain old records and none from the fatal accident? Is it possible that the CVR from a Cessna 560XL is just showing old records and nothing from the fatal accident when the plane crashed?

I´m talking about the airplane crash in Brazil, where a election candidate was killed. The investigation team said that the data in the CVR contained only old data and they could not pinpoint from when those recordings were.

If the fuse was pulled between the previous flight and the crash then yes, the CVR can only record when it has power

Pulling the fuse or going behind the panel and cutting the wire will prevent it from recording.


Is my understanding of magnetic variation and declination (for a VOR) correct? AIXM (the XML-based exchange model for AIP by Eurocontrol and european AIS) defines the following properties for a VOR:

codeTypeNorthA code indicating the direction of the "zero bearing" provided by the station, values: "TRUE" (true north) or "MAG" (magnetic north).
valDeclinationThe angular difference between the direction of the "zero bearing" indicated by the station and the direction of the True North.
valMagVarThe local angular difference between the True North and the Magnetic North.
dateMagVarThe year the magnetic variation was measured.

Now say we have a VOR with the following values:

<codeTypeNorth>MAG</codeTypeNorth>
<valMagVar>0.27</valMagVar>
<dateMagVar>2018</dateMagVar>

Am I correct with the following statements?

valMagVar is a positive number, therefore magnetic north is east of true north.
codeTypeNorth is "MAG", therefore the VOR is aligned towards the magnetic north. Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

Please don't comment on the practical relevance of this question. I'm developing and documenting OFMX (a derivative of AIXM) we will use as our future well-defined exchange format for https://openflightmaps.org and I want to be sure to be 100% sure I explain the terms "magnetic variation" and "magnetic declination" accurately in the OFMX documentation.

Thanks a lot for your help!

Since the magnetic variation is +0.27°, the "zero bearing" is equally +0.27° east of true north and therefore the (undeclared) valDeclination must be +0.27° as well.

You are assuming that the VOR alignment will always be kept bang-up-to-date with the actual measured magnetic variation. Sadly, this is not the case! There is actually a considerable amount of work involved in re-aligning a VOR (it is not just a matter of tweaking a knob!) and re-alignment is only performed rather rarely.

Also note that the terms 'variation' and 'declination' are NOT synonymous in this context - 'variation' refers to the actual measured difference between true north and magnetic north; 'declination' refers to the difference between VOR zero-bearing and true north. These are NOT generally exactly the same.

For example: UK AIP ENR 4.1 (PDF)


Do ATC displays show the target altitude that the pilot has set in the autopilot? When cleared to climb/descend to a level, the pilot enters that level in an altitude preset window, allowing the autopilot to climb/descend and level off at that level. Is this level, preset by the pilot, visible on an ATC radar screen?

Yes, our radar is indeed capable to display data of the Mode S Enhanced Surveillance Transponder like Selected altitude, IAS, Mach Number, Heading and more.
After issuing a climb/descend clearance and entering it into the sytem it would also warn us if the selected altitude is different from the cleared level.


Can a plane fly backward if its propeller is in reverse? I would like to know if a prop plane, for example the P-51, could be able to fly backward. Is it possible through some trick like changing blade angle or reverse the prob rotation direction? It would have been hard for piloting, but is this theoretically possible?

Fly? No.
Wings generally only produce enough lift to keep an aircraft aloft when air is flowing over them in their intended direction. If you reverse the airflow over the wing (by moving backwards in the air, for example) the wing would no longer be producing the necessary lift, and the aircraft would "descend at an extremely rapid rate" until a normal airflow over the wings was restored. (That's the polite way of saying "the darn thing falls out of the sky like a rock!")

Move? Yes. At least on the ground.
This has been demonstrated on more than one occasion by Fat Albert, the C-130 that supports the Blue Angels.  While the propellers themselves do not reverse, the blade pitch is changed to produce reverse thrust (slowing the aircraft the same way thrust reversers do on a jet engine, and in this case reversing its direction of travel).

Note that there is one notable exception to the "airplanes can't fly backwards" rule, which the article Ethan linked to alludes to: If the wind velocity exceeds the aircraft's stall speed it's possible for an aircraft to "fly" at its minimum airspeed, but be moving backwards relative to the ground.
This is a bit of a cheat though: The airplane still thinks it's flying forward (the relative velocity of the wind over the wings is in the "normal" direction, and the airspeed is fast enough to create enough lift to sustain flight). It just happens the the airspeed includes a headwind component sufficient to give the aircraft a net "negative" groundspeed.
The hypothetical P-51 in your question would require sustained winds of about 83 knots to make this trick work (we generally call that a hurricane), but something like a Piper Cub can do it at much more reasonable wind speeds.


What are the general problems faced for maintenance in fuel inerting systems? Currently most of the nitrogen generation systems have Air Seperation Modules (ASM) and other associated mechanical components to generate Nitrogen Enriched Air.

Hence, are there any inherent problems with the system in terms of maintenance, weight or the controller?

As far as commercial airliners go, the inerting system is relatively simple and lightweight; see the below diagram from Airbus for a relatively representative architecture (most systems also have some kind of compressor to inert the tanks more quickly in certain situations).

With a total weight of just a few hundred pounds and the ability for most of the architecture to be colocated near the fuel tanks, the inerting system is far less complex and heavy than, say, the air-conditioning system. The air separation module (ASM) is practically the only unique component; most other maintenance issues with inerting systems are similar to those of other systems: mechanical/thermal fatigue and failure, software bugs, accidental damage, etc.

However, there have been some issues with the reliability of the ASMs, which is currently the biggest concern of aircraft operators. I can't find any public information on why so many ASMs are failing at half their predicted service life, but I imagine it has something to do with their being a relatively new mechanical technology on commercial aircraft.


In which countries a medically unfit US pilot can fly? For example can one get another license in a foreign country?

Which countries allow that?

It completely depends on the condition. Just because you're denied a FAA medical it doesn't automatically mean you can't get one to operate in another country.

For example, colour-blind pilots can fly up to First Officer level in Australia, and we have no mandatory retirement age. But we are stricter on things like head injuries.


What is the cabin noise hertz in typical commercial aircrafts? In cabin, we hear noises in commercial aircrafts. Wondering if it is in general over or under 100 hz. There should be a band width. Thanks!

In a jet it's going to be pretty much all above 100hz. Most of the noise at cruise is broad spectrum "white noise" from the air flow and the engine core exhaust.  The fan however, running at, typically 4000 rpm, with 60 blades say, will be producing a 4000 Hz noise, and this along with the exhaust white noise will be the dominant sound prior to cruise.
In a propeller driven aircraft however, you will have noise below 100 hz from the prop.  A 2 blade propeller spinning at 2200 RPM is imparting a 73Hz pressure wave pulse on the windshield and this is one of the dominant forms of noise in that case.  Next is the exhaust pulse from a 4 cyl engine, 2 per revolution, also giving 73 Hz at 2200 RPM.  Then random frequency mechanical noise from the engine and white noise from the airflow.
So in a Cessna 172 at take off, running at 2700 rpm, you're going to have mostly a 90 Hz sound from the prop and the engine, then some random noise from other mechanical noise from the engine plus airstream white noise.
This is a problem for pilots who use foam earplugs only when flying.  Foam plugs only attenuate sound above maybe 5000 Hz and don't help all that much with the prop and engine noise. ANR headsets are a huge help here because ANR systems work best on low frequencies although not so good on high frequencies.
So when I'm flying I use both an ANR headset and ear plugs.
After I tried out a set of  Isolate Earplugs (and promptly lost them, although they do work), I've also developed my own low frequency earplugs by wrapping about 3" of half inch lead tape (used for ballasting golf clubs) around the shaft of a 3M Push-In earplug (the kind with the little yellow bulb on a blue plastic shaft) and covering the tape wrap with heat shrink tubing.  I put them in with a bit of lanolin on the foam bit to lubricate and improve the seal.  They do a fantastic job of attenuating low frequencies below 200 Hz, and make the ANR headset nearly redundant.


Where are jack points typically located on an aircraft? I realised that full gear swing tests require an aircraft to be fully jacked up; and I think it would make sense to locate the jack points close to the landing gears themselves, on the same load-bearing structures (e.g., main wing spar), since those structures are already designed to support the weight of the aircraft.

Is that generally the case, and are there considerations where designers would locate the jack points elsewhere (and presumably have to specially reinforce these other locations)?

Yes indeed, at strong points such as landing gear structure, engine support, or wing strut. The jacks need to be out of the way of the swinging doors of course, but as close as possible to the fuselage.

This one has a special design man leg jack.


Why is the alternator connected only to one electrical bus in the C152? I am a noob in electrics so pardon my ignorance. What I see from an electrical system of C152 (pasted below): there are two busses, connected to each other, but alternator is fed only to the first bus. Does it mean that each component connected to the second bus takes energy from the battery and due to that battery is constatly discharging and alternator must additionally load the battery? If that is true (correct me if I'm wrong please) why don't connect alternator to both busses?

There's a direct connection from the primary bus to the other bus, so they are essentially one. If you have a wire connecting the two, the current will flow from one to the other.


As of June 2019, can a DPE in the United States renew a CFI's certificate based on completion of a FIRC? Flight instructors need to renew their CFI certificates every 24 months. One of the ways to do that is by completing a FIRC. In the past, you had to take your completed FIRC to the local FSDO and have an inspector approve it.

Lately, I've heard rumors that FSDOs are so overworked that they're not doing CFI renewals any more, and are sending them to DPEs instead.

Is this rumor true? Can I take a completed FIRC to a DPE in June 2019 and re-up my certificate?

Yes they can
I asked a DPE to renew mine a couple days ago and they did.  Sometimes asking nicely is the best way to learn.


Is landing the 'wrong way' possible on an aircraft carrier? I wonder if the arresting wires would work the 'other' way?

You're going to run into a few issues.

Below is a diagram of a typical aircraft carrier layout. Normally aircraft would land from right to left. The four cables are located closer to the right end than the left. The extra length after the cables is both to allow a "bolter" (YouTube) time to accelerate in case of missing the arresting hook; and to allow an aircraft that caught a cable time to decelerate.

Notice how far an aircraft can continue down the deck (YouTube) after catching the cable. Even if you catch the first cable, you might go off the aft end of the ship.

Another issue is the way that the arrestor cables work. The housing where the cable enters the flight deck is curved to allow the landing aircraft to pull on the cable as it decelerates. The cable is not designed to be pulled in the opposite direction, which could cause it to break (YouTube).

Aside from these issues, you would also lack the FLOLS system that provides physical guidance during the landing. And as voretaq7 also mentioned, you have to either touch down late, or catch the cable late, both of which could pose problems.


Why do some canard aircraft have dihedral on their canards? For instance, Rutan's Solitaire and Steve Wright's Stagger-EZ. What are the benefits of having a dihedral on the canard?
It seems like the canard dihedral increases yaw instability, and requires a bigger vertical stabilizing surface. Yet there are flying GA airplanes and sailplanes using canards with dihedral, and they've surely thought of this. What's the reason for that?
(My opinion-based hypothesis is that it's about very stalled AoA behavior, where canard dihedral may naturally slow down some flat spin, and help with recovering from it; still, I cannot find any references about this, and it may also be a very wrong statement as long as it's opinion-based.)

(Rutan's Solitaire)

(Wright's Stagger-EZ)

(Saab Gripen)
Edit:
Gripen also has dihedral on canard surface, however since those are full moving surfaces with high sweep angle, depending on the angle of attack, those surface viewed from the front, display either dihedral, no dihedral, or anhedral (if very positive canard AoA)

If it is a canard, the assumption of a well thought out design is doubtful. Yes, dihedral will add to directional instability and require a bigger tail, but at least in case of the glider the vertical is sized for countering adverse yaw, so the bit of canard instability is easily compensated.

My direct response would be: To lift the core of the canard wake above the wing. But from looking at the Solitaire I wonder whether that is really true. At least on the Stagger EZ you can see that the designer tried his best to separate canard and main wing vertically.

Regarding flat spins: The spin axis normally runs through the forward fuselage, so there is not much horizontal airspeed at the position of the canard. The tail will help to dampen the yawing moment much more. A bit of canard dihedral will not make a difference.

But maybe wing dihedral was found to be lacking during flight test and changing dihedral on the canard seemed to be an easier fix? That would be my best guess, at least for the Solitaire.


How many staff are needed to support aviation on an aircraft carrier? An aircraft carrier is the largest type of warship and is used to transport, operate, and maintain military aircraft.

The staff which support aircraft in the carrier can be divided into 2 groups.

Some "variable" staff are directly related to the aircraft operation like Pilots, the number of those staff is proportion to the number of aircraft.

On the other hand the number of the "fixed" staff is fixed, regardless the number of aircraft, like Air Traffic Control.

How many of the 2 different types of staffs are employed? To simplify the answer, please use the biggest Nimitz-class as example.

The ship's complement for a Nimitz class carrier is 3200. That is the crew required to maintain ship operations.

A further 2480 crew comprise the Air Wing - everyone needed to perform duties directly associated with flying activities, including maintenance, ordnance handling, emergencies, etc. It's treated as a separate entity because any one of ten Air Wings could be embarked on a carrier at any given time.

That is a total crew of 5680.

I couldn't find a breakdown of specific trades or ranks for either entity.

Source: Wikipedia

Note that the newer Gerald R. Ford class carriers make extensive use of automation, and so have several hundred fewer crew members in the ship's complement.


What property of a wing allows it to flex? Is it the material used that allows the wing to flex or is wing flex caused by the structure of the wing?

The flexing of a structure like an aircraft's wing is generally due to the materials used, but is also related to the nature of the structure itself.

In very simplified form, a wing is a beam that is supported at one end (the fuselage). Any forces such as from aerodynamics that act on the beam will cause it to bend.

All materials have a characteristic way in which they behave when load is applied. This can be expressed as stress (from applied forces) vs. strain (deformation of the material). Once the material reaches the yield strength point, it will be permanently deformed. For stress below this point, though, the material will return to its original shape. The wing can then flex as long as it remains below the yeild strength in all required loads.

See here for more about this graph

Of course there is much more detail to the subject. The whole structure is designed together so that it will be as light as possible, but the amount of flexing is limited to prevent related issues. The wing may need to be made more stiff to prevent flutter. Farhan mentions fatigue, which can also limit the amount the wing may flex. The panels on top and bottom of the wing must also be prevented from breaking or permanently buckling. There are also fuel tank seals that should not be broken. Adding material in the right places can help to spread out the loads, which reduces the stress on the material and therefore reduces the amount of flexing.


How can an aircraft owner/operator track unauthorized flights? Is there a product or some other way to track an aircraft's use, even if the pilot doesn't file an IFR flight plan, to let the owner know if it's used without authorization?

The owner could do regular checks of the aircraft's Hobbs meter, and if the times don't match what the log is saying, then there could be some red flags being thrown.

If someone it out to deceive, I'm sure they could get around this, but maybe then, the problem is giving access of expensive equipment to people who aren't trustworthy.

There are GPS tracking systems that could keep a movement log in memory.


Where can I get a copy of my notice of disapproval from a failed checkride? I can't find the notice of disapproval that I was given after I failed the oral portion of my private pilot checkride. I have looked EVERYWHERE.  What do I need to do?  Is there something on IACRA or can I go back to the examiner?

I am fairly close to taking the checkride again (I switched instructors), and I am under the impression that I need to take that paper with me.

Any suggestions?

As far as I'm aware you can't pull the "completed" application from IACRA (with the examiner's notice of disapproval & failed areas on it). Much like with your knowledge test report you need to hang on to that original bit of paper.

I don't believe you NEED to bring the Notice Of Disapproval with you to a retest though.
61.43 says, in part:

  f) If a practical test is discontinued, the applicant is entitled credit for those areas of operation that were passed, but only if the applicant:
  (1) Passes the remainder of the practical test within the 60-day period after the date the practical test was discontinued;
  (2) Presents to the examiner for the retest the original notice of disapproval form or the letter of discontinuance form, as appropriate;
  (3) Satisfactorily accomplishes any additional training needed and obtains the appropriate instructor endorsements, if additional training is required; and
  (4) Presents to the examiner for the retest a properly completed and signed application.

So you only need it if you want to skip over the parts of the test you passed. If you don't have it, or your retest is more than 60 days after your original failed checkride, the examiner will have to give you a full exam, with no credit for the stuff you already completed.

Given that you failed the oral there's probably not much "credit for those areas of operation that were passed" you can claim, so it probably doesn't matter much that you lost the original notice of disapproval. You're almost certainly getting a "full checkride" anyway.


How should I request flight following when I'm not taking a direct route to my destination? I want to plan a flight from KMSN - KDAY. My plan avoids the ORD airspace as much as possible, so something like:

KMSN - JVL - KARR - JOT - EON - KRZL - OKK - MIE - KDAY

What would be the phraseology to use to get VFR flight following to let ATC that I'm not planning the most direct route?

I would make the request as:

(controller) "N12345, Say your request"
(you) "N12345 request VFR Flight Following from Madison via the Joliet VOR to Cox Dayton; Altitude 5,500"

I don't think all the intermediate points matter, and I think the controller will quickly figure out what you intend.

They may ask for some clarifications, such as:

"Say your equipment"(they're asking if you have a Mode C transponder)
"Spell waypoints"(they want the phonetic alphabet of your waypoints because they are unsure)


Is TCAS II required in the US? I would like to know if, in addition to any MEL requirements, there is any regulation or requirement about having an operational TCAS II while operating in US airspace?

TCAS II is required for most airline flights. This is not a requirement for certification, and the TCAS may or may not be on the MEL. However, most flights under part 121 (scheduled airline service, as well as some freight) require TCAS II.

As noted in FAR 121.356, "Turbine-powered airplane of more than 33,000 pounds maximum certificated takeoff weight" must be operated with equipment that meets a list of standards, but is essentially equivalent to just requiring TCAS II. Smaller aircraft with 10-30 seats can use TCAS-I.


Where is the only Aerostar 800 ever built? The Ted Smith Aerostar has a very interesting history. Wikipedia has a list of all the models that were built. There's a note there about an Aerostar 800, only one of which was ever built. Does anyone know where that 800 is today?

According to this page the airframe was donated to Embry-Riddle Aeronautical University:

  The Superstar airframe was later used as the prototype Aerostar 800. The prototype 800 was the Superstar airframe with two more seats (due to a 32” extension of the cabin) and featured two 400-hp. Lycoming engines. The 800 was under development when Ted unexpectedly passed on in 1976. Piper continued development of the airplane as a replacement for their P Navajo but never certified it and eventually donated the airframe to Emery Riddle [sic] Aeronautical University.

This is the only reference I can find to the ultimate location of the Aerostar 800.


Do GA pilots routinely request "flight following" from ATC, and why? My understanding is that private pilots are able to contact air traffic control and request "flight following", as it may help them with their situational awareness. Beyond that, however, I'm curious:

Is this a common procedure? What sort of "situational awareness" benefits can be realized by a "flight following" request? What are other pros/cons of such a request?

I typically like to get VFR flight following if I haven’t filed a flight plan for longer distance flights. The biggest advantage here is that somebody knows where I am if something should happen.

I don’t typically find the chatter too bad, but you do have to be on the ball, especially when handed off from a local airport to an Air Route Traffic Control Center (commonly referred to as just “Center”) frequency. Often on flights out of my airport (KGRB) I will get handed off to Minneapolis Center, which is relatively busy frequency.

Another nice thing is that it gives you some practice talking in busy airspace. You really have to follow the rules and jump in when able. Sometimes they won’t hear you because you are transmitting over other aircraft that you can’t hear, so the frequency will appear open. It’ll sound like they are talking to themselves.

So here is how I see it:

Pros

You are talking to somebody and working with other traffic in the airspace
You are on a frequency you can broadcast an emergency to. Yes we all know about 121.5, but you are not guaranteed to get somebody on the other end.
ATC knows where you are, and if you fall off the radar and don’t respond, they will initiate a search. If you just file a VFR flight plan they won’t even start until 30 minutes after your planned arrival, and then they start calling airports and contact numbers, then start a search. They may not know where you fell off your flight plan so the search area is much bigger.
They can help route you around active MOAs, TFRs, and other airspace/weather issues that don’t show up well on paper maps.
Traffic advisories (as others pointed out)
Can provide a vector to where you want if you get lost.
Don’t need to open a flight plan if you can get following all the way.
Good radio practice

Cons

Flight following is “workload permitting.” They may call up and say Radar services terminated; squawk VFR at any time, turning your flight back into normal VFR. If you didn’t file a flight plan, now you should call up FSS and open one en route, which is more difficult.
Requires you to be on-the-ball with your radio transmissions on busier frequencies. If you frequently fly from uncontrolled airspace, this can be intimidating.
You may get vectored around a bit, but I usually don’t have that problem. I’ve typically contacted the destination airport by 20 miles out and either got clearance or vectors anyway.


I have bad eyesight, can I still be a commercial pilot in Malaysia? So, I have really bad eyesight (short-sighted): without my glasses, I can't read text unless it's near. So can I be a commercial pilot?

According to the Civil Aviation Authority of Malaysia's medical standards:

  (c) Distant visual acuity, with or without correction, shall be:
  (1)
  in the case of Class 1 medical certificates, 6/9 (0,7) or better in
  each eye separately and visual acuity with both eyes shall be 6/6
  (1,0) or better;
  (2) in the case of Class 2 medical certificates, 6/12
  (0,5) or better in each eye separately and visual acuity with both
  eyes shall be 6/9 (0,7) or better. An applicant with substandard
  vision in one eye may be assessed as fit in consultation with the DCA
  subject to satisfactory ophthalmic assessment;
  (3) applicants for an
  initial Class 1 medical certificate with substandard vision in one eye
  shall be assessed as unfit. At revalidation, applicants with acquired
  substandard vision in one eye shall be referred to the DCA and may be
  assessed as fit if it is unlikely to interfere with safe exercise of
  the license held.

The key words appear to be "with or without correction", i.e. it's completely fine if you wear glasses to correct your vision. That seems to be standard around the world, e.g. the US standards also allow correction.


How has FAR 61.57 changed since 1969? An answer to another question on Aviation.SE asks about the historical development of FAR 61.57.

How has FAR 61.57 changed over the years?

Glad you asked!

I referenced historical FARs from 2013, 1993, 1988, and 1969.  Here's what I found:


L/D -Bell vs Eliptical spanload? look at this   video from 18:00-21:00 ,Al Bowers talk about benefits of bell spanload compare to classic eliptical..Can someone explain what does it mean in practice examples when design aircrafts:

1)So if we have plane with 3000kg and we must have best L/D  and if wingspan  is not constrain,then we must choose bell spanload so our new wing will be 22%longer and it will has 11% smaller induced drag?

2)Why Al Bowers  talk like airplane overall weight and wing bending moment are  same thing?If some plane have bigger wing bending moment,that dont neccesery mean that it has larger weight than plane with lower wing bending moment...

3)Why all gliders use eliptical spanload if bell spanload has better L/D?

4)What do you think,when will aircraft  industry  switch to the bell spanload solution(if ever)?

1)So if we have plane with 3000kg and we must have best L/D and if wingspan is not constrain,then we must choose bell span-load so our new wing will be 22% longer and it will have 11% smaller induced drag?

All other parameters except span held constant this is true.

  2)Why Al Bowers talk like weight and wing bending moment are same thing?If some plane have bigger wing bending moment,that dont neccesery mean that it has larger weight than plane with lower wing bending moment...

Wing root bending moment corelates to the load factor N * weight. Also spar cap sizing is done with REF to the the root bending moment which in turn drive the wing weight.

  3)Why all gliders use elliptical span-load if bell span-load has better L/D?

To be precise, all gliders do not use elliptical span-load. However, it's not wrong to say all gliders tend to use span-load distributions which are very close to elliptical loading. What elliptical span-load gives is the lease induced drag per given span. Say you start with an airplane with a given span and increase the span by 22%. Yes, you will get 11% decrease in induced drag but if you use elliptical loading on that increased span, your induced drag will be even smaller. (32% decrease)

  4)What do you think,when will aircraft industry switch to the bell span-load solution(if ever)?

I am not qualified and/or knowledgeable to answer this.
Just for the info, there are many more viable and attractive solutions available with lower risks involved, such as the ones investigated in the NASA N+3 program.


Does training adolescents require a different approach with regard to the level of neurological development? There is some evidence of brain under development in adolescents, those from 13 through as high as 23 or 24, that is linked to judgment, decision making, risk appetite, etc., mostly in an unfavourable way as it relates to properly evaluating a flight.

See:  http://www.nimh.nih.gov/health/publications/the-teen-brain-still-under-construction/index.shtml

Does this suggest that CFIs training an adolescent should approach the training differently than how they train adults?  Are additional training and scrutiny indicated in order to minimize the effects of an under developed brain in this population group?  Or does doing so somehow cause some level of unwarranted discriminating practices to an adolescent training for his pilot certificates?

No, that article doesn't suggest a different training approach with teenagers.

What it does suggest is that the teenage brain has potentially more neuroplasticity, and that "the capacity of a person to learn will never be greater than during adolescence." It does point out a possible factor to watch out for in young trainees: sleep deprivation, but it doesn't say that the effects of sleep deprivation are different than in older trainees, only that they may be more likely to stay up late.

The article mentions "the importance of creating an environment in which teens can explore and experiment while helping them avoid behavior that is destructive to themselves and others". Flight instruction seems to me to be exactly such an environment.

Instructors already should be adjusting their training approach to the individual. (Transport Canada - Flight Instructor Guide: Individual Differences). The advice given is based on symptoms, and there is no indication that a teenage slow learner (for example) should be treated differently than an adult slow learner. There is a column in that guide relating to "immature" trainees, which might apply more frequently to adolescents, but unless a student is showing immaturity, there is no need to treat them so.


Why is the relation of height and temperature different when calculating altitude? I'm trying to grasp the essence of altitude calculations. One thing I can't figure out in my head is this:

In the International Standard Atmosphere (ISA) the temperature of the air decreases with the altitude, which is called the lapse rate. This is an approximate of 1.98 C (degrees Celsius) or an even more approximate 2 C per 1000 feet. This is usually described through dry adiabatic lapse which means simply that when you go up the pressure rises and the air expands thus through the ideal gas equation it makes sense for the temperature to drop as well.

However when calculating the density altitude we need to "correct" the pressure altitude with this formula:

DA = PA + (118.8 ft/C) x (OAT - ISA temperature)

(from https://en.wikipedia.org/wiki/Density_altitude#Calculation)
Here the height change per degree Celsius is only 118.8 feet (approx 120 ft).

My question is this: Why is the relation of (change rate) height and temperature different? With just "going up" it changes 2 C / 1000 feet (ie 500 ft/C) but when adjusting for temperature it changes 120 ft/C).

These both are simple linear equations but have different coefficients.

The equation is saying, that for every degree difference from standard temperature, the difference between pressure and density altitude grows by 118.8 ft. A completely different factor from the lapse rate.


What does this black triangle placard on board an A320 denote? Just out of interest whilst travelling aboard a British Airways A320-232 G-MIDT on the 9.45 BA shuttle service (LGW-GLA) in seat 10F I noticed a placard above the window  of seat 9F. Given this seat was two rows forward of an overwing exit I was wondering if it was related in any way.

If there is a problem on-board and the crew is unsure about the condition of the aircraft, the triangle indicates what window should be used to inspect flaps, slats or engines or look out for icing.

This saves important time in case of an emergency, as the crew doesn't has to search the right window to look out.

For example if there is a failure of the flaps condition display, it has to be verified visually that the flaps are extended properly. From the window with the triangle, this can be seen:

Source, added square

Now the crew can compare the position of the flaps switch in the cockpit, with the actual extension of the flaps.

There are usually four of these triangles in the cabin. One to inspect the forward of the wing for icing, the slats and the engine on each side and one at the back of the wing for the flaps.

I have searched for cabin pictures and found this 360° cabin view by SAS.

The following picture shows the triangle on an Airbus A330 at the front of the wing. You can see a part of the engine through the window.

Source, added squares

And this picture shows the rear triangle.

Source, added squares


What instruments or avionics do balloons usually have? For normal sightseeing flights in a typical hot air balloon, what instruments or avionics does the pilot usually carry? Do balloons have a minimum equipment list?

The magic regulation for minimum equipment on a balloon (in the USA) is 14CFR31.85 ("Required Basic Equipment" for manned free-air balloons).

The FAA requires that balloons be equipped with:

For all balloons

  [Reserved]
  An altimeter.
  A rate of climb indicator.

For hot air balloons (additional equipment):

  A fuel quantity gauge.
  An envelope temperature indicator.

For captive gas balloons (additional equipment):

  A compass.

Beyond what's required by regulation, in most free balloon operations I've seen they also carry an aviation radio and/or a CB radio (to communicate with their ground crew) - this is usually a handheld transceiver.

The FAA Balloon Flying Handbook also makes for interesting reading. They talk about required and optional instruments and equipment found on various types of balloons in Chapter 2.


What is the largest plane ever to have been stolen by a single pilot? This guy stole a few small planes like two c172s, a 182 and a Columbia 400.

Also, in 2003 two persons stole a Boeing 727.
Has there been other cases like this one where someone stole a large aircraft? If so, what is the largest aircraft to ever been stolen, preferably by a single person?

(To clarify, I'm doing some research for a story I'm writing. I don't plan to beat the record, rest assured.)

Probably the one you stated in your question.  Though, I should note, that there is some credible evidence that only one person was actually flying the plane and the other may have been forced into coming along.  So you could probably consider it to have been stolen by one person, and that person appears to have not been remotely qualified to fly it.  When it was leaving the plane seemed to meander down the taxiway to the runway, like the pilot had no idea what they were doing, and then it took off.  It hasn't been seen since, and most credible theories think it crashed in the ocean.  But conspiracy theories abound.

A close second would probably be this ATR-42-320(wikipedia plane reference), also stolen in Africa.  This time by an employee of the company who was, unfortunately, suicidal.  He flew the plane around the airport for a few minutes demanding to speak to everyone from his boss, to his significant other to the president of the country.  He eventually crashed it into a couple of other ATR's the company owned.

In reference to the "I'm writing a story" line.  I would suggest that a 727 probably isn't the optimal plane for a single person to steal.  They're older and difficult to manage on your own.  I think a much better option would be a more modern aircraft, like an Airbus A350 or a Boeing 787.  They have avionics that allow for one person to reasonably fly the plane.

Granted, they are very new and may be harder to steal.  Probably the sweet spot between "new enough to be flyable by one person" and "old enough to have one just sitting around to be stolen" would be something like a 737-800 or an Airbus A320.  Both are still quite large, but a skilled pilot could probably operate one on their own if they needed to.  In fact, it has happened before.


How many actuators does the F-35 have for each primary control surface? How many actuators does the F-35 have for each primary control surface like rudder, stabilator or aileron? And is it active/active or active/standby?

Just one per control surface.  Though the airplane is unique with using a single Electro-Hydrostatic Actuator (EHA) per control surface as opposed to a centralized hydraulics system.

https://www.f35.com/in-depth/detail/up-in-the-air-understanding-the-f-35-air-vehicle

I don’t know what you mean by active/standby but the surface actuators are commanded by inputs from the flight control computers, which respond as needed to stick and rudder inputs from the pilot to achieve a desired aircraft reaction to a control input.


What is the difference between a rating and a certificate? I did some research on what it would take to become a private pilot. I am totally green to the aviation world, so bear with me on my question.

I see the licenses also referred to as certificates but I also see the term "rating" listed on many things like "twin-engine", "turbo-prop", etc.

So what is a "rating" vs a "certificate"? Am I over-thinking this?

I'm referring to US regulations.

@CrossRoads is sort of correct, but is off on the details. The comments cover some of it, and I’d like to clarify and add some detail.

For purposes of airmen certification aircraft are divided into Categories and within Categories there are classes.

14 CFR §61.5 Certificates and ratings issued under this part.

  (1) Aircraft category ratings—(i) Airplane.(ii) Rotorcraft.(iii)
  Glider.(iv) Lighter-than-air.(v) Powered-lift.(vi) Powered
  parachute.(vii) Weight-shift-control aircraft.

  (2) Airplane class ratings—(i) Single-engine land.(ii) Multiengine
  land.(iii) Single-engine sea.(iv) Multiengine sea.(3) Rotorcraft class
  ratings—(i) Helicopter.(ii) Gyroplane.

A typical student pilot will train in an Airplane with a single engine that is equipped with tires—not floats. When pass their checkride they get their certificate with a Private Pilot, Single-Engine Land rating.

Once they have their certificate, they may begin to fly different airplanes that need endorsements in their logbooks. CFR §61.31 covers these. You would need an endorsement in your logbook to act as PIC for an  airplane with over 200 horsepower; a complex airplane—controllable pitch, flaps, and retractable gear; or with a tailwheel.

A pilot may also get additional ratings to fly multi-engine aircraft, seaplanes, etc. Note that ratings require additional training and testing by a DPE while endorsements require training and logbook endorsements by a CFI.

A type rating is required by §61.31  for Large aircraft (I think this means over 12,500 lbs), turbine-powered aircraft, and other aircraft specified by the FAA.

Earlier I mentioned that when you pass your checkride you will have a Private Pilot rating. There are several other ratings that apply to your certificate, starting with your Student Pilot rating.

14 CFR §61.5   Certificates and ratings issued under this part.(a) The following certificates are issued under this part to an applicant who satisfactorily accomplishes the training and certification requirements for the certificate sought:
(1) Pilot certificates—
(i) Student pilot.
(ii) Sport pilot.
(iii) Recreational pilot.
(iv) Private pilot.
(v) Commercial pilot.
(vi) Airline transport pilot.
(2) Flight instructor certificates.
(3) Ground instructor certificates.

Sport pilot and Recreational pilot certificates have some limitations on the type of aircraft you can fly and require logbook endorsements for some privileges (e.g. flying in Class B for recreational pilot).

The instrument rating (§61.65   Instrument rating requirements.) is kind of odd in that it isn’t listed in the types of certificates, but it is an add-on to your pilot certificate. So a commercial pilot usually has an instrument rating and a CFI must have an instrument rating in order to qualify as a CFI.

Completely separate from all of this, aircraft themselves are classified as well.

Certification of Aircraft is covered by PART 23–AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES Among other things it sets out the four types of aircraft that are normally flown in Part 91 operations.

14 CFR §23.3 Airplane categories.
(a) The normal category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for nonacrobatic operation. Nonacrobatic operation includes:

(1) Any maneuver incident to normal flying;
(2) Stalls (except whip stalls); and
(3) Lazy eights, chandelles, and steep turns, in which the angle of bank is not more than 60 degrees.
(b) The utility category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for limited acrobatic operation. Airplanes certificated in the utility category may be used in any of the operations covered under paragraph (a) of this section and in limited acrobatic operations. Limited acrobatic operation includes:

(1) Spins (if approved for the particular type of airplane); and
(2) Lazy eights, chandelles, and steep turns, or similar maneuvers, in which the angle of bank is more than 60 degrees but not more than 90 degrees.
(c) The acrobatic category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for use without restrictions, other than those shown to be necessary as a result of required flight tests.
(d) The commuter category is limited to propeller-driven, multiengine airplanes that have a seating configuration, excluding pilot seats, of 19 or less, and a maximum certificated takeoff weight of 19,000 pounds or less. The commuter category operation is limited to any maneuver incident to normal flying, stalls (except whip stalls), and steep turns, in which the angle of bank is not more than 60 degrees.
(e) Except for commuter category, airplanes may be type certificated in more than one category if the requirements of each requested category are met.


Definition of aircraft longitudinal axis? I know that longitudinal  axis connect nose and tail, but at what exactly point?
What definition says?
I have a glider and I want to draw with pen longitudinal axis to measure angle of incidence  for wing and stabilizer, but I don't know where exactly I need to draw longitudinal axis.

Is airflow parallel with longitudinal axis during cruise, for well design real aircraft(to reduce fuselage pressure drag)?

Aircraft designers use many different axis frames, depending on what is the most convenient under the circumstances.
In the context of the question, I need to mention only two of them, or rather even - two classes of them: body axes and wind axes.
The body frame is tied, well, to the body. It is rigidly fixed to the body and moves with it. By convention, the X axis points "forward", to the nose.
The wind frame is aligned with the streamflow "at the infinity", and its X axis points against the free stream. If you rotate the wind frame by the wind angles - angle of sideslip and angle of attack - you get to the body frame.
The term "longitudinal axis" refers specifically to the body X axis.
But where is the origin? It depends!
For most calculations, especially in flight mechanics, it is best to put the origin at the centre of mass. Furthermore, the direction of the longitudinal axis, over which the designer has some liberty (that arbitrary "forward"), should be such that it formed (one of) the principal axis of rotation.
This is not always easy, nor necessarily convenient. Centre of mass moves, the aircraft may not be symmetric, and the airframe designer will not like the reference axes moving about.
To avoid ambiguity, it is often necessary to state in which system the calculation is done, and how exactly it is defined. Oftentimes, engineers in a certain sub-field (e.g. flight mechanics) assume a certain frame and understand each other well, but this assumed knowledge is not carried to other fields, leading to errors and misunderstandings.
The situation is exacerbated by the fact that in many practical cases, the difference between the frames seems to be small, and the result may be "approximately correct" in most of them.1
So, as a designer, you decide how you define the longitudinal axis. If the aircraft is relatively symmetric, draw it as the axis of symmetry.
Your choice will affect how you read angle of attack (AoA). There is nothing strange about that: AoA of the aircraft is also "conventional": it is with respect to an essentially arbitrary axis. It will likely be different to the AoA of the wing, which is of the most concern for the aerodynamicist. (One of the conventions in pure aerodynamics is to choose the X direction such that it resulted in zero lift at zero AoA).
Whether the airflow will align with the axis at cruise depends on your choice. You will need to choose the angle of incidence of the wing to minimise drag, but the AoA may not be zero if you chose the axis direction with other considerations in mind.
Note that the choice of origin will not affect the wind angles. Essentially, you can leave the choice of origin until you need to decide on the other two axes.

1 A common example: Lift is defined in the wind axes. It is normally linearly dependent (~proportional) on AoA, but this only holds perpendicular to wind, and not with respect to "up" of the aircraft. But the difference is small for small angles.


While flying, how do I contact someone who is on the ground? As a regular General Aviation pilot, how do I contact or relay a simple message to someone on the ground? For example, to update a friend, family member, or taxi company with an ETA or a meeting point. Assume that asking them to follow a tail number on something like FlightAware would not suffice.

Cell phones are not an option. FAA AC 91-21.1B prohibits cellular communication while in flight (Correction: see comment below; it's really the FCC that prohibits cell use), and cell reception above 5000ft AGL is pretty poor.

Commercial operators have access to more sophisticated services like ACARS, ARINC, or satellite communications, but these are usually too expensive or complicated for the regular GA pilot.

Can Flight Service Stations (FSS) call someone and relay a short message for me? Has anyone tried this?

Once, in a pinch, I persuaded Phoenix Approach to call my mother-in-law and advise her to meet me at an alternate airport since I would have had to hold for two hours to get into PHX where we'd originally planned to meet. This is definitely not something that ATC does on a regular basis.

Other options include carrying an Iridium handheld satellite phone (not cheap, but certainly a possibility for light GA) or installing an amateur radio transceiver.