What is the motivation behind designing a control stick that does not move? According to Wikipedia (and various other online sources), the control stick on the F-16 does not move. The input values are determined by the amount of force acting on the stick, rather than the deflection angle/distance away from its neutral position like the majority of sticks and yokes. After pilots reported the design as "unnatural", the stick was modified so it would move a little.

Clearly the designers must have thought that having a very stiff control stick is advantageous. Why?

The stick does not need to move in order for the pilots to sense their inputs!
Humans have very accurate force sensors in their fingers, and no direct position sensors. Without looking, we only have a very vague idea where our hand is due to muscle tone sensing, but we don't need to look at our hand to know exactly how hard it is pushing at an object. For the human, force is the imminent control feedback factor.
The F-16 can do 9G in manoeuvres, and the pilot is inclined backwards with their arm on an armrest. If this arm needs to move a stick it shifts position, and during a manoeuvre can get grabbed by the load factor - precise control is much easier if the hand, wrist and arm can stay in one position.
A pilot who has flown both the F-16 and the centre stick F-18 has this to say about it:

Flying a side-stick control takes a while to get used to, but once you do, it's a joy. The conformai stick's shape feels very natural (it fits in the hand like a melted candy bar), and it allows easy access to nine of the 16 hotAS controls. Two fully adjustable forearm rests on the right cockpit bulkhead stabilize and isolate the pilot's arm and wrist, so when rattling around the cockpit during turbulence or going after the bad guy, the pilot's arm won't accidentally move and initiate unwanted control inputs.

Initially the stick was fixed completely and pilots would get comfortable with the novel concept of no stick movement pretty quickly. But there were two issues:

Cross-coupling. The control input must be separated in two axes, pitch and roll, and this is difficult to separate if feedback is from force only. In order to aid this, a small bit of movement was introduced with a breakout force in the centre: upon release, the stick moves back to neutral and requires a bit of force to be deflected from there. From the article linked above:

However, after initial F-16 flight tests, a ¼ inch of stick movement was incorporated to give a small dead band and a nominal breakout force to give better "feel" of a neutral stick because otherwise it was entirely too sensitive. The control harmony is quite good (the pressures required for pitch and roll mix well), but without the capability to physically position the stick, it's easy to contaminate roll inputs with unwanted pitch inputs, and vice versa

F16s kept getting back with bent sticks. During chasing another plane and pulling, pulling on the stick, they pulled excessively hard while the fly-by-wire was already generating the deflection for maximum load factor. With the stick now moving a little bit, the pilot can feel that the limit is reached when hitting the travel stop at 25 lbs. At this force the acceleration is 9G and pulling harder has no additional effect.


Why does the ICAO alphabet use "Charlie" for C? Why does the ICAO alphabet use "Charlie" for C? Specifically, why choose "Charlie", which has a "Ch" sound, rather than a word with the hard "C" such as "Carl"?

A cursory Googling turns up nothing.

Hard C sounds too much like K.  Ch (Charlie) will not be confused with K (Kilo). And soft C sounds too much like S (Sierra).


Is it allowed to operate an aircraft when the Emergency Locator Transmitter has been removed? I purchased an aircraft from which the previous owner removed the ELT before I took possession; it was present during the test flight. I need to know what the minimum legal requirements are for a replacement. Can I use a personal unit which are sold everywhere or do I need a specific unit for general aviation?

Assuming that you are in the US, you need to comply with

§91.207   Emergency locator transmitters.

(2) … there must be attached to the airplane an approved personal type or an approved automatic type emergency locator transmitter that is in operable condition,

and

(ii) No person may operate the aircraft more than 90 days after the ELT is initially removed from the aircraft; and

So in the next 90 days you need to install an ELT. It must meet the TSO standards for ELTs.

The cheapest way to comply would be to purchase a used 121.5 MHz unit so you are legal and use a personal unit in case of an accident.

The newer 406MHz units require a GPS source and a panel switch. Installation costs can be more than the unit. However, since the previous owner removed the ELT, I would assume that the wiring is still intact so you can probably save money on the install.

Which type of attached transmitter you choose depends on your budget and the terrain over which you fly. On February 1, 2009, the international COSPAS-SARSAT satellite system discontinued satellite-based monitoring of the 121.5/243-MHz frequencies, in part because of a high number of false signals attributed with these frequencies. AOPA

They also frequently fail to activate and since they are only monitored by pilots who are following the AIM and monitoring 121.5 even if they do go off, they often are not heard. Not to mention the problem of triangulating the signal to find the source. If you fly over populated areas and are in contact with ATC via flight following or IFR flight plans, they are still adequate and most pilots I know have not upgraded to the 406 MHz units.

The newer 406 MHz are satellite monitored and because they transmit GPS location data it is easier to find a downed aircraft. They have an activation switch in the panel so that they can be activated before the pilot hits terrain which increases the odds of finding you. If I frequently flew over mountains, remote areas, or at night I would certainly install one of these.

Personal beacons have lots of advantages over installed ELTs. If you survive the landing, chances are that they will as well. They are also incredibly cheap and many of them have web-tracking or texting that can be used by those interested in tracking your progress. You can also take them with you if you leave the airplane.


Why are/were there almost no tricycle-gear biplanes? A friend recently asked me: are there any tricycle-gear biplanes? After hunting around, we found the Waco AVN-8, but that's all. This causes me to ask: is there an aerodynamic or other design consideration which makes a tricycle gear a bad choice for a biplane, or is it just an evolutionary coincidence that the move to single-wing designs shortly preceded the move to tricycle gear?

It is a bit more than a coincidence. You might also ask why there are so few biplanes with retractable landing gear. The reason is similar.

Biplanes are propeller-driven (except for this one), and in almost all cases the propeller is in the front. Efficient propellers are large and require a longer landing gear when a tricycle gear is used. Granted, the plane will lift its tail during the ground run so the prop comes down a bit, but that happens over a flat and clean area, so lower tolerances can be afforded during the ground run than during all other ground operations.
Tricycle gears produce substantially more drag. The nose gear sits directly behind the propeller and in its slipstream, so its drag contribution is typically 40% of the whole gear drag. Tricycle gears became less of a drag (pun intended) when gear retraction became standard.
Biplanes are built light and the heavy engine cannot have much distance to the center of gravity. Where do you mount the nose gear? There is preciously little room for another piece of structure. And once that nosegear is crafted on the plane, its distance from the main gear is rather small. Any bump on the field will produce strong pitch oscillations when hit by the nose wheel. Better to have a tailwheel or skid at some distance from the main wheels.
Tricycle gears became a necessity with higher wing loadings and higher landing speeds. Biplanes don't need powerful brakes. On the contrary, on a grass field a taildragger is easier to handle. And with their short ground runs, biplanes often operate from grass fields.

Not that tricycle gears weren't tried. See the picture of a Breguet 6 below (source), conveniently proving that pusher biplanes were also designed. Gun synchronisation removed the last reason not to place the propeller in front.


Why does the Hinch Mountain VOR (as an example) have void areas below certain altitudes published in the U.S. Chart Supplement? I was digging around the SE region Chart Supplement, and I was reading KSRB (Sparta, TN), and this "VOR unusable" listing is contained in it:

The sectional chart shows that it is nearly on top of a hill. I've made some visual marks on this to help everyone visualize.

Hench Mountain is a Low Altitude VOR, so by normal standards, service should be clear all the way around at 1,000 ft above it's altitude:

Why are there unusable areas more than 1,000Ft above the elevation of the VOR?  I've flown over it, and there are no obvious obstructions, it sits on top of a flat hill as you can see in this video I found on Youtube.

Either multipath including refraction induced  errors  or blocking/masking by terrain.


Does uncoordinated control affect stall speed? Does an uncoordinated turn increase stall speeds? When you do cross control for crosswind landings in GA aircraft like C172, should you be worried about higher stall speeds during flares?

First question, yes.  For two principle reasons: instrument error and a change in relative wind across the wing. The increase in indicated AS would be small.

Second question, not a factor.  During the flare, you are close to the ground, and the AOA is mostly controlled by feel, sink rate and all the other things that factor into that complex maneuver.  So if the speed were slightly higher, it probably wouldn't make a difference.

But let's talk practicality.  When landing a C172 in a direct crosswind of 20k or so, I would add 5k to my speeds, and if the winds were gusty I would add accordingly (like 1/2 the gust factor approximately).

It is better to focus on having a stable approach with good AS and pitch control, than it is to fret over a potential 2k indicated stall speed differential.  Besides, in ground effect the numbers change as well.


Can single engine planes glide from a falling position? Near the end of the 50s Sci-Fi movie This Island Earth, the protagonist is escaping from a flying saucer which is crashing to Earth. As he was abducted while flying a plane (specifically a Stinson 108-1), it's sitting in the cargo hold on top of a retracting hatch:

The protagonists then start up the propeller, and once it gets up to speed, the entire hatch seems to disappear and they're dropped vertically out of the bottom of the flying saucer:

They then manage to continue moving straight without going into any kind of dive, before banking out of the shot.

Is this even remotely possible? I would have assumed that when they leave the system of the flying saucer, they will lose the speed which the saucer has and will basically be falling in a stationary position as they have no lift due to the air only hitting the bottom of the wings.

It's remotely possible, assuming you can find a suitable UFO :-) If you drop an object from an aircraft it starts falling with the same airspeed that the aircraft has, until drag or other factors slow it down. If you've ever seen a flour bombing competition (or just watched some WW2 movies or documentaries) you may have noticed that you drop bombs before you reach the target, not when you arrive directly over it.

So if the UFO is moving forward at an airspeed greater than the Stinson's stalling airspeed (65mph) - let's say 80mph - then the Stinson will fall into the air at an airspeed high enough to generate lift immediately and it will fly. If you already have the engine running and throttle up as soon as you fall out of the UFO, you will generate thrust and maintain the flight.

In reality - if there is such a thing when a UFO is involved - the Stinson is almost certain to lose some altitude and the wake turbulence from the much larger UFO will have a significant (and potentially very dangerous) effect if the Stinson can't stay ahead of it.


Helicopter retreating blade stall: Increasing the fwd speed decreases the tip speed on retreating side and angle of attack. So why limit on V_n.e? On Retreating side: Speed of wind over blade tip  = blade rotation speed - forward speed.
So, with an increase in forward speed, the speed of the wind over tip is getting reduced. In airflow diagram, this will decrease the angle of attack. So, why is there a limitation on forward speed in form of V_ne?
Let's consider a case where rotor of radius 10m rotating at 300 rpm anti-clockwise and chopper is moving forward at 30m/s. So, now the retreating side has got a larger angle of attack of 15 degrees (critical angle is 17 deg) due to blade flapping down. Blade tip speed being = RPM x radius - forward speed = 314-30 = 284m/s.
"Now" if the helicopter fwd speed increases to 50m/s, then the blade tip speed will decrease to 264m/s. Consequently, the angle of attack should reduce.

For the same reason aircraft have $V_{S}$ — you don't want to get into stalled condition, because while recovery is possible, it is difficult and far from certain.
Due to the gyroscopic effect, the rotor blades reach the lowest point 90° behind the point of minimum lift. As the helicopter accelerates, the lift would decrease on the retreating side, which would push the aft side of the rotor down, pitching the helicopter up, and reducing the speed again (that is, unlike hover the helicopter is stable in forward flight).
Except the pilot wants to accelerate, so they keep pushing the cyclic forward, which increases the angle of attack on the retreating side (90° ahead of the desired tilt to compensate for the gyroscopic effect), compensating the loss of lift due to the lower air stream velocity.
So the angle of attack is higher on the retreating side because of pilot input, not inherent property of the rotor.
Now when the helicopter reaches the point of stall, the tendency to pitch up will increase, and it won't be possible to compensate it with more forward cyclic, since the retreating side is not able to generate more lift. In this sense, the condition is actually self-correcting as the helicopter will start climbing and losing speed. However

the accompanying vibrations from the repeated stalling and unstalling of the blades as they rotate will stress them significantly and might cause structural damage and
the violent pitch up accompanied with some roll to the retreating side (which it is depends on type) might lead to the pilot losing spatial orientation and consequently control over the usually low flying vehicle.

So you still want to avoid getting into the retreating blade stall condition in the first place.


Are "Tally-ho" and "no joy" acceptable ATC terms for civil operations? I keep meaning to ask this question. I heard "Tally-ho" used for the first time by a pilot on Liveatc.net. I know these are pretty standard phrases in military aviation but I wondered if civil pilots use them. Are they acceptable under either FAA or ICAO standards? If not, what's the likelihood a controller will know what you mean?

As a private pilot, I have heard "Tally-ho" and other pseudo (British?) military phrases used when talking to ATC.  I understand "Tally-ho" to be equivalent to "target(s) in sight" or "inbound" or even in some cases "affirmative", and that's the problem.  It is important to be clear and precise when communicating on the radio, however, the folks working at ATC seem to understand these phrases.

FAA:

I've read the AIM - Aeronautical Information Manual and Pilot's handbook of aeronautical knowlage (Both available for free download from the FAA's website) and I don't remember seeing these phrases anywhere.

Other texts caution against using these colorful phrases instead of standard phraseology because problems stemming from varied interpretations.  It is important to be clear and precise when communicating on the radio.

I think the correct terminology is: "Traffic In Sight" and "Negative contact"


How should this Bendix course deviation indicator from the '50s be read? This is a Swissair photo from a DC-7C panel.

Source

I have seen photos on airliners.net of these in DC-6s, VC-118 and Constellations. It appears that other companies besides Bendix made these indicators, but they are similar in appearance and function.

I've searched unsuccessfully for patents, or any other information that would describe what I am asking about:

What are these "45" scales for?
What is the flag in the window used to indicate?
What are these two lines for?
What happens when the test button is pressed, e.g., the loc, glideslope needles move to some position, #5 rotates to some position, #2 flag appears?
What does this "airplane" indicate?

This is an early version of an HSI.  The heading (depicted by the airplane) is either set manually, or it's slaved to a remote compass.

Responding to the question as numbered:

Depicts wind correction angle when tracking a course.
To/From flag
Glide slope reference (too low)
Push to test the warning light.  Also push to illuminate while turning to adjust brightness.  This may light without a valid heading source from a remote compass/flux valve.  It may also be to indicate failure of an internal gyro if so equipped for heading source.
Depicts aircraft the heading


Does runway centerline lighting provide information about distance left before end? I asked Does runway center lighting increase runway lifetime? and this video was was mentioned.

I don't really understand the significance and I first assumed centreline lighting in night conditions helped crews know half the runway is done and they don't accidentally overshoot the runway.

But I was explained centreline lighting helps in directional and alignment guidance for the aircraft. How is that possible?

Runway centreline lights serve two main purposes:

Inform the pilot about where the centre of the runway is (they are installed on the runway centreline.
Inform the pilot about how far away the end of the runway is (by colour coding the final 900 m metres of runway centre line lights).

Their safety objective is to help prevent misalignments when landing and taking off, as well as to help prevent undershoots and overruns.

They are not always required, and in fact they are considered a complementary system to other visual aids such as runway edge lighting or runway markings, amongst others.


Does every Safety–of–Flight Test (SOFT) include full testing of each and every equipment? Does every Safety–of–Flight Test (SOFT) include full testing of each and every equipment with the full EMC or is it just a basic quality test for some of the affected systems?

Safety of Flight (SOF) Testing is a sample, a subset, of a more exhaustive set of tests.1 It evaluates critical functions that "assures basic" levels of safety for critical components of the system.

The FAA further defines "critical" in this way:
   Usually a function whose loss would prevent the continued safe flight
   and landing of the airplane.2

Though the definition is somewhat circular, it is usually clear when a function is not critical, not required for safety of flight.  For example, a Terrain Awareness and Warning System (TAWS) is not required to be installed, therefore, it's function would not be part of the SOF testing.  However, attitude display, engine instruments, and airspeed are all defined in 2 as critical and would be part of SOF testing.


Has anyone used a 1/4" Y adapter and 3.5mm-to-1/4" converter to pipe in music into their aviation headset? I have a standard aviation headset and would like to splice in some music for longer flights.  The aviation-specific adapters are overpriced at around $40.  Can i just use a 1/4" Y adapter and a 3.5mm to 1/4" converter, such as these?   Has anyone tried this with results?

https://www.sweetwater.com/store/detail/YPP118

https://www.sweetwater.com/store/detail/CMS103

Commercial 1/4 Jack audio headsets are stereo, signals to top and ring, body is ground... so you need to fiddle the wires so both signals go to the tip... Else you will.only hear one channel .. I don't advocate this use because if you are listening to.music you might miss calls about other traffic etc.. Your call.


Inviscid aerodynamics: why is circulation zero at the wingtip? I read that the circulation at the wingtip for a finite wing is $0$. I am a little bit confused about this statement. First, it is said that drag due to lift (aka induced drag) generates vortices at the wingtip. But on the other hand in Prandtl's lifting line theory the circulation at the wingtip dies off. I thought about some explanations but I would like to know which one is really true. I also welcome any additional explanations.

Pressure difference at wingtip is $0 \implies $ no lift $\implies \Gamma = 0$.
Indirect: If the circulation at the wingtip is not equal to $0$, then the downwash due to wingtip vortex is infinitely large, which is not possible. This gives $\Gamma =0$ by contradiction.

Prandtl's lifting line assumes that the form of the solution for its equation is similar to an elliptical solution with small variations.

$$
\Gamma(\Theta)=4sV_{\infty}
\sum_{n=1}^{\infty}A_nsin(n\Theta)
$$

Which, essentially, assumes that the circulation at the tips is 0, so the tip is not producing lift at all in any assumemd solution. The theory assumes that the tip pressure difference is 0 and actually no speed normal to the surface.

This theory explains that the drag due to lift is NOT a drag produced at the tip, is drag of the complete wing as a single element and the tip vortex is not produced at the vortex, is produce down the wing due to the circulation variation along the wing.

Which drives the typical picture (via helicel)


On average, how often (in flight hours) will a pilot face an emergency situation? Experienced pilots (e.g. those with over 10,000 hours) are more likely to have faced an emergency situation some time in their career, compared to a new pilot (with say 40 hours). My question is, how often can one expect to encounter an emergency situation (e.g. one every 5,000 hours) ?

Here, an emergency situation is defined as a situation where the use of a "Mayday" call is justified, for example engine failure (in single / multi engine aircrafts), loss of electronic instruments in the cockpit etc. Medical emergencies are not counted.

The answer is going to vary wildly according aircraft type, pilot rating and proficiency, number and type of engines, aircraft size, and part of the world.

In this 2016 study by the FAA for GA pilots, the median period of time for non instrument rated pilots was over 3 times as often as instrument rated. 250hrs vs 823hrs. The report describes some of the hardships involved in arriving at the less than precise determination.

I have asked this question to many airline pilots and about half of the 4000-6000hr pilots have a story to tell. Just guessing but I would say modern airliners are perhaps every 10,000 hours.

I must be doing something wrong, because I averaged about every 500-600hrs but then I was mostly flying very old run down Beech-18's in Alaska.


what is the earth resistance to be maintained while aircraft is on ground and during ground checks for military aircrafts While aircrafts are under servicing on ground, the external dedicated earth wires are connected to the aircraft to avoid static hares effect to human beings. How much resistance value has to be maintained?.Is there any standards available for the same.

For the US Navy, according to this instruction:

  The maximum resistance allowable for the cable (MS27574) itself shall
  not exceed 0.5 ohms.

  The cable’s operational (in use) maximum resistance shall not exceed
  10 ohms.

The difference in values corresponds to the loss of quality in the electrical contact at both ends of the wire (clips). Contacts are allowed to be as resistive as about 5 ohms. For comparison hand to foot impedance is about 500 ohms at 100 Hz, meaning the current will be 50x lower in the body than in the wire in case the aircraft is not at the potential of the ground.

You'll find a similar requirement of maximum 10 ohms in: TO 00-25-172, Ground Servicing of Aircraft and Static Grounding/Bonding from USAF.


What is the effect of the Load Factor on wing Structures? I am quite confused with the definition of Load factor and how it applies to the wing structures of an aircraft.

For example, you have an Aircraft being subjected to a Load Factor of 2 and whose total mass is 5000kg and wings + fuel mass is 1500kg, and are asked to calculate the Inertia Loading of Said wings.

Would you multiply the weight of the wings by 2? Or does Load factor only affect the lift and not the weight?

https://en.m.wikipedia.org/wiki/Load_factor

As a start for an aircraft the load factor is the ratio of total lift to total Weight

An easy way to imagine this concept is to understand the idea that when the aircraft is flying at high speed during cruise with a small angle of attack the load factor is very close to 1, this corresponds to the first case on the left of the picture here below

Now what is lift, the picture here below is self explanatory, so it is important to understand that the lift is not always an upward vertical force; when the aircraft is under a coordinated bank angle the lift has two components ; horizontal and vertical.

Now to simplify we shall consider the lift to be totally produced by the wings, while in real flight it is the result of the wing, the body, and the tail.

Since the Aircraft is subjected to a load factor of 2 this means the aircraft as a complete body is subjected to a total lift( not vertical) equal to twice the total weight of the Aircraft

The lift produced is therefore: $2\cdot(5000 + 1500)\,\mathrm{kg}\cdot g = 13000\,\mathrm{kg}\cdot g = 128\,\mathrm{kN}$

If we consider we are in the steep banked turn indicated on the right of the picture above:

The weight of the aircraft body without the wings is a downward force on the root of the wings

Also the wings have their own distributed weights, that produces a torque to the root of the wings

Also the distributed fuel in the wings produces a torque to the wings roots

Also each wing is subject to a distributed lift of $\ 6500\,\mathrm{kg}\cdot g = 63.8\,\mathrm{kN}$

The exact computation are difficult because

we don’t know the bare weight of the wing nor the centre of gravity of the wing
we don’t know the distribution of the fuel in the wing, so we don’t know the centre of gravity of the fuel of each wing.

Therefore it is impossible to calculation the shear forces and the torques at the wings roots.

Nevertheless I imagine the main question concerns the relationship between lift and load factor, And here the most important point is to understand that the lift is not always a vertical force


Can UHMWP be used as composites for aircraft impact susceptible areas? Can Ultra High Molecular Weight Polyethylene (UHMWP), be used as composites for aircrafts impact susceptible areas?
I did some research but could only see UHMWP being used as a hybrid with graphite/ carbon.
I have two questions is it possible to make composites purely of UHMWP and epoxy?
And, why use UHMWP with carbon, as it already has good characteristics alone?

Short answer: Yes, when used properly. It does have its limits (low melting point, low adhesion with epoxy), but if you design around those limits it helps to raise energy absorption considerably.
Long answer: I've used Spectra for improving the crashworthiness of glider cockpits in a study. That was back in 1987-88 when that stuff was brand new.
I molded models of forward fuselages in glassfiber-epoxy as a baseline and then added more models using graphite, aramide and Spectra woven fibers. UHMWP is great in tension but poor in compression, so using it all by itself for the fuselage cannot be recommended. I used 100% Spectra layups only for the seat pan of the models but covered it on both sides with glassfiber or graphite fiber for improved compression and shear strength when using it for the fuselage shell.
Next, I set up a dynamic and a static test. The dynamic test used a swing to let the models crash into an angled, greased steel plate while the static test used a hydraulic testing machine which slowly lowered a steel plate on those poor, unsuspecting models until they were completely crushed. Using a video camera and strain gages, I could prove that the quasi-static test delivered equivalent results to the dynamic test (since the speed of sound in the material is much higher than the impact speed), so the much easier to measure static test could be used for the majority of the tests.

Left: Quasi-static test in progress, right: Model mounted on the swing. Own work.
While the glassfiber and carbon baselines produced a strong force spike at the beginning of the impact and then developed cracks which greatly reduced the stiffness and impact energy absorption, the Spectra samples would not crack in the same way: The UHMWP fibers prevented those cracks. Instead, the epoxy matrix crumbled over large areas and the cover layer delaminated, producing a nice, continuously high force over the duration of the impact. I could improve energy absorption by 200%.

Lineup of some models (after the test). On the far right is a graphite shell with an aramide seat pan.
Those strain gages you see in the pictures were used to calibrate a FEM model using ADINA, a nonlinear code which allows for large displacements. Running the FEM model on a Microvax took days but produced very convincing results after I had measured the material properties of the materials employed in the models. Never use manufacturer-supplied data but gather your own!
The results were published in the Journal of Aircraft.


Why are wings load tested upside down? Why are wings load tested by putting weights on the underside of the  wing (placed upside down)? A wing is being pulled into the air from its upper surface in flight, so shouldn't it be load tested in the other direction?

A wing can be tested in any orientation as long as the load is applied correctly. The classic wing test photo is the 787 in a fixture showing its extremely flexible wings.

I thought it might be fun to add an ultimate load test, so here is the 777 tested to failure. Sorry for the early 1990s video quality.

Boeing 777 wing fail at 154% design load


Does adding flaps during ground roll perform better in short-field takeoff? In most POHs, the flap configuration for short-field is usually recommended as partial flaps, for instance 10˚ for C172.

I remember the arguments are that full flaps would reduce the acceleration rate, and in no flaps the airplane would have to accelerate to higher speed to off the ground. In either case, the ground roll distance is longer than the optimal partial-flap configurations.

However, this assumes a fixed flap setting during ground roll. How about let the airplane accelerate to a certain speed (in zero flaps) before adding to full flaps? We could achieve best acceleration as well as best lift-off speed?

I found this answer is a good analysis.

13.3 - Obstructed-Field - Skimming versus Wheelbarrowing or Flap-Popping
"Another possible procedure (which is usually not recommended) is to keep the flaps retracted until you are ready to leave the runway. Less flaps means less incidence. A big disadvantage is that “popping” the flaps like this increases your workload at a time when there are lots of other things you should be attending to. Another disadvantage is that you run the risk of extending the flaps past the takeoff position to the landing position, creating lots of drag, which is really not what you want in this situation. If your POH calls for this procedure, go ahead, but be careful. Make sure you have some sort of detent to block inadvertent over-extension."
-- John S. Denker, See How It Flies


Why does an airliner have a shallow descent when heavier, opposite to gliders with ballast? Gliders utilize water ballast to, among other things, descend faster:

But sometimes you need to get down fast: This is when water ballast is added.

In airliners however, the heavier the plane, the shallower the rate of descent.
Why is that difference? What is the physics involved?
Some 737 figures can be found here. And from the Airbus performance publication:

1.3.3. Weight Effect
Green dot speed (minimum gradient) is a function of weight. In the standard descent speed range (from green dot to VMO), the rate and gradient of descent magnitudes are reduced at higher weights. [emphasis added]

For the site readers based on some comments:
Descent – does not refer to landing or approach, rather the idle-power descent from cruise in clean (no flaps, speed brakes, or landing gear) configuration. The heavier an airliner, the longer it takes to get down.

If you can forgive my limited Paint skills:

It's the variable/constant speed that makes the difference.

Weight changes endurance (time to ground) and not range (distance to ground) if speed is adjusted to match the new weight.
Weight changes the speed for best L/D, but does not ever change the best L/D ratio (and thus best range)

For gliders (which are adjusting speed to new weight):

ballast allows them to fly faster while maintaining best L/D. Pilot will increase speed to keep same (best) gliding ratio, albeit at an increased descent rate (fpm). You get there faster, in an increased rate, but same gliding distance (to the ground eventually).

For powered airplanes (flying at set speeds regardless of weight):

pilots usually descend from cruise at fixed airspeed, usually much above the best L/D. Higher weight means the aircraft is much closer to best L/D speed, so the flight path is much shallower. You get there in the same time, but you will be much higher when heavy.

Or if it is easier to think this way, a light aircraft is a lot further away from its best L/D speed than a heavy one, when flying an approach. Naturally this makes the light one descend more steeply.

Note that this all refers to a normal descent from cruise. Final approach speeds, just before landing, are indeed corrected for weight in a large airplane, and heavy ones will indeed have a higher Vref and land slightly faster.


Why can't pilots drink alcohol after a flight? Why are pilots told not to drink alcoholic drinks even after landing, even when their next flight is more than 24 hours later?

Is this some kind of precautionary rule?

Commercial pilots (in the United States) are usually not allowed to drink while on duty regardless of their flight status because, among other things, it would be frightening to any prospective passengers who might see them drinking. For example, American Airlines strictly forbids consuming alcohol or any intoxicant publicly while in uniform. Here is the exact text of their company policy:

  Rule 25

  Unauthorized possession or using any intoxicants on Company premises
  at any time, or drinking intoxicants in public while wearing a uniform
  with Company emblem or insignia is prohibited.

Note that this does not disallow drinking as long as it is (1) not on company premises, and (2) is in private. Obviously, if the person is a US crewmember then they must adhere to FAA rules in addition to company rules (see FAR Section 91.17 for details on those rules).


Why would a flight take a detour on clean weather? I was in a flight recently (march 22nd) from SBCF to SBGR (Brazil). It's usually a 1h10m flight, but mine took roughly 20 minutes more. I saw the flight path in FlightAware and there's an unexpected (to me) detour mid-flight:

I looked for other instances of this scheduled flight and there are more extreme cases, such as this one:

In this case, the actual flight distance was 833km whereas the straight-line distance is measly 496km.

Obviously it would be difficult to pinpoint the exact reason why this happened, but what factors could lead to taking such a different than expected route?

There are lots of reasons flights may not take a direct route, some are but not limited to:

It's just the route ATC assigned (for whatever reason they see fit)
There is an active military operations area or live firing area they are avoiding, these are not always "active" and may be avoided only sometimes.
Some kind of natural event on the ground, wildfire, volcanic eruption, etc.
There is a TFR they can't get through these are typically not hard bans on overflight but can be.
If it's an international flight they may not be allowed to overfly the airspace.
For smaller aircraft it may be a terrain avoidance issue (generally this is for general aviation sized planes).


How do pilots determine the takeoff distance of an aircraft? While making a flight plan, depending upon the weight of the aircraft, I'm assuming pilots determine the takeoff distance. How do they do so (is there a formula to calculate the takeoff distance)?

Most airplanes have a POH (Pilot Operating Handbook) with a table on aircraft distances.  Here I have included one for the C172.


What is the chime that can be heard roughly 30 seconds after takeoff? This may be confirmation bias on my part, this might be impatient passengers pressing the call button, and it might be me imagining things, but it's one of those I've wondered about. I'll try to describe as best I can.

Of all the times I've flown recently on an airliner, of which most of the time it's a short haul carrier often flying smaller Boeing/Airbus aircraft*, I have been aware that there is an audible chime which can be heard throughout the cabin (ie, not only when sat in the vicinity of crew seats) very soon (30-45 seconds) after take-off.

It is heard well before the passenger seat-belt signs are switched off.

Can anyone describe what I am hearing and what its purpose is? Who is this a signal for, and who is sending the signal?

* the two times I've flown on an Embraer I don't recall hearing it

It's signalled from the cockpit (the cabin chime) to let the cabin crew know it's OK to move around.  If you watch, this happens before the seat belt sign goes off which at the earliest, is at 10,000 feet.


Would a flying-wing drone fly farther if filled with helium? I'm wanting to build a flying wing designed drone and I'm wondering if I were to fill the fuselage with helium if it would improve it's range.

If the end result of the sealed wing filled with helium is lighter than the same wing filled with a normal atmosphere (nitrogen, oxygen, and assorted other gases), then yes - a lighter aircraft will, all else being equal, have better performance and thus longer range than a heavier one.
If you'd actually be able to achieve economic benefit is doubtful - the delta in weight will be offset by everything required to seal the wing up, and the gain in efficiency may be pretty tiny. Beyond that, if mechanics need to access stuff inside the wing, you'll be paying to replenish the helium whenever that happens. Plus, the volume inside the flying wing is normally where things like fuel, passengers, crew, cargo, engines, and etc would go; you may find that you don't have all that much available space left to fill with helium.


Do mono spar wings need compression strut(or ribs)? Loyolksip

Regarding airplane wings, are compression struts needed in a mono spar wing?
Secondly, are anti drag and drag wires needed in a mono spar wings?

The wing in your picture has a tubular spar. In addition, smaller tubes are used at the leading and trailing edge, and one is running diagonally through the inner half to add in-plane bending stiffness.

Tubes have the same stiffness in all directions, so using a tube gives already some in-plane and torsional stiffness. For the outer part of the wing in your picture, this stiffness seems to be sufficient to not use drag or anti-drag wires. Since loads accumulate from tip to root, the root has the highest loading, and here a bracing tube is added in the inner wing so in-plane stiffness and strength are sufficient. I expect that this wing also uses bracing for the out-of-plane loads, and I checked with the manufacturer's home page: Yes, it does.

Kolb Fire Star II side view, from the manufacturer's home page.

The ribs are supplemented by half ribs placed between the full ribs, so the forward part of the airfoil, where more aero loads and more curvature require more support, is adequately shaped and strengthened. The ribs themselves seem also to be made from rather thin tubes, and this being a rectangular wing, need only a single jig to be manufactured. An N-type truss allows the ribs to transmit shear to the spar.

In all, this is a very efficient and pragmatic wing design. There is nothing missing (well, except for the fabric covering, of course)


What is the reasoning behind Kamov twin-rudder design? Several helicopters, namely the Ka-226, the Ka-25, and the Ka-27, have twin vertical stabilizers that are canted inwards at an angle of about 15 degrees, similar to the feathers on a shuttlecock.

The stated purpose of this design is to increase directional stability during auto-rotation, but I cannot find any information as to why the inwards angle is used in general, and more specifically, why it seems only these models of helicopter need them.

My best guess is that much like the shuttlecock, when in straight flight, both stabilizers will have a positive Angle of attack and will be generating inwards lift, which is a more stable condition for low forward speeds.

It's kind of a hard one. It looks like in this helicopter directional stability is a challenge because:

the tail arm is quite short;
there is no tail rotor, which provides directional stability just like a vertical tail does.

An aerodynamic surface is most effective around zero angle of attack of the air stream - slipstream angle $ \beta $ in this case. Most profiles stall at angles around 15 degrees, and are therefore most effective at -15º < $ \beta $ < 15º.

If we have a look at lift L and drag D of the vertical surfaces, with dynamic pressure q in N/$m^2$, A = total vertical tail area in $m^2$, and $C_L$ and $C_D$ dimensionless coefficients:
$$ L = C_L * q * A$$
$$ D = C_D * q * A$$

The two images above are from Principles of Helicopter Aerodynamics by J. Gordon Leishman Figure 7.51. They show the coefficients over a full range of 360 degree, and we can see that:

In the $C_L$ plot, after the stall point at about 15º there is still quite some lift left, that increases again but to a lower peak at about 45º according to flat plate lift theory. That means that between 15º and 45º, there is not much increase/decrease in lift: $dC_L/d\alpha$ is almost zero and the surface is very ineffective in stabilising in this range. There is even a little dip in the curve just after the stall where the angle might get stuck.
In the $C_D$ plot, there is a deadband between -15º and 15º, after which the drag jumps up dramatically and has a relatively high $dC_D/d\alpha$. Since drag is defined as the force in line of the air stream, at higher angles of attack drag actually helps in getting the surface back to neutral (At 90º lift is zero and all force is drag)

So with the above in mind, what it looks like to me is that due to the short tail arm the vertical surfaces must deal with helicopter slipstream angles > 15º. By canting them inwards, one surface operates in the effective $C_L$ range, and the other surface operates outside the $C_D$ deadband, in doing so channeling the drag forces into a stabilising moment.


What were DJI drones spying on? What data did they capture? In late 2017, it was claimed that DJI (a very popular Chinese drone company) was working with the Chinese government to spy on the United States via their drones, or their drone controllers, which are owned by many Americans. This claim was made by the Los Angeles Office of the Immigration and Customs Enforcement Bureau, which cited "moderate confidence" in their findings. This led the military to discontinue the use of DJI drones.

My question is - What specific information could be gleaned from these drones? I would presume navigational data, but perhaps video and audio could be recorded? The data that was collected, what benefit would it provide to China?

Regarding the above, my only assumption would be that the Chinese could track military operations that DJI drones were used in, but I'm not sure if this is accurate, or the only potential purpose. I'd be appreciative of any well-sourced answer.

LA-ICE Analysis

I doubt anything was gleaned per se, but there was recognized a significant security risk here and the military discontinued the use of the produce for fear of integral malware which could give an unfriendly foreign power a readily available means to spy on other nations.

If you have large numbers of these drones in operation all over the United States and other nations of interest equipped with a GPS receiver, and they make use of a controller linked to a smartphone or other device with internet access, you now effectively have an unwitting air force of spy aircraft operating within your target's airspace where the information transmitted from the drone is readily accessible to intelligence agencies within China.  It probably cannot gather sensitive national security information, but these kinds of drones could be used to uncover non-classified government or military activities which could be of use to Chinese data analysts.  We use to do this on a regular basis using spy satellites over Russia and Eastern Europe and thinking several levels ahead, considering how interference with more mundane activities could have a real impact on.

Analysts in China can glean through users until they find ones doing something of interest.  People operating drones near military installations and ranges, etc can be useful and their video can be analyzed e.g. a kid in Las Vegas who lived near Nellis AFB may have his drone targeted when he/she flies it.  If the video captures images of the airbase, analysts can look at day to day differences for things like base maintenance, vehicles in the motorpool, movement of aircraft on the flightline, etc. and the data could be used to predict patterns of behavior, operations protocol, etc. that can be useful to military intelligence.  It's no wonder the US military stopped using DJI products for that reason.


Could maintenance data be downloaded via ATC facilities? Could aircraft send maintenance data to the airport via ATC facilities (e.g., tower, TRACON, enroute)? Or are they only used for air traffic control?

Not through ATC.  The airlines have "company" frequencies they can use for such data.  Outside of airlines, General Aviation can use FBO frequencies for the same communications.


Can pilots fly circling approaches at non-towered airports? So the answer in my mind is "of course pilots can fly circling approaches at non-towered airports" (seriously, I could swear that I've done it before, but then again I can't think of any specific examples....).

That is, until I ran across this little tidbit in the Air Traffic Control Order while researching another question:

  4-8-6. CIRCLING APPROACH

  a. Circling approach instructions may only be given for aircraft landing at airports with operational control towers.

So then the question becomes, why do they have circling minimums at non-towered airports??

No tower here.
ATC can't clear me to circle.
Why do we have circling minimums??

The way I understand that ATC order is that it is about "circling approach instructions" which is different than "(circling) approach clearance". They cannot tell you to enter a left downwind, do a right 360, or follow some other specific path to the runway. If they clear you to execute a circling approach to a given runway at a non-towered airport, then the path to the runway is for the pilot to determine, as necessary to separate yourself from other aircraft in the terminal area and land safely.


What are the registration requirements for US aircraft? Where can the requirements for US aircraft registration be found?

What is the penalty for flying an aircraft without a valid registration?

What should someone do if they believe an aircraft to be operating without valid registration, and how could they determine this?

Somewhat related to this other question:
What is the pilot's obligation to verify a registration has not expired?

Where can the requirements for US aircraft registration be found?

The registration procedure is outlined in FAA Form AFS-750-94, Information to Aid in the Registration of U.S. Civil Aircraft. See also the full list of FAA registration forms.

  3. The owner of an aircraft must apply to the Aircraft Registration Branch, P.O. Box 25504, Oklahoma City, Oklahoma 73125-0504, to obtain a certificate of registration. The applicant must submit an Aircraft Registration Application, AC Form 8050-1, accompanied by evidence of ownership and a $5 registration fee.

  Requirements for registration of aircraft are contained in the Federal Aviation Regulations, Part 47, “Aircraft Registration.” (For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.)

Basically, the registrant (whether an individual or a corporation, trustee, etc., all of which are defined in Part 47) must be able to prove legal ownership of the airplane.

  What is the penalty for flying an aircraft without a valid registration?

The violator would be subject to any of the standard FAA enforcement actions: administrative action (a "Warning Notice" or "Letter of Correction" which goes on your record), reexamination ("709 ride"), certification action (suspension or revocation), or civil/criminal penalties. The exact penalty would likely depend on the circumstances: whether the violation was intentional and the degree of the pilot's negligence under the FARs.

  What should someone do if they believe an aircraft to be operating without valid registration, and how could they determine this?

The FAA's website recommends reporting FAR violations by calling the FAA's toll-free telephone number, 1-866-TELL-FAA.


Does an uncovered naval aircraft suffer any weathering affects? This question is for WW2-era naval aircraft.

I noticed that some cruisers and battleships carried a couple seaplanes, launched by catapult and recovered with cranes.

Obviously these are unprotected by a hangar. So it raises the question, is a hangar really necessary to protect naval aircraft from the weather? Naval aircraft should be more stalwart against seawater at the very least, and I can't imagine that freshwater rain would do anything to even a land-based plane. Dirt and sand also seem a remote possibility. Only thing left I can think of is sunlight over long periods.

The question pertains to seaplanes as well as wheeled aircraft, if they are carrier aircraft. (I understand that carriers have a hangar for maintenance and/or an armored roof, but my question is about the need, or not, for weather protection.) Can naval aircraft just be left on the deck without ill-effects?

Naval aircraft cannot be just left on deck without maintenance and expected to operate after an extended period of time. I will restrict this to storage on deck and avoid going into operational issues like spray effects on engines as OPs question does not ask that.

There are a multitude of technologies in use to protect metal parts from corrosion, but in the end these all fight a losing battle against entropy and their effectiveness is more accurately measured in how long they can stave off corrosion.

Painted parts are probably the most resistant, as organic coatings provide a hard barrier over the vulnerable surfaces. Paint is not foolproof, however, as it can chip and crack, due to thermal stresses, FOD, etc. Furthermore, not all parts can be painted and corrosion can develop underneath paint if it has an entry point, like a rivet hole , a static discharge connector or a crack in the paint.

Where organic coatings are not suitable, for example on shock absorber struts, other coatings can be applied. For steel a common solution is a chrome layer; for aluminium, chromate conversion and anodizing. These solutions are not as effective as a coat of paint, and an aggressive-enough environment will attack even chrome layers. As an example, take landing gear struts, which have multiple exposed moving parts, many crevices to accumulate moisture and diverse metals ready to form galvanic cells.

Finally, there are places where no coating can be used at all: static dischargers. These do exactly what the name implies, ground the aircraft to allow for built up static charges to flow into the ground, so they need to be exposed and to contact (almost)bare metal on the airframe to ensure conductivity. Their connection points are typically covered in sealant after installation, but these sealants do not last forever: they weather and end up cracking after some years in the field, turning into a moisture trap and destroying the part they are on if not monitored.

Finally, a note on composite airframes.
CFRP parts are conductive and very noble, so they can easily corrode whatever metal is attached to them. This can be mitigated with careful engineering, but can come as an unexpected problem for systems suppliers that are used to installing equipment on metal airframes: an aluminium airframe will corrode before a steel tray attached to it without insulation, but the opposite will happen with a composite airframe.


Why do you need to subtract when using a climb/descent performance chart for an altitude other than sea level? If you look at this chart you see we subtract 3 minus 1, 9 minus 2, and 30 minus 4 in the example calculation. My question is, why do we subtract these numbers? Do we subtract the same numbers each time? Nowhere on the performance chart does it give any explanation!

If you look closely at the example, it's describing a situation where you're cruising at 5500 feet, and landing at an airport with a "pressure altitude" (elevation) of 680 feet. The chart, however, is indexed to zero-altitude. You can't just reduce your cruise altitude by the elevation of the airport, because performance varies with altitude (indicated by the curvature of the arcs for fuel, time, and distance).

To find what you'd need to descend to the airport, you must run the numbers twice - once for descending to sea level from cruise (3 gallons, 9 minutes, 30 NM), and once for descending from the airport to sea level (1 gallon, 2 minutes, 4 NM). Note the two sets of dashed arrows on the chart, providing the two example calculations. You then subtract the numbers for the airport from the numbers for the cruise altitude, as you would hopefully not descend below the altitude of the airport!


Why don't airliners use noise cancelling? Some cars have started using noise cancelling in their cabins, through the speaker system. It's a well established technique for dealing with low frequency noise like engine rumble and wind noise.

So why don't modern airliners use it?

First, some airliners do use it - the Bombardier Q400 uses a NVS (Noise and Vibration Suppression) system to reduce cabin noise. Basically, it uses devices called Active Tuned Vibration Absorbers (ATVAs) mounted on the fuselage frames to 'cancel' the vibrations from propellers and outside noise, thereby quieting the cabin.

However, there are some issues with using active noise cancellation in aircraft:

The aircraft cabin is quite big and complex, and it would be very difficult to achieve noise cancellation over the entire volume; as active noise cancellation depends on the cancellation of sound waves using opposite phase, it works best in small, confined spaces. If the waves turn out to be in phase in a region, the sound would be doubled - not a good result.
A significant contributor to the noise inside the cabin is the wind outside; this is usually random and the frequency spectrum is also quite wide, resulting in difficulties in noise cancellation.
Noise cancellation is effective mostly when the frequency of the source is constant, which is not usually the case (the props are an exception - they lend themselves well to noise suppression)

Instead of active noise cancellation, aircraft manufacturers are putting more energy in reducing the noise itself:

Better engines, which has lesser noise through the use of chevrons, etc.
Better modelling of aircraft, so that the fuselage design minimizes the noise due to wind.
Better damping and cabin design, so the noise produced (for example, due to air conditioning and ventilation) is reduced.


What was in the noses of airliners before the weather radar? At the time of the first Lockheed Constellations, airliners fitted with a weather radar were not common, and the navigator was not placed in the airplane nose. Thus, the nose seems to be mostly dead space.

Was the nose dead space with no useful equipment in it, or was it used for equipment I don't know about?

(Source)

Externally, the Connie (shown above) had lights, with the red being the now obsolete passing light.

From the high-res DC-3 and DC-7 cutaways, all sort of stuff, from autopilot oil and air filters, wiring, windshield spray systems, to hydraulic reservoirs.

There wasn't one main object like there's now, nor was it completely empty. Its main aerodynamic function remains though.


What is the future of manufacturer independent jet engine maintenance repair and overhaul (MRO) services? I have noticed that Air France KLM offer a Maintenence (MRO) service for aircraft and jet engines. This sounds like a potentially big threat to engine manufacturers like Rolls Royce who makes around 50% of its revenue from its TotalCare MRO servicing contracts.

I would have thought the price of buying saying a Rolls Royce jet engine would be far higher if an airline customer did not take the MRO TotalCare package from RR and in addition the airline may suffer from reduced engine warranties if they employ a 3rd party to service the engines.

Do you think engine manufacturers business will get competed away by independent MRO businesses like Air France KLM's?

Background:
These days, Rolls-Royce is not any more in the business of selling engines, but selling thrust. The engines send telemetry data to four centers, the biggest one in Derby, in Great Britain, where every engine is under surveillance. An inspection can be scheduled at or spare parts can be directed to the right destination even before the pilots or the airline know that one of their engines has a problem.

By now, 80% of Rolls-Royce engines are sold at a loss which is recouped by a hourly fee paid under the TotalCare (.pdf!) program, and that center in Derby is monitoring maybe 4000 of them at the same time. Rolls-Royce has accumulated a treasure of engine operations data, which enables it to consult airlines on best practices. This makes it very hard for a third party to steal maintenance business from Rolls-Royce. All maintenance cost is covered by the TotalCare fee which now accounts for more than half of Rolls-Royce's revenue.

Other engine manufacturers have begun to copy this approach, and both Airbus and Boeing now offer similar programs for airframe surveillance and maintenance to their customers.

I wonder how Air France KLM can get at the data from the engines. My impression is that Rolls-Royce is in a different position, and third parties cannot compete on an equal basis. Airlines can rent out their shop capacity to others, but the specific business model of Rolls-Royce is directly coupled to them being the manufacturer of those engines and owner of the data it creates.


Is there any class G airspace that extends to 14,500 MSL? The U.S. Aeronautical Information Manual, part 3−2−6, lists a variety of airspace areas which may be designated type E.  From reading this section, I get the impression that "typical" class E airspace consists of regions near non-towered airports, plus everything from 14,500' MSL to 18,000' MSL.

The word on the street, however, is that this is not an accurate picture: In reality, class E airspace typically goes all the way from 1,200' AGL to 18,000' MSL, with dips down to 700' AGL around non-towered airports.  I.e. apart from a thin layer of class G airspace near the ground, almost all airspace away from towered airports is class E.  This is hard to notice on sectionals, since the only place it is mentioned is in the legend, where there is a footnote stating "Class E Airspace exists at 1,200' AGL unless otherwise designated as shown above."  I gather this widespread sort of class E airspace falls under the provisions listed in AIM 3-2-6e4 and 3-2-6e5.

So I have two questions regarding this:

Is there anywhere in the Coterminous U.S. where class G airspace extends up to 14,500' MSL?  (Excluding the trivial case of mountainous terrain above 13,000' MSL; c.f. AIM 3-2-6d1b.)
Since the existence of class E airspace from 1,200' MSL upwards is so ubiquitous, why is this not codified like the 14,500' rule is?

Historically there was much more uncontrolled airspace in CONUS. There is less but it exists. 14500 is the old continental control area, sometimes called the continental control shelf.  It was designed with the highest natural obstruction in CONUS in mind.

In uncontrolled airspace IFR operations could be conducted without ATC involvement.  I used to do this frequently.

The idea was that in the Continental Control Area, IFR ops were controlled.

For the second question, some Class G exists because there is inadequate service coverage and traffic density and the blanket classification to Class E is inappropriate.


Does an aircraft equipment suffix denote how many VOR radios are installed? I'm studying IFR. Does an aircraft equipment suffix specify if and how many VOR radios are installed?

I'm guessing that the suffix is used by ATC to know if they can give you an RNAV or a an approach that requires DME, so maybe the number of VOR radios is irrelevant? Or maybe ATC assumes that you have a certain number of VOR radios installed?

If you look at the AIM TBL 5−1−3 Aircraft Equipment Suffixes it is split into two parts. The first part has the codes for RVSM which don’t apply to you since I presume you aren’t flying above FL 290. The second part doesn’t mention VORs at all, presumably because historically VOR and DME were almost always used together for IFR navigation. So ATC doesn’t know if you have any VORs based on your equipment suffix. However, if you use /A then they would assume you have VOR capability.

If you have a GPS installed in your plane, then you would be a /G and ATC would know that they can give you an RNAV approach. If you have a GPS installed, then you can use it in lieu of DME. The footnote to Table 1-1-6 gives the conditions when it is applicable.

  3 Requires current database or verification that the procedure has not
  been amended since the expiration of the database.

ATC wouldn’t know if your database is current, so it is up to you to accept or reject the clearance.

AOPA has a good article describing when you can use GPS in lieu of ADF and DME.

There is also a StackExchange question that discusses it as well.


Do you need strong muscles as far as applying the yoke is concerned, in order to be able to deal with some emergencies? In some of the plane accidents, the pilot had to use all his might to apply the yoke in certain direction to save the plane from imminent crash; in some cases even crew members had to help with the force. So it seems certain degree of physical strength is needed to be a pilot in view of possible emergencies, though it doesn't seem to be explicitly required?

For the Airbus it is not actually require any strength, all the technology is fly by wire, this means that the actual control (Side stick) is not directly connected to any cable, so been this said, there is no actual force to fly any airbus plane. For the Boeing plane, at least for the 737, that's the one I know, when it loses all the hydraulic systems, the aircraft will become on a manual reversion mode and it will need strength from the pilot to move the aircraft controls that are linked to the surfaces by cables, but this is a remote possibility to happen, and if it is the case both pilots can apply force to the control column


Constant turn radius with Δ airspeed and bank Can someone explain how speed impacts rate of turn and and degree of bank required?

I'm trying to understand this practice problem but can't get the concept: Say you have 2 vehicles travelling at different speeds and carrying out a turn at the same radius. The example states that the faster vehicle will have to bank at a shallower angle and will have a lower rate of turn.

In my mind, the faster vehicle would have to maintain a steeper bank angle to compensate for the higher relative speed, otherwise their radius would increase. If you're moving faster, and banking at a lower angle, wouldn't that mean you're travelling at greater forward distance and less lateral distance thus increasing you're radius?

You are right on all accounts.

In a steady-state turn, from circular motion, we have, with $F_c$ being centripetal force, $V$ being airspeed in zero wind, $R$ being the radius of turn, and $m$ being the aircraft mass:

$$F_c=m\frac{V^2}{R}$$

The centripetal acceleration in a coordinated turn is created from the banked lift vector ($L$), therefore, with $\phi$ being the bank angle:

$$L\sin\phi=m\frac{V^2}{R}$$

Therefore, the radius of turn is:

$$R=\frac{V^2}{g\tan\phi}$$

and the rate of turn is:

$$\dot{\psi}=\frac{g\tan\phi}{V}$$

As speed increases, you would need an ever larger bank angle in order to achieve the same rate of turn. Of course, there's a limit to how much lift you can get out of the wing at a given speed, especially at higher Mach when transonic buffet becomes the limiting factor.


What is the function of variable inlet guide vanes in terms of pressure change? Nowadays in the turbofan Variable Inlet Guide Vanes are more and more used. My question is: which is their function in terms of pressure change?

The question is very specific: I know that they allow to give the most proper direction to the fluid, but do they act also to increase the pressure? So: what happen between the inlet and the outlet of each blade, talking in term of pressure?

I need some reference, if possible.

You need to be careful in understanding the significance of the variable guide vanes (VGVs) being variable.  You could think of them as a bit like venetian blinds -  when they they are most open they present the least obstruction to the air flow, much as you get the most light through a set of blinds when they are at the right angle to the incoming light. As you close them, you get less flow (light) through them, as they present a greater obstruction to the flow.

The VGVs are only variable to maintain stable flow in the compressor at conditions away from the design condition.  The design condition for jet engines is always high power during cruise (or at the top of the initial climb to be pedantic), so the VGVs only start to close at reduced powers.

At high power the vanes are at their most open to maximise flow through the engine, minimising pressure losses across them and maximising thrust.   As they are gradually closed when power is reduced, they present more of an obstruction to the flow (acting as effectively a variable area throttle valve in the path of the flow being closed down), so presenting an increased pressure ratio loss (or if you prefer reduced pressure recovery).

The increased use of VGVs really shows designers pushing at the limits of airflow stability in pursuit of better efficiency by maximising pressure ratio per stage at the design condition, at the cost of the increased weight of the mechanisms to drive the VGVs.

Not sure I expressed that very well, but first post on stack exchange.


Why would an airplane's nosewheel bounce during landing? A pilot recently had an issue involving the nose wheel bouncing several times before settling down during landing.  Setup was as follows:

Cessna 162 Skycatcher
Simulated short field landing.
Full Flaps, per POH
Approach speed 50 KIAS per POH
Headwind 6 knots
Good condition, paved runway
Pilot has 30 hour in this specific airplane, ~100 landings in this specific airplane and has not experienced this before.

During the touchdown, the following happened:

Pilot flared at correct altitude.
Plane touched down, main gear first.  Main gear remained on runway.
Nose wheel then bounced 3-5 times before settling down on the runway.

Note: Bounce was not pronouced enough to cause a tail strike

This felt to the pilot like a PIO, but it was not a Porpoise as described in this post.

Questions:

Is this a PIO?
Why did the nosewheel bounce?
How can the pilot prevent this from happening in the future?
Should this occur again, how can the pilot minimizes the nose bounces?

Sounds most like the pilot released back pressure  on the control stick abruptly once the main wheels touchdown. The idea here is that you gently lower the nose wheel to the pavement once the mains are on the ground in order to prevent that kind of a bounce.   As I recall the Cessna 162 has a tendency to bounce on the nose wheel as well as to porpoise if the landing attitude is not done correctly it's just a quirk of that airplane.


Did AirFone ever make money? I remember when the first AirFones were put into service in 1990s and marveled at who would be rich enough to pay $7/min for air to ground phone service.

Even though every seat eventually had an AirFone, I cannot once recall seeing one in use (except in movies) in twenty years of flying (in economy).

I know they're being pulled out now, but did AirFones ever make a profit? Who paid for their installation, was it the airlines or did GTE etc pay for everything?

Not according to the authors of The 4 A's of Marketing 1

  Despite impressive technological achievements and great expectations all around, the service’s business performance ultimately proved an abject failure. In the ten years before Verizon finally pulled the plug, Airfone generated only 50 million total calls, a fraction of the calls carried daily by cellular companies. While the Airfone service was heavily used when bad weather caused significant delays, the system’s utilization at other times was extremely low. A typical plane was equipped with as many as 60 phones. From these, the average large jet generated fewer than 100 calls per day in about 16 hours of flying time. As a result, the expensive system, with its heavy load of fixed costs, remained idle well over 99% of the time.

  ...

  While Airfone claimed that some customers’ monthly bills topped out
  at $4,000, there were not nearly enough of them to sustain the service’s sky-high fixed
  costs.

  ...

  Within a few years of the service’s launch, airlines started to remove the expensive phone
  infrastructure from their cabins, since the Airfone cost them more in increased fuel
  consumption (because of the system’s weight) than it generated revenues through their
  revenue sharing arrangement with Verizon.

My emphasis.


Emergency landing at Air Force airfield What would happen if I perform an emergency landing at an Air Force airfield?

Would they send fighters planes to intercept me?

Would they arrest me?

Would they detain my aircraft?

There are some bases that are joint-use anyway, and in that case, it would be almost exactly like an emergency landing at a civilian field. Then most bases will do little if you are truly an emergency. The times this would come into play are sensitive bases such as Nellis AFB or Groom Lake, or bases that have significant national security interests, such as Andrews AFB, or potentially a base with nuclear bombers.

In these cases, you would probably get a fighter escort if you reported intentions soon enough or entered restricted airspace. Most bases will not provide a fighter escort, as there is little they can do to help, and you should not be a threat to anything from the air.

If you declared an emergency and nothing was actually wrong, you would almost certainly be arrested. If there was an actual emergency, you would probably be detained at a sensitive base, but at most bases they would just keep you confined to unsensitive visitor's areas. Here is an incident where the pilot was forced to land at Travis AFB (which only has C-5, C-17, and KC-10 aircraft). The pilot followed emergency procedures and everything turned out well. But the report notes:

  "Had he not made contact with RAPCON, security forces would have to respond as if the aircraft was hostile," Wilson said. "His proper communication allowed us to clear the area for him and notify the appropriate agencies."

As your aircraft is not flyable at the moment, it would have to stay there until it was repaired, but in most cases you should be able to fly it out after it was fixed. In a sensitive area, I have heard reports of airplanes being disassembled and shipped off the base, but nothing concrete about it.

However, at most of the most prohibited areas the prohibited airfield is rarely going to be you best option (e.g. it would be unlikely for Groom Lake to be your only option, unless you were snooping around the edges, which they would not take kindly to). But just remember - in an emergency, you are in control until your plane is on the ground. So make
the conservative decision and land safely. If it is an emergency, and you weren't pushing limits before the emergency, the most you should get is a stern talking-to and a short detainment.


Is there any plan to make flights more interactive using smartphones? This question might be a little bit off-topic, but I'll give a try.

Me and many people are aeronautics "aficionados" that would love to know about flying data during flight. Today we all have smartphones that run apps for everything: why couldn't airplanes have an internal WIFI for only broadcasting flight data for being displayed in our smartphones (i.e. as if they were cockpit simulators) as well as video streaming from external cameras? Of course this kind of internal network's purpose would be to broadcast data, so security wouldn't be an issue.

Does anybody know if aircraft manufacturers plan to develop such kind of project?

Actually some airline did it already.

Many Airways including Qantas, Virgin American, Delta, Lufthansa and DragonAir (but Cathay not) has installed Wireless inflight entertainment system which allows passengers to access entertainment system and track flight via their mobile/tablet.

Surprisingly some LCC like Virgin Australia and Southwest also provides wireless on-flight entertainment system, though some contents is not free of charge.

On the other hand, several airline use Tablet to replace the original inflight entertainment system in upper class like La Compagnie (All business class airline), as well as some paid onboard entertainment services in LCC.


What does a passenger aircraft galley ('kitchen') look like? What equipment is installed in passenger aircraft for catering the passenger?

An aircraft Kitchen is known as the Galley and the picture is from a Lufthansa A380.

Some equipment you will see here are:

Food Oven, the door with a black knob in the middle of the picture.
Coffee Machine, with the pot, in the right side of the picture.
Large Trolleys, filled with food and/or drinks, on the floor. Weight about 25kg empty.
Trash Bin, the darker grey unit on the right on the floor. Wheeled off like everything else. Some have proper trash electric trash compactors as well in trolley format.
Fuse Panel, located in the middle along the top row. There is in general a fair bit of power going into this area.
Smaller Storage Containers, that can handle drinks or bottles for instance.
General Storage Space: The galley also provides a bit of space for anything from the children's toys and newspapers to emergency equipment. You can see some compartments for this in the upper-left corner and a symbol that a smoke hood is stored in one of them.

Quick design comment:

Aircraft galleys are hotspots for corrosion due to the liquid in the area and is the cause for particular attention during maintenance. Most aircraft galleys are generally very similar in design.

  Galley equipment comes in 2 basic universal standards - namely ATLAS and KSSU. These have their origins in legacy maintenance resource pools (ATLAS = Alitalia, TAP, Lufthansa, Air France, Sabena and KSSU = KLM, Swissair, SAS, UTA) and have subsequently developed into industry standards to enable modular equipment interchange.

These standards for instance specify the height, width and depth of trolleys. I gather that ATLAS standard is more prevalent (80% of galleys by one source) but both exist. Other ideas and concepts have popped up but since changing the stuff is very expensive there's not been a real push for it. I think I've also read that the original standard was pretty well thought through.

Naturally, this is a long-distance galley that will serve usually two meals. Equipment and size are scaled to the needs of the airline and can vary greatly between operators.


Why is FAC 1 cycled OFF to ON during a dual engine failure? I was reading in the Airbus A320 FCTM regarding the dual engine failure procedure. The ECAM tells us to recycle FAC 1 OFF THEN ON. Why do we have to recycle FAC 1 specifically? I opened the QRH and it shows this also.

Resetting FAC 1 enables the recovery of characteristics speed displayed on the PFD.
  Resetting FAC 1 also enables rudder trim recovery, even if no indication is available.
  When the hydraulic power is lost, the right aileron is lost, and is in the upfloat position.
  Rudder trim may be used to compensate for this upfloating aileron.

This is from the QRH under 'ENG DUAL FAILURE'. That answers why FAC 1 is reset.

FAC 2 on the other hand (unlike FAC 1) is not powered by the emergency electric system, see § Flight Control System - Electrical Supply. And that's why you don't cycle FAC 2.

From the same section FAC 1 is not on a hot (stand by) supply, so after a power interruption it requires a reboot (my interpretation).

(Red highlight mine)

Do not use for flight.


SR-72 --- is the speed reported in this story wrong? I was just reading this story from Sky news and reported the following

  Lockheed Martin says it is close to developing a military plane that flies at Mach 6 - six times the speed of sound, which travels at around 762mph (1,236km per hour).

Am I missing something, or is that a misprint?

6 * speed of sound at sea level = 4567.2423 miles per hour

is the speed reported in this story wrong?

No
No it is not a misprint that the SR-72 is intended to be capable of Mach 6
From the horses' mouth:

Envisioned as an unmanned aircraft, the SR-72 would fly at speeds up to Mach 6, or six times the speed of sound. At this speed, the aircraft would be so fast, an adversary would have no time to react or hide.
The SR-72’s design incorporates lessons learned from the HTV-2, which flew to a top speed of Mach 20, or 13,000 mph, with a surface temperature of 3500°F.

Yes
Yes it is not true that Mach 6 is necessarily 4567.2423 MPH
From elsewhere:

Cmglee. License:CC by SA 3


What kind of flight termination measures are taken for supersonic aircraft? Having recently read  an article that hyper-sonic airplanes carry enough energy that just flying into some target makes them as powerful as some missiles and I wonder if the supersonic planes have been around for so long, what measures were taken if they misbehaved or went out of control. I've heard of crashes with pilots, but what about terminating the aircraft mid-air after getting the pilot to safely eject. With more UAVs and drones being used, what measures are taken to terminate the flight of such a system in case of a loss of control? I know of such measures for rockets but am unaware for flights.(Commercial ones have FAA guidelines or other national ones to follow)

P.S. Are there any books on such design or discussions about such situations. Please recommend.

Aircraft like the B-58 and XB-70 Valkyrie were equipped with escape pods to aid in super sonic ejection. The pod its self came down around the seat when activated the only movement that appears to have been required was bringing you legs up a bit. Heres some video of the capsule in testing.

(source)

You can find some more info on the only XB-70 capsule ejection in the answer here. There is also some interesting info on the topic in this Physics.SE answer.

Ultimately getting lower and slower may be your only option as was the case during an SR-71 ejection.

  Since the chances to survive an ejection at Mach 3.18 and 78,000 feet
  weren’t very good, Weaver and Zwayer decided to stay with the aircraft
  to restore control until they reached a lower speed and altitude, but
  the cumulative effects of system malfunctions exceeded flight control
  authority. Everything seemed to unfold in slow motion, even if the
  time from event onset to catastrophic departure from controlled flight
  was only two to three seconds.

  It was at that point that the pressure suit proved to be very effective protection for Weaver. In fact, once it was inflated, an emergency oxygen cylinder in the seat kit attached to the parachute harness was functioning. It not only supplied breathing oxygen, but also pressurized the suit, preventing Weaver’s blood from boiling at the extremely high altitude. In this way the suit’s pressurization had also provided physical protection from intense buffeting and g-forces. That inflated suit had become like a tiny escape capsule.

There is some interesting footage of wind blast testing on flight gear and seats for the F-106 aircraft you can find here. According to some of this data the helmet and flight suits were capable of surviving wind forces in and around Mach 1.3 and the seat was capable of ejecting at those speeds. However in the case of the F-106 line of aircraft it was ultimately deemed that 0-0 ejections were more likely (and a higher priority) than high altitude high speed ejections. Since no seats were capable of both ejections the 0-0 seats were ultimately favored. (check out this for more info)


How is a runway inspection carried out? During the Concorde's takeoff run, this piece of debris still lying on the runway, was run over, cutting a tyre and rupturing it.  A runway inspection, scheduled for 15h, had not been carried out.

How is a runway inspection carried out? How often is a runway inspection done? And, what all is checked in a runway inspection?

The answer will differ a bit from airfield to airfield but here is a typical inspection procedure for East Midlands in the UK(EGNX).

The inspection is carried out first thing in the morning and last thing at night, then at 2 hour intervals throughout the day.

It is carried out by a vehicle in co-ordination with ATC who might shut the runway down for a few minutes.  The vehicle must get permission from ATC to enter the runway and must report on leaving.

Approach will co-ordinate the arrivals, by extending them slightly, so that no-one has to hold or go around (hopefully).  It is also possible to do half the runway as a departing aircraft lines up, leave the runway at a turn off, wait for the departing aircraft, then complete the inspection before the next departure.  There are several ways to do it without causing disruption.

The vehicle enters the runway and drives the complete length visually checking the items on the inspection list.

The document gives you a complete list of items to check but essentially it is:

Clear of FOD.
Markings and signs in good condition and visible.
Lighting working and undamaged.
Runway surface not damaged.
Oil and fuel spills.


How to change 737 Max to 737 NG? With the on going grounding of 737 Max, if some airlines want to change their 737 Max to 737 NG what modification is needed? The main change would be the engine and control system. Any other change is needed in order for the plane to be re-certified for flight?

This page lists all the functional differences between the MAX series and the previous 737 variants. There are quite a bit of substantial changes like the reshaped tail cone and electronic bleed system. Not only are the engines different but the mounting pylon was changed to account for the new engines so you likely cant just "bolt on" the old engines. Even the cockpit has a different panel.


How many significant digits should I use for simulating aircraft equations of motion? I have coded the equations of motion in Matlab, and I am using the ode45 function to simulate the nonlinear dynamics of F/A-18 Hornet without any controller (just the airframe). I have given initial conditions as trim conditions.

d2r = pi/180;           % degree to rad

V       =  438.653328;  % Airspeed , ft/s
beta    =  0*d2r;       % Sideslip Angle, rad
alpha   =  10*d2r;      % Angle-of-attack, rad

p       =  0*d2r;       % Roll rate, rad/s
q       =  0*d2r;       % Pitch rate, rad/s
r       =  0*d2r;       % Yaw rate, rad/s

phi     =  0*d2r;       % Roll Angle, rad
theta   =  10*d2r;      % Pitch Angle, rad
psi     =  0*d2r;       % Yaw Angle, rad

pN = 0;                 % X position in Earth Frame, ft
pE = 0;                 % Y position in Earth Frame, ft
h = -25000;             % Z position in Earth Frame, ft

During the first iteration, C_m has a very positive small value (1e-10), which leads to some pitching moment when multiplied by S, c, and qbar (even though it's small). But, as time keeps on increasing, this value of C_m goes on increasing and as a result, a large moment is created. Due to the high precision of Matlab, the pitching moment coefficient is not for the given trim condition.

My question is: should I round off the values of C_m to 3 or 4 digits? By rounding off, the C_m will be zero and I get M as zero.

The below figure shows how velocity is varying with time when the aforementioned initial conditions are given.

This figure is obtained after rounding off C_m to 4 significant digits.

This figure is obtained without rounding off C_m to 4 significant digits.

Alpha and theta values without rounding off C_m.

From your description, it's not an issue with the aerodynamic coefficients. If as you have described previously that you're modeling F-18, then the airframe should be unstable longitudinally. A 1e-10 deviation (assuming it applies to body rates, body speeds, etc. as well) is well-within typical trim error bound; in fact, a deviation is unavoidable when dealing with computational floating point. But if it's longitudinally unstable, any deviation will magnify over time exponentially.


When to use beacon, anti-collision, strobe, logo, and navigation lights? During taxi, the taxi lights are used, and I think the landing lights are required to be used up to 10.000 feet for all commercial flights.

However, my question is regarding the abovementioned lights: beacon, anti-collision, strobe, logo, and navigation lights. When are they to be used, and when should they not be used? What is the meaning and purpose of each one?

The Aeronautical Information Manual (AIM) is a great place to look for questions like this, and has this on the subject:

  4-3-23. Use of Aircraft Lights

  a. Aircraft position lights are required to be lighted on aircraft
  operated on the surface and in flight from sunset to sunrise. In
  addition, aircraft equipped with an anti-collision light system are
  required to operate that light system during all types of operations
  (day and night). However, during any adverse meteorological
  conditions, the pilot-in-command may determine that the anti-collision
  lights should be turned off when their light output would constitute a
  hazard to safety (14 CFR Section 91.209). Supplementary strobe lights
  should be turned off on the ground when they adversely affect ground
  personnel or other pilots, and in flight when there are adverse
  reflection from clouds.

  b. An aircraft anti-collision light system can use one or more
  rotating beacons and/or strobe lights, be colored either red or white,
  and have different (higher than minimum) intensities when compared to
  other aircraft. Many aircraft have both a rotating beacon and a strobe
  light system.

  c. The FAA has a voluntary pilot safety program, Operation Lights On,
  to enhance the see-and-avoid concept. Pilots are encouraged to turn on
  their landing lights during takeoff; i.e., either after takeoff
  clearance has been received or when beginning takeoff roll. Pilots are
  further encouraged to turn on their landing lights when operating
  below 10,000 feet, day or night, especially when operating within 10
  miles of any airport, or in conditions of reduced visibility and in
  areas where flocks of birds may be expected, i.e., coastal areas, lake
  areas, around refuse dumps, etc. Although turning on aircraft lights
  does enhance the see-and-avoid concept, pilots should not become
  complacent about keeping a sharp lookout for other aircraft. Not all
  aircraft are equipped with lights and some pilots may not have their
  lights turned on. Aircraft manufacturer's recommendations for
  operation of landing lights and electrical systems should be observed.

  d. Prop and jet blast forces generated by large aircraft have
  overturned or damaged several smaller aircraft taxiing behind them. To
  avoid similar results, and in the interest of preventing upsets and
  injuries to ground personnel from such forces, the FAA recommends that
  air carriers and commercial operators turn on their rotating beacons
  anytime their aircraft engines are in operation. General aviation
  pilots using rotating beacon equipped aircraft are also encouraged to
  participate in this program which is designed to alert others to the
  potential hazard. Since this is a voluntary program, exercise caution
  and do not rely solely on the rotating beacon as an indication that
  aircraft engines are in operation.

  e. Prior to commencing taxi, it is recommended to turn on navigation,
  position, anticollision, and logo lights (if equipped). To signal
  intent to other pilots, consider turning on the taxi light when the
  aircraft is moving or intending to move on the ground, and turning it
  off when stopped or yielding to other ground traffic. Strobe lights
  should not be illuminated during taxi if they will adversely affect
  the vision of other pilots or ground personnel.

  f. At the discretion of the pilotincommand, all exterior lights
  should be illuminated when taxiing on or across any runway. This
  increases the conspicuousness of the aircraft to controllers and other
  pilots approaching to land, taxiing, or crossing the runway. Pilots
  should comply with any equipment operating limitations and consider
  the effects of landing and strobe lights on other aircraft in their
  vicinity.

  g. When entering the departure runway for takeoff or to “line up and
  wait,” all lights, except for landing lights, should be illuminated to
  make the aircraft conspicuous to ATC and other aircraft on approach.
  Landing lights should be turned on when takeoff clearance is received
  or when commencing takeoff roll at an airport without an operating
  control tower.


What should be the minimum time spent in cruise (for e.g. a B737)? Having a short sector to fly, it would not be convenient to climb all the way up to the best cruise altitude, since the time in climb would burn more fuel than that necessary for a flight at lower altitudes.

Having said that, what would be the minimum time spent at the best cruise altitude that will make it reasonable to climb to that pressure altitude with a medium range twin jet transport airplane (B737 or similar)?

Above is a simple model to break down the problem into its simplest form. Of course the model can be refined. Assuming for a short flight that the ground speed doesn't change much from after takeoff to before final approach, it becomes apparent that reaching the top and descending right away gives the shortest time burning fuel (blue line) and longest time with the engines idle during descent.
Operationally you need a few minutes in cruise for ATC purposes, approach briefing, etc. So the 5 minutes mentioned by @CptReynolds in a comment makes sense.
The same model should hold true for long haul flights, only if aerospace materials and propulsion would efficiently allow a sub-orbital flight from say London to LA, but alas that's not the case.
Refining the model

* Half-climb and half-descent.
Here the flight is divided into equal time segments (maintaining the 'same ground speed' assumption). I've taken into account the shallower descent rate, and the decreasing climb rate / thrust / fuel flow the higher the jet climbs. The numbers indicate the fuel flow (FF) ratios per time-segment.
With real FF figures, further refinements can be made.
Real Boeing 737 FPPM manual (flights <500 NM)

(Click to view)
Here it is very clear that the longer the short flight (even by a little), the higher the cruise altitude. There is not a point where there is a cut-off. And from the right-hand side: the lighter the plane, the higher it should cruise.

Note regarding the cruise FF ratio being fixed at 1.25: looking at a 737 AFM, the FF doesn't change by much from FL250 to FL410 at the same weight (2374-2458 lb/hr/eng; the Mach number rises though). So, for short hops with lower cruise levels, I've maintained the cruise FF for that simple model.