Why can't jet engines operate with supersonic air and how do they slow it down? Typically jets cannot operate when intake airflow is supersonic relative to the engine. Why is this so? Also, why are scramjets able to use supersonic air?

To slow down the air to subsonic speeds, the air passes through a shockwave (if I understand correctly). How does this slow down the air?

In a nutshell

A compressor blade works best in subsonic flow. Supersonic flow introduces additional drag sources which should be avoided if efficiency is important. Thus, the intake has to slow down the air to a Mach number between 0.4 and 0.5. Note that the high circumferential speed of a large fan blade will still mean that its tips work at around Mach 1.5, but the subsequent compressor stages will operate in subsonic conditions.

A scramjet is possible with fuels with supersonic flame front speeds and rapid mixing of fuel and air. If the engine would burn regular kerosene, the flame would be blown out like a candle if the internal airspeed would be supersonic, and even if flame holders keep the flame in place, most combustion would take place only after the fuel-air mixture has left the engine due to the slow mixing of kerosene and air. By using hydrogen, a stable combustion can be achieved even in supersonic flow. Due to the high flight speeds, compression is possible by a cascade of shocks, so no moving turbomachinery is needed in ramjets and scramjets.

Background: Maximum heating of air

All jets decelerate air in their intake in order to increase air pressure. This compression heats the air, and in order to achieve a combustion which produces thrust, this heating must be restricted. If air is heated above approx. 6,000° K, adding more energy will result in dissociation of the gas with little further heat increase. Since thrust is produced by expanding air through heating, burning air that enters the combustion process already at 6,000° K will not achieve much thrust. If the air enters the intake at Mach 6, it must not be decelerated below approx. Mach 2 to still achieve combustion with a meaningful temperature increase - that is why scramjets are used in hypersonic vehicles.

Full disclosure: Oxygen starts to dissociate already between 2,000° and 4,000° K, depending on pressure, while Nitrogen will dissociate mainly above 8,000° K. The 6,000° K figure above is a rough compromise for the boundary where adding more energy starts to make less and less sense. Of course, even a 6,000° K flame temperature is a challenge for the materials of the combustion chamber, and ceramics with film cooling are mandatory.

The equation for the stagnation temperature $T_0$ of air shows how important the flight speed $v$ is: $$T_0 = T_{\infty} \cdot \frac{v^2}{c_p} = T_{\infty} \cdot \left(1 + \frac{\kappa - 1}{2}\cdot Ma^2 \right)$$

$T_{\infty}$ is the ambient temperature, $c_p$ the specific heat at constant pressure and $\kappa$ the ratio of specific heats. For two-atomic gases (like oxygen and nitrogen), $\kappa$ is 1.405. Temperature increases with the square of flight speed, so at Mach 2 the factor of heat increase over ambient is only 3.8, while at Mach 6 this becomes 26.3. Even at 220° K air temperature, the air will be heated to 5,800° K when it is ideally compressed in case of a hypersonic vehicle traveling at Mach 6. Note that real compression processes will heat air even more due to friction.

Compression with shocks

Supersonic flow is slowed down by a pressure rise along the flow path. Since no "advance warning" of what is coming is possible, this pressure rise is sudden: Pressure jumps from a fixed value ahead to a higher, fixed value past the jump. This is called a shock. The energy for the pressure rise is taken from the kinetic energy of the air, so past the shock all other parameters (speed, density and temperature) take on new values.

F-16 air intake (picture source)

The simplest shock is a straight shock. This can be found at the face of pitot intakes like the one of the F-16 (see the picture above) in supersonic flight. More common are oblique shocks which are tilted according to the Mach number of the free flow. They happen on leading and trailing edges, fuselage noses and contour changes in general: Whenever something bends the airflow due to its displacement effect, the mechanism for this bending of the flowpath is an oblique shock.

straight and oblique shock (own work)

The index 1 denotes conditions ahead of the shock, and 2 those downstream of the shock. For weak straight shocks the product of the speed ahead of the shock $v_1$ and the speed past the shock $v_2$ equals the square of the speed of sound: $$v_1\cdot v_2 = a^2$$
If $Ma_1 > 1$, then $Ma_2$ must be smaller than 1, so the flow is always decelerated to subsonic speed by a straight shock.

The same equation works for the normal speed component $v_n$ ahead and past a weak oblique shock: $$v_{1n}\cdot v_{2n} = a^2$$
Note that the tangential component $v_t$ is unaffected by the shock! Only the normal component is reduced. Now the speed $v_2$ is still supersonic, but lower than $v_1$, so a weak oblique shock produces a modest increase of pressure, density and temperature.

The angle of the oblique shock wave is determined by the Mach number ahead of the shock.

Supersonic intakes

Weak shocks are desired, because they produce only small losses due to friction. Pitot intakes with their single, straight shocks work well at low supersonic speeds, but incur higher losses at higher Mach numbers. As a rule of thumb, a pitot intake is the best compromise at speeds below Mach 1.6. If the design airspeed is higher, more complex and heavier intakes are needed to decelerate the air efficiently. This is done by a sequence of weak, oblique shocks and by means of a wedge intake. The picture below shows the intake of the supersonic Concorde airliner:

Concorde intake (picture source)

Gradually increasing the angle of the wedge is causing a cascade of ever steeper, oblique shocks which gradually decelerate the air. The design goal is to position this cascade of shocks caused by the wedge on top such that they hit the lower intake lip. This is done by a moveable contour of the upper intake geometry and/or the lip. The goal is to achieve a uniform speed over the intake cross section and not to waste any of the compressed air to the flow around the intake. See the picture of the Eurofighter intake below for an example of a moveable intake lip (which admittedly is mainly for increasing the capture area at low speed and for avoiding flow separation even with a small intake lip radius).

Eurofighter intake (picture source)

Once the air has entered the intake, it is only mildly supersonic and can be further decelerated by a final, straight shock at the narrowest point of the intake. After that point, the intake contour is gradually widened, such that the air decelerates further without separation. To achieve this, a very even flow across the intake area is mandatory, and even the slight disturbance caused by the boundary layer of anything which is ahead of the intake must be avoided. This is achieved by a splitter plate which is clearly visible in the pictures of the F-16 and Eurofighter intakes. The splitter plate of the Eurofighter intake is even perforated to suck away the early boundary layer there.

The deceleration of the intake flow results in a significant pressure rise: In case of the Concorde at Mach 2.02 cruise, the intake caused a pressure rise by a factor of more than 6, so the engine compressor had to add "only" a factor of 12, such that the pressure in the combustion chamber of the four Olympus 593 engines was 80 times that of the ambient pressure (admittedly, this ambient pressure was only 76 mbar in the cruise altitude of 18 km).

This pressure increase means that a supersonic intake must be built like a pressure vessel, and the rectangular face of the intake must quickly be changed to a round cross section downstream to keep the mass of the intake structure low.

Intakes at higher speed

Going faster means the intake pressure recovery increases with the square of flight speed: In case of the SR-71 intake at Mach 3.2, the pressure at the engine face was already almost 40 times higher than the ambient pressure. Now it becomes clear that going faster than Mach 3.5 does away with the need for a turbocompressor: At these speeds a properly designed intake can achieve enough compression by itself for the combustion to produce enough thrust, and going above Mach 5 will need restraint in slowing down the intake flow in order to have enough temperature margin for combustion, requiring supersonic flow in the combustion chamber.


What causes a plane to lose its roll momentum I am currently developing a flight simulator but there is one force I can't figure out how to simulate using real world physics. When the pilot pushes the stick to the right, the plane will reach a roll rate that it will keep; and when the stick comes back to its neutral position, the plane will stop rolling, more or less quickly.

Is there a formula, or an approximation that explains how the plane will stop its roll? On what does it depend? Apart from wing area, does airspeed and lift impact this? Or does it depends on angular velocity?

I need a formula that I can compute to slow my roll down.

What you want is the roll constant $\text{T}_R$. This is basically one of the characteristics wich determines the equations of motion of an aircraft. It gives the slope of the roll speed increase over time with full aileron deflection and an ideally stiff wing, and equally the rate of decrease once the ailerons are set to neutral during a rolling maneuver.

With p the dimensionless roll rate, m the aircraft mass, i$_x$ its radius of the momentum of inertia about the roll axis, S its wing surface, b the wing span, v the flight speed, $\rho$ the density of air and $\text{c}_{lp}$ the roll damping coefficient, the formula is

$$T_R = \frac{2\cdot m\cdot\left(\frac{2\cdot i_x}{b}\right)^2}{\rho\cdot v\cdot S\cdot c_{lp}}$$

When the pilot moves the stick, the aircraft will accelerate into the roll but the acceleration will diminish with the square of roll speed until an asymptotic value is reached. This acceleration grows with increasing air density, flight speed, roll damping coefficient and lower wing loading and smaller square of the ratio of roll inertia over wing span. Same in reverse: Stopping will be fastest under the same conditions.

For the roll damping coefficient, use this approximation for wings with an aspect ratio AR larger than 4:

$$c_{lp} = -\frac{1}{4}\cdot\frac{\pi\cdot AR}{\sqrt{\frac{AR^2}{4}+4}+2}$$

Since this is a damping coefficient, it makes sense to be negative.

The asymptotic value is reached when the propelling moment from aileron deflection equals the retarding roll damping:

$$c_{l\xi} \cdot \frac{\xi_l - \xi_r}{2} = -c_{lp} \cdot \frac{\omega_x \cdot b}{2\cdot v_\infty} =  -c_{lp} \cdot p$$

For an explanation of this equation and all the terms used please see this answer.

Please note that this is all only valid for a stiff airframe. Increasing dynamic pressure reduces aileron effectiveness because the wing will warp when ailerons are deflected. Assume a linear decrease with dynamic pressure until only a fraction of the ideal roll acceleration remains at top speed and low level flight.

Now you ask about roll damping torque and that cannot be explained in the comments. Look at the last equation – it is already there, albeit dimensionless. To get from there to an actual torque, multiply by wing area, semispan and dynamic pressure:

$$T=\frac{\rho}{2}\cdot v^2_\infty\cdot S\cdot\frac{b}{2}\cdot c_{lp}\cdot p = \frac{\rho}{8}\cdot v_\infty\cdot S\cdot b^2\cdot c_{lp}\cdot\omega_x$$

with $\omega_x$ the actual angular speed in rad/s. Do the unit check - this is actually a torsion moment [Nm]. Please note that I used the reference length for lateral moments used in Germany; the US uses the full span instead of the semispan. So make sure you check which reference length your sources use!


What are the differences between pre-transistor and transistor era jet engines? What are the differences between pre-transistor and transistor era jet engines? Towards the end of the second world war, most of the air powers had started development on jet engines. From what I understand, all of this predated any practical application of transistors. The transistor era obviously gave way to integrated circuits and microprocessors. On any piston engine I can usually spot the evolution of components because I'm well versed in the operation of piston engines. For jet engines it is less obvious to me, there are for example differences between turbofan and turbojet engines. I superficially understand the difference between them but I've never even operated one, much less been hands on with it.

What are the significant changes from pre-transistor to transistor-era jet engines? I've never seen vacuum tubes used on jet engines so it doesn't seem like it is just a matter of "they replaced the tubes".

The vibration levels on jets preclude the use of vacuum tubes. They may work if everything is fine and all rotating parts are well balanced, but once you have a defect and an out-of-balance condition, the electronics would fail quickly, just when you need their help the most. Especially the afterburner adds a lot of vibration when it works as designed already.

Earlier jet engines used hydraulics for adjusting things like compressor vane angles (litte stationary "wings" positioned between the rotating discs of the compressor, which help to convert rotational flow energy into compression) and the nozzle size. Like on piston engines, the amount of adjustment was much lower back then, and all adjustments were done mechanically.


Is there a cockpit indication if the B737 flap load relief is not working properly? On a 737, does the pilot get any cockpit indication that his Flap Load Relief is not working properly? Say for instance due to a Flap Lever Position Switch failure?

The cockpit indication would be that the flaps don't retract as expected. The subsequent flap overspeed would then be captured by the Flight Data Recorder, the same as any other exceedance. But I'm unaware of any specific indication on a cockpit display that indicates directly "the Flap Load Relief isn't working".


Why ETOPS 207 minutes? There are many ETOPS certification levels, such as 120 (2 hours), 180 (3 hours), or 300 (5 hours).  However, what is the reason behind ETOPS 207?  Is there anything specific at that point?  Why not make it ETOPS 210 (3.5 hours) altogether?

ETOPS 138 and 207 are 15% extensions of ETOPS 120 and 180.

The reason for that is that with the ETOPS 120 rule, most of the Atlantic could be covered when using Kangerlussuaq (Greenland), Gander (Newfoundland), Lajes Field (Azores) and Keflavik (Iceland) as alternative airports. Only some small triangles of the Atlantic were outside the 120 minutes flight time limit from these airports. To cover the whole Atlantic, the next step up in ETOPS (180) would be necessary. But since that is such a big difference, a 15% extension of 120 minutes, ETOPS 138, was approved which allows for covering all of the Atlantic with few extra requirements.

For ETOPS 180 it is similar, but then for the Pacific. Normally ETOPS 180 is sufficient to cross the Pacific, but ETOPS 207 allows the 777-200ER/-200LR/-300ER to cross the Pacific ocean even when Eareckson Air Station on the western end of the Alaskan Aleutian Islands is closed.


Do experienced pilots look at the inclinometers during flight? I have been researching on the use of rudder during flight e.g. during turns and noticed that several novices have issues using the rudder to achieve coordinated turns.

Often, I see comments stating that the "sense" on the use of rudder will develop over time / rudder use becomes second nature sooner or later / you should turn based on seat of pants etc

Question: Do experienced pilots of single engine propeller aircraft actually look at the inclinometer or do they solely use their "senses" to make coordinated turns? If they do use their senses, does that mean that "stepping on the ball" is no longer applied when you become experienced?

Yes to some degree but it depends a lot on your seating position.  If your torso is fairly upright and not too well supported, slips and skids become easy to sense, beyond a certain point, as your waist muscles have to work to keep up upright as you "lean into the ball".

With experience it's possible to stay within, say, half a ball width of centre, without looking at it.  It's tougher if the airplane has reclined seats or has seats with strong side bolsters that hold you in well.

That being said, you still need to glance at the ball it to keep it really centred while maneuvering and I tend to use both my leaning sense and glances at the skid ball at the same time, leaning sense for large yaw excursions and the skid ball for fine tuning.

It's a good skill to master when VFR flying because you can keep your head outside the cockpit where it belongs (in VFR) a lot more.

Gliders use a yaw string for skid control and because you are steeply reclining the leaning sense is fairly useless and you rely mostly on the string, which is easy because it's right in front of your face and you don't have to put your head down.

In any case, you still "step on the ball" or "step toward the lean".  It helps to develop the automatic foot action so you squeeze the pedal with your foot instinctively whenever you move the ailerons.  Pilots who learn on gliders have this action fully automated.


How fast can an F-22 scramble? The startup process in many military aircraft is, complicated, to say the least.
It takes a lot longer than most would think. An F-16 from cold is required to be able to scramble within 5 minutes if on alert (armed, fueled, and pilot ready), 15 minutes if not.
I also know that during the Cold War when the situation was expected to deteriorate, they could deploy B-52's to all runways (civilian airports included) that were longer than 9,000 feet, just sitting there with engines running ready to go. (Terrifying to think about...)
I also found this:

Default NATO QRA alert time is 15 minutes (in DEFCON 5), although the local CRC can lower that to ten (DEFCON 4), five (DEFCON 3) or even two minutes (DEFCON 2) depending on current state of affairs.
At 5 minutes (DEFCON 3), the engines would be running idle 24/7, at 2 minutes (DEFCON 2) there are pilots in the cockpit 24/7.

Anyways, an F-16 can scramble in 5 minutes. But an F-22 has an automated start process. IIRC, the checklist is something like...

Master Switch On.
Throttles Forward to idle.

I understand that this may be classified, but I was wondering if anyone can put me in the ballpark.

For military aircraft on "alert", there is a position called cocked on (or as some news sites call it, "hot-cocked").  This procedure is run before the aircraft is placed on alert, and does a pre-flight check of all the instruments and then places them in a state that allows power application in a rapid manner.  This way, only minimal steps are required for the aircraft to be ready.  In addition to cocking on the aircraft, they can be parked in special areas (some are called Christmas Trees because of their appearance). This allows the aircraft to reach the runway quickly. Also, in the old SIOP days, there was an Alert Facility near where the aircraft were parked, where the crew would stay in a state of readiness.

Currently the full SIOP mission isn't active (it was ended in 1992 by President Bush, but there are still forces dedicated to it in a new capacity). However, the facilities still exist on many bases.  They can use these facilities still for whatever alerts are required.

With an aircraft in cocked on position, with a pilot on alert (cockpit or facility), an aircraft can take off in a very, very short time (from seconds to minutes).  In general, the mission requirements will dictate the amount of time that the pilot is given to take off (as you note in your question).  So sadly, I can't give a specific answer to your question, because it depends on parameters you haven't specified.


Why and how do atmospheric properties sometimes vary from the ISA? I'm curious as to why sometimes atmospheric properties don't match the ISA's. One reason I am considering is due to calibration discrepancies. By atmospheric properties I mean temperature, Density, Pressure etc.

An ISA standard atmospheric day is really more like a global average.  Locally and regionally, there will always be deviations from the "norm" based on the natural cycles of the sun.  Overnight the temperature will cool, then the sun will rise and things will warm up again.  This phenomenon also produces variations in the pressure and density over larger areas.

Otherwise, what specifically do you not understand about how and why local and regional conditions are not always "standard"?


What are the engine-out implications of giant turbofans? It seems that turbofans for civil airliners are improving fuel efficiency by building larger and larger fans that spin slower.  For example, the RR Trent 800 introduced in 1993 had a diameter of 2.8m, while the RR Trent Ultrafan expected in 2025 may be 3.56m in diameter.
In the case where both engines fail in flight, I imagine that wider engines would create much more drag, reducing glide distance.  Is this true, and if so, what do aircraft manufacturers and airlines do to mitigate it?

Both-engine-out glide performance isn't accounted for and isn't a certification requirement (of any airplane that I'm aware of).  Airliners have good glide performance as a happy side effect of design optimization for cruising at high altitudes.
What does have to be accounted for is single engine performance issues; things like departure performance with an engine failure at V1, single engine ceilings, etc.  Gliding ability comes into it in a secondary way, in terms of what is called "driftdown".  Driftdown is when you are forced to descend from a both engines cruising altitude to the single engine ceiling following an engine failure or shutdown when your power is cut in half, and  could be thought of as "semi-gliding" or "power assisted" gliding performance.
Driftdown speeds are speeds to use to get the flattest descent with maximum continuous thrust on the good engine, and you may use these speeds where terrain clearance means you need to stay at high as possible for as long as possible. Driftdown performance is measured during certification for the purpose of providing data for the pilots in the QRH, but there is no minimum driftdown "flatness" or glide ratio requirement per se.
As far as designing around geared TFs, well, the effects are what they are, and since nobody makes feathering fan blades, you are stuck with whatever the drag penalty is. A windmilling GTF is going to have more windmilling drag than a direct drive TF. But a direct drive TF has more windmilling drag than a traditional turbojet.  So what did the manufacturers do when TFs came out?
They made allowance for the differences, and maybe an airliner would need somewhat more thrust from a given engine in TF form, all else being equal, to accommodate the higher windmilling drag of a failed engine on the other side, compared to the same airplane with pure turbojets. The drag penalties are not as bad as windmilling propellers, and don't necessitate radical solutions like feathering fan blades.
Geared TFs just take the same problem a little farther, but still get away without needing things like feathering fans (which would have killed the entire GTF concept) by making sure the thrust reserve is there to deal with a windmilling fan.  But if a GTF equipped airplane ends up with a both-engines-out glide ratio of 12:1 instead or 14:1 because of the drag of its super-size fans, that's neither here nor there.


Why don't bombers have close-in weapon systems (CIWS)? Besides the TU-22 radar-controlled rear-cannon, why aren't there close-in weapon systems (CIWS) on planes to defend against incoming missiles? The tech seems to be there.

The main reason is the same reason why CIWS are not used on ground vehicles - anything smaller than a ship just doesn't have enough space/weight allowance to fit one, let alone two. The lightest CIWS I found, an outdated version of Soviet AK-630 system, weighs in at 1 ton (later versions are much heavier), has a turret ring 1m across, is 1 meter high above deck and needs 2m of space below (which means that installing one would also going to be aerodynamically awkward) - and all of this does not include the ammo supply (2000 shells for this version translate to additional 0.2 tons, later versions, again, had more ammo). All modern CIWS weigh at least 6 tons, are considerably bigger, and some of them also need an external targeting radar - and I assume not all bombers have the equipment needed by default. Aircraft also has more approach vectors to cover - a ship only needs CIWS installed on top, but a bomber would also need defence in the lower hemisphere; so for complete coverage we would need even more guns, which would mean even less speed and payload.

Even if we would manage to fit some in - gun-type CIWS effectiveness is disputed. Their limited range means that even a succesfull kill on an incoming missile might not protect the ship from an impact - and what would be a minor inconvenience for a warship, will be a deathblow to an aircraft. Missile-based defences are usually considered a better choice, as they can engage and destroy incoming missiles at significant range from the protected object - but they are even heavier and bulkier than guns.

And last, but not least - modern strategic bombers tend to eschew defensive armaments at all, because their main weapons are missile-based, and imply a launch at safe distance from the enemy.

Thus - modern CIWS on a bomber are impractical. They are heavy, bulky and do not provide immunity to missiles, so might as well stay on the ground.

P.S. On a closer inspection, several other bombers had or have radar-controlled tail guns - Tu-95 or early B-52, for example. B-52's tail turrets were removed after a friendly-fire incident where radar-homing HARM missile locked onto the gun's targeting radar - another downside that I haven't noticed before.


Other than fighters, which airplanes can fly straight up? I would think, for example, all modern fighter jets can maintain 90 degree climbs due to having thrust to weight ratio greater than one.
Are there any other air planes which can do that?

The general answer to your (general) question is: all airplanes can do that. With every air plane you can accelerate to maximum speed pull the stick until you reach 90°.
The problem is that not all planes will be able to recover safely from that maneuver.
Most planes will not maintain enough speed to continue the flight in a controlled manor.
That's why the flight management software of modern Airliners will prevent that from happening.
On the other hand there are quite a lot planes out the able to do acrobatics like this. They have specially designed  aerodynamic behavior and, if powered,  mass/thrust ratio. The powered versions also need special engines that keep running at zero or negative G.

Are you certain that every plane has enough inertia at Vne and enough elevator authority to reach 90° before stalling? – StephenS

My claim holds, when any aircraft can reach a pitch of 90° before its speed is 0 and the pitch rate does not exceed the structural limits of the plane.
When flying strait upward gravity is slowing down the aircraft with 1G (9.81m/s²).
The time to decelerate an aircraft to 0 with 1G is=
t = v/1G

While "rotating" from horizontal flight to 90° angle this deceleration gradually raises.
Therefore real time til full stop is longer. But lets keep the calculation simple.
For a Sailplane (no thrust) like an ASK21 top Speed is 250km/h = 70m/s
t =  70m/s / 9,81m/s²
t =  7,135s

The ASK21 must be able to pitch to 90° in roughly 7 second, which is a pitch rate of 12° per second (and it is, I've done it already... ;o))
Lets recalculate for an A380:
Cruising speed 900km/h = 250 m/s
250m/s / 9.81m/s² = 25s
90° / 25s = 3,5°/s

On a quick search I could not find a resource giving the maximum pitch rate of an A380, but I'm pretty sure it is higher.
Conclusion: The faster the plane the more time is available and the less pitch rate is needed to rotate to 90°.
And yes, I keep my statement that (from a physical point of view) every aircraft can reach 90° pitch. But may not safely recover from it.


What are the consequences of flying GA without a license (certificate)? As far as I can tell, in the U.S., 49 USC 46317(a) makes flying without a proper certificate a crime punishable by fine and/or imprisonment.  However, the FAA is a regulatory agency, which means it can only assess civil penalties (e.g., civil fines or revocation of certificates).

In practice I've heard (and would expect) the FAA gets quite exercised over "airmen" found to be carrying passengers for hire without current and proper certificates.

What I'm wondering is whether individuals have ever been, or are routinely, subject to any scrutiny or regulatory or prosecutorial action if they fly without certificates and don't commit any other egregious violations of FAA regulations?  E.g., if you only fly registered aircraft for your own amusement, and you never break a serious airspace rule, but you never get an FAA certificate, could you expect to go your whole life without any trouble from the FAA?  If not, what sort of trouble might you expect and how would you get into it?

And has anyone ever faced criminal prosecution for flying without proper certificates who did not also commit a gross violation of safety while operating an aircraft for hire, or some other felony (e.g., smuggling)?

First, if you fly privately without a license can you avoid trouble with the FAA? Yes, if you don't attract attention to yourself.

It's essentially the same as driving a car without a license or insurance: it happens because some people just don't see why they should have to get a license, and it's possible because no one regularly checks every car and every driver. Anecdotally, there are plenty of pilot stories about unlicensed pilots in rural, remote areas who fly out of private strips or small, uncontrolled airports. Alaska in particular is notorious: I've heard the 'joke' several times that the FAA used to have the goal of getting at least 50% of Alaskan pilots certificated.

Second, if you do get caught by the FAA or by law enforcement, what happens? As you said, the FAA can revoke your certificate but state or federal officials can go after you with criminal charges. This is from the criminal indictment of someone who was accused of lying about his pilot's and mechanic's certificates:

  At sentencing he faces a maximum of five years in prison and a
  \$250,000 fine for each of the two criminal counts of making false
  statements to the FAA, and a maximum of three years in prison and a
  \$250,000 fine for each of the three counts of flying without proper
  authorization.

I couldn't find out what happened with that case, though, so I don't know if he was found guilty or not and penalties - if any - he really faced. @mins found another example that did result in a prison sentence.

Finally, there is one legal way to fly in the US without a license: fly an ultralight under part 103.


Why complex landing gears are used instead of simple, reliable and light weight muscle wire or shape memory alloys? Muscle wires actuators are really simple, have a high power density and are reliable. Landing gears on other hand are used only twice per flight while taking off and landing (if everything goes well though). During flight these machines are not useful and there are numerous gear up landings because of landing gear failures.

What is so complex in landing gears?

Why are electric motors/hydraulic actuators used instead of muscle wire actuators in landing gear which are simple, light weight, reliable and cheap?
(If there are electric power requirements, that can be achieved by supercapacitors.)

Shape-memory alloys change their shape with temperature, with control usually achieved with electric heating in one direction, and back with convective heat transfer to the environment. There is one specific temperature where the change in shape occurs. There are two huge problems with this:

The operating temperature range for gear actuators is huge. It may easily be below -40°C on a cold arctic night, but on a hot tropical day the sun can easily heat the structure to upward of +60°C. That means the transformation temperature must be pretty high, but that means a lot of energy has to be put to heating it, especially if the ambient temperature happens to be low on the day.

Note that the actuators are not very energy efficient. A super-capacitor can give you high peak power if you need that, but will not help at all with the total energy. The work the actuator has to do is significant, so the efficiency matters.
Worse, a shape-memory alloy actuator requires constant power to remain at one end of the motion range and spontaneously returns to the other. However, gear must be stable in both end positions without requiring energy, so there would still have to be uplocks and separate actuator for extension or retraction (or maybe extension would be gravity-only, but then you have to ensure it gets reliably locked). Not really simple anymore.

I also don't think they actually scale to the required forces and displacements well. A 2 mm wire bends quite easily, 2 cm rod, not so much. The thicker will of course need more force to bend—which is OK—but it will also break much sooner, because the difference in strain is bigger.


What are these FAA facilities in Seattle doing? While walking in Discovery Park I discovered this radar dome with FAA notices on its fence.

And not far away I cam across this small FAA signed radio antenna and shack.

Any idea what these facilities do?

My guess would be the radar is used by Seattle Center and that the small antenna is a marker beacon for southbound approaches to SeaTac.

Seattle’s "golf ball" in Discovery Park was once part of the country’s Nike Hercules air defense system of missiles and radars in the 1950s. While most of the structures have been torn down, this golf ball still stands as a relic of the Cold War.
Discovery Park used to have an array of radars at its peak. It was a Nike Ajax air defense site in 1953 before becoming an Air Force command center for the Nike Hercules system. The remaining radome was built in 1959. It is not currently in use.

https://blog.seattlepi.com/thebigblog/2013/04/09/what-is-that-giant-golf-ball-in-seattle-sky/#photo-226558

Initially assigned a Permanent ID of RP-1 and later a Sage ID of Z-1. Abandoned by the Air Force in 1963 and turned over to the FAA, now known as Fort Lawton FAA Radar Site.

https://seattlediscoverypark.org/faa-tower/

More information:

https://en.wikipedia.org/wiki/Project_Nike


Why is it forbidden to wear glasses when leaving the plane during an emergency? On my last flight with Iberia I noticed in the safety guidelines that you are not allowed to wear glasses during an emergency.

If I have to take off my glasses I would have serious problems seeing what is going on. So this makes no sense to me.

Why would I have to remove my glasses?

This is because they don't want people holding up the evacuation by searching for their glasses.   It does not mean you are not allowed to bring glasses if you are wearing them but passengers blocking the aisle to search for their glasses would slow the evacuation and risk lives.   They do want you to actually take off your high heels and leave your luggage because these can actually slow you down.   This should be portrayed more clearly.


Why is it advised to turn the aileron away from quartering tail-wind during taxiing? This question is related to a single engined cessna tricycle gear airplane. e.g, C152/C172.

The theory states that if the quartering tail-wind is from the right, the aileron needs to be all the way to the left with elevator held down. Can someone explain the physics behind this?

I understand this when you have a quartering headwind from the right. In this case you set the aileron on the right thereby reducing the lift on the right wing and increasing the lift on the left wing. This helps keep the right wing pressed down. But somehow the picture is not clear for the quartering tailwind.

Another way to look at this is to look at the wind as a vector. For a quartering headwind, the wind can be thought of as a wind component from the front and a wind component from the right. The wind component from the front is not a problem and the wind component from the right is balanced by the ailerons as discussed earlier.

Taking a similar approach for the taiwind from the right, the wind component from behind is taken care of by the elevator. That leaves only the wind component from the right. So the same aileron action should be required. But the books state the complete opposite.

The mnemonic that I use is, "climb into headwind, dive away from tailwind."

To address your question.

When the wind is a quartering tailwind, the tailwind component tends to want to lift the tail of the airplane, which could cause a prop strike or the airplane to be flipped over if the wind is strong enough. So by having the elevator all the way down, the wind hitting the tail will push the tail down instead of lifting it up.

The quartering tailwind hitting the trailing edge of the up-wind wing can lift the wing, so by diving away from the tailwind, the upwind aileron will be in the down position, which will help hold that wing down.

With regard to the physics of how this works, it has to do with the direction in which the wind is being deflected. When wind is coming from the front (headwind) an up aileron deflects the air up and pushes the wing down, but when the wind is coming from behind the wing, such as in a quartering tailwind while taxiing, the wind would catch an up aileron and lift the wing, so it is necessary to put the aileron down, so that the wind is deflected upwards thereby pushing that wing tip down.


Is skin friction drag affected by material? Normally, friction between surfaces is affected by the materials. Can skin friction be reduced by a low friction material like Teflon?

Indirectly, yes. Friction goes up when dirt and smashed bugs accumulate on the wing. A low surface energy coating might well reduce their stickiness, so the wing collects less dirt and has less roughness. This is especially helpful when it helps to delay the transition from laminar to turbulent flow.

Another way to delay that transition would be a soft surface which could help to dampen the characteristic Tollmien-Schlichting oscillations from which the transition of boundary layer flow from laminar to turbulent springs. Accelerated flow is naturally damped (the amplitude decreases relatively to flow speed), so it stays laminar on a smooth wing. Once the damping turns negative (i.e. the oscillation is increasing), the turbulent transition is imminent. When the wing is not only smooth, but also soft and helps to dampen those Tollmien–Schlichting oscillations, even unaccelerated flow might stay laminar for longer, which again reduces friction drag. So far, I am unaware that this effect has been exploited successfully on an aircraft, though.

The friction between gaseous substances and solids is not affected by the surface energy of the solid, so a direct reduction in viscous drag due to a polytetrafluoroethylene coating will not happen.

I trust you know how drag depends on surface roughness. If not, please feel free to post another question.


What kind of dihedral would be most stable? What kind of dihedral would be most stable?

An airplane with wing span x ft and whose dihedral begins from the root(attachment point to the fuselage) of the wings and whose wing tips are y ft above ground level.
An airplane with wing span x ft and its dihedral begins from the mid point of the wings from the wing root and its wing tips are y ft above ground level.
An airplane with wing span x ft and its dihedral begins at a distance 2/3 of each wing from the wing root and its wing tips are y ft above ground level.

Note: They are all low wing airplanes of equal size, power, weight, wing span, wing chord etc. Only the starting point of the dihedral and the angle of dihedral  vary.

I also believe a polyhedral wing will be most stable(everything being equal). Am I right?

More dihedral doesn't necessarily make an airplane more stable. With too much of it, oscillations like the Dutch roll are not damped any more.

A polyhedral wing can be chosen for different reasons than just stability: With the dihedral of the outer panels, any angle of attack change there is reduced by the cosine of the dihedral angle. This means that the lift curve slope is flatter there and stall happens later. This is a great way to combine an almost elliptical circulation distribution with pleasant stall characteristics, similar to a raked wingtip, but using dihedral instead of sweep!

For the stability derivatives of partial dihedral I recommend to turn to no other source than Sighard Hörner's Fluid Dynamic Lift. In chapter XIV it shows a plot of a rectangular wing with an aspect ratio of 6 where the location of the onset of the dihedraled wing fraction is plotted over the X axis. This data is taken from NACA's technical report 548:

Figure 13 from Chapter XIV of Fluid Dynamic Lift by Sighard Hoerner. Note that it plots the derivative of a derivative on the Y axis!

The $c_{l_{\beta}}$ curve can best be approximated by a sine curve. Note that the Y axis shows the $c_{l_{\beta}}$ and $c_{n_{\beta}}$ derivatives over changes in the dihedral angle $\Gamma$. To arrive at your expression with constant tip elevation for different dihedral onset locations, the curves need to be modified by dividing them by the cosine of the partial span (assuming the root is 0 and the tip is $\frac{\pi}{2}$). This would yield a constant value for $c_{l_{\beta}}$, so the dihedral onset is of secondary concern and the dihedral effect depends on the tip elevation. Note: This is valid only for rectangular planforms - as soon as taper is involved, things get more complicated. This PDF has more on effective dihedral of tapered wings.

NACA TR 548 supports your opinion:

  The tests also showed that with dihedral only the outer one-fourth of
  each semispan, the dihedral effect was maintained to a much higher
  angle of attack than when the complete wing had dihedral.


What needs to be considered when changing a tire during boarding/deboarding? Passenger aircraft tires are physically inspected by the cockpit crew member or technician after each landing or before take-off, that the tire may have been cut or punctured as it may have hit something on the ground (FOD) while taxiing. If the tire needs to be replaced for any reason, the tire is lifted with a jack in the parking stand and the tire replacement is carried out less than half an hour (similar to changing a car tire).

Since airline companies are commercial enterprises and the time the aircraft will spend on the ground cause extra cost, can the tire replacement and passenger boarding/deboarding and/or cargo offloading/onloading operations be performed simultaneously?

What needs to be considered in order to replace the tire and perform the other mentioned operations safely at the same time?

I have never seen an airline (or the military) have a maintenance procedure where a tire can be changed with pax on board the aircraft. Normally, if the problem is found when pax are on the aircraft, they are asked to deplane, the maintenance is done and the pax are then boarded once more. The biggest reason is that pax moving inside the aircraft can cause the aircraft to move and the jack to tilt and cause the landing gear to hit the ground. Without a tire, that could be really bad news.


Why are the runways on Denver Int'l very separated from the terminals? As you can see in the airport diagram the runways are very separated from the terminals:

I've  found a similar question about Schiphol runways and one of them is very separated because of the noise. But at Denver Airport there are no towns or cities near the runways so, what could be the reason?

Source

FAA has published Advisory Circular 150/5300-13A, Airport Design, which includes recommendations for parallel runway separation for US airports.

It summarises:

  Quadruple simultaneous precision instrument approaches are currently
  under study by the FAA. In the interim, the FAA, on a case-by-case
  basis, will consider proposals utilizing separations down to a minimum
  of 5,000 feet (1524 m). Quadruples may require special radar,
  monitoring equipment, etc.

  Simultaneous non-radar departures require a parallel runway centerline
  separation of at least 3,500 feet (1067 m), and Simultaneous radar
  departures at least 2,500 feet (762 m).

Back to the case, Denver airport is one of the busiest airports in terms of aircraft movements. It needs 6+ runways to support 550K aircraft movements every year. However, the more runways an airport has, the higher the chance air routes will interfere with each other. More separation between runways (and thus the terminals) can reduced distributions.

In fact the airport masterplan (PDF) shows that the operator is planning to build runways further separated from the terminals.

PS1: Denver is not the only case. Many mega airports have been built or are planned to be built with runways far from the terminal buildings, including Dallas/Fort Worth, Washington Dulles, Houston George Bush, as well as Dubai Al Maktoum Airport, Seoul Incheon airport and Shanghai Pudong airport.

PS2: Even though built in less populated area, Denver airport still have noise issues. More details can be seen here and there.


Is there a parameter that links the areas of the fuselage and control surfaces? Do aircraft designers look at the ratio of how much fuselage surface area can be loaded onto a given unit of wing and tail surface area including elevator, aileron, rudder etc? (Possibly, the surfaces of propellers and internal parts of jets, may be excluded while making such calculations.)

Is there such a parameter, and if so, what are the normal values?

This question is for normal fixed-wing and rotary-wing aircraft, and mostly commercial aircraft.

This question is NOT about fighter jets, space-shuttles, canard-wing designs, flying wings, lifting body and other cutting-edge-technology stuff.

I do not know a method that relates the fuselage area to empennage sizes, but there is rule that can be used to estimate the size of the vertical and horizontal tail. It is using the so called tail volume coefficient. For the horizontal tail given by:

$$ V_h = \frac{S_h \cdot l_h}{S c} $$

With $V_h$, the horizontal tail volume coefficient, $S_h$ the horizontal tail surface, $l_h$ the horizontal distance between the c.o.g. and the tail, $S$ the wing area and $c$ the average wing chord.
Basically, this coefficient says that the moment induced by the wing scales with $S$ and $c$, whereas the counteracting moment given by the tail scales with the area of the horizontal wing multiplied by the distance to the c.o.g., and that there should a certain ratio between the two.

A well behaved aircraft typically has a $V_h$ which falls in the following range $^{[1]}$

$$V_h = 0.30 ... 0.60$$

  If $V_h$ is too small, the aircraft’s pitch behavior will be very
  sensitive to the CG location. It will also show poor tendency to
  resist gusts or other upsets, and generally “wander” in pitch
  attitude, making precise pitch control difficult. $^{[1]}$

We can check the values for some commercial aircraft here (I chose Airbus, but it should hold for others as well) :

$$\begin{array}{|c|c|} \hline
 \text{Model} & \text{$V_h$} \\ \hline
\text{A330-200} & 0.957 \\ \hline
\text{A330-300} & 0.791 \\ \hline
\text{A340-200} & 0.733 \\ \hline
\text{A340-300} & 0.791 \\ \hline
\text{A340-500} & 0.729 \\ \hline
\text{A340-600} & 0.729 \\ \hline
\end{array}$$

You can see that the values are already higher than given by the guideline, showing the issue of the generalization.
Another fact that causes the deviations, is the fact that the Airbus aircraft are designed in families. Thus, they design an aircraft, and then add some extra fuselage length to generate a larger family member. This will possibly drive the tail volume coefficient away from the ideal case.

For the vertical tail there is an equivalent form:

$$V_v = \frac{S_v l_v}{S b}$$

With $V_v$, the horizontal tail volume coefficient, $S_v$ the horizontal tail surface, $l_v$ the vertical distance between the c.o.g. and the tail, $S$ the wing area and $b$ the average wing span.

A well behaved aircraft typically has a $V_v$ which falls in the following range $^{[1]}$
$$V_v = 0.02 ... 0.05$$

  If $V_v$ is too small, the aircraft will tend to oscillate or “wallow” in yaw as the pilot gives rudder
  or aileron inputs. This oscillation, shown in Figure 5, is called Dutch Roll, and makes precise
  directional control difficult. A Vv which is too small will also give poor rudder roll authority in an
  aircraft which uses only the rudder to turn. $^{[1]}$

Again checking the values:

$$\begin{array}{|c|c|} \hline
 \text{Model} & \text{$V_v$} \\ \hline
\text{A330-200} & 0.057 \\ \hline
\text{A330-300} & 0.059 \\ \hline
\text{A340-200} & 0.055 \\ \hline
\text{A340-300} & 0.059 \\ \hline
\text{A340-500} & 0.049 \\ \hline
\text{A340-600} & 0.049 \\ \hline
\end{array}$$

Here the values are quite close to the general guideline.

Another interesting plot is shown by This tail design page by Stanford

Here it shows that the tail volume correlates quite nicely to fuselage height and length.

Image showing correlation of aircraft vertical tail volume as a function of fuselage maximum height and length.

It should be noted that aircraft aren't really designed by these guidelines anymore, all the control surfaces are designed such that they provide the control authority necessary, not to match a general rule.

$^{[1]}$: Lab 8 Notes – Basic Aircraft Design Rules Link


Why doesn't the black box also record video? Why does the black box still only record the cockpit's audio today, rather than also recording video of the cockpit? Wouldn't a video of the pilots' actions and what they were seeing outside the plane be very helpful in determining what caused an accident?

Why does the black box still only record the cockpit's audio

The Black Box

There isn't a single black box. There is normally a Flight Data Recorder (FDR) and a separate Cockpit Voice Recorder (CVR) in large civilian transport aircraft. These are not mandatory in military transports or smaller aircraft.
Under proposals made in the last decade or two there would probably be a separate video recorder that records the flight instrument panel, recording what information the displays are showing the pilots and the positions of switches. So far as I can find, there are no official proposals to record external views - which are usually likely to be much less useful in determining the cause of an incident if you have FDR and CVR data.

Opposition

  the National Transportation Safety Board in the United States has proposed the idea of featuring cockpit video recording in airliner cockpits several times since the year 2000. However, such proposals have largely been opposed, especially by pilot unions who view such a stipulation as an invasion of privacy. NTSB has never been met with support for such a requirement from the FAA, since first proposing it in the year 2000.

(my emphasis)

From ICAO is Not Calling for Cockpit Video Cameras in New Airplanes. Aviation Today. 2017.

  Pilots don’t see the disappearance of Flight 370 as a reason to embrace cameras. They cite two reasons for their opposition: Video surveillance will almost certainly be misinterpreted or get into the wrong hands, and it can adversely affect how they do their jobs. “What a camera can capture can be so easily misunderstood and misconstrued,” says Doug Moss, a former test pilot and accident investigator.

From Why Pilots Hate the Idea of Cameras Watching Them Fly. Wired. 2014.


How does radar receive Mode C information? I know primary radar of ASR can determine only slant range, so by using Mode C transponder, ATC can know the altitude of aircraft.

And I found that if altimeter setting of radar is changed, displayed altitude also be changed.

So I guess that Mode C transponder can deliver only pressure information not altitude information. So Radar convert this pressure information to altitide information referring to radar altimiter setting. And it is not important what altimeter seeting pilot set.

Is it correct?

Correct.  Mode C responses include the pressure altitude.  This means that many installations use a separate altitude encoder that has no barometric settings.

Whether it is integrated or a separate unit, the kollsman setting does not change the pressure altitude reported.

From http://www.faraim.org/aim/aim-4-03-14-133.html

  Mode C Veil. [...]  Unless otherwise authorized by ATC, aircraft
  operating within this airspace must be equipped with automatic pressure
  altitude reporting equipment having Mode C capability.


Where can I get information about the FAA "restricted like" license to fly a N aircraft in Italy / Europe? I have recently bought a N-REG C172 in Italy since 2008 and in fact I have been told by the Italian NAA that my EASA License will be valid limited for flights in Italy. I understand if I wish to fly in other EU countries I should apply for a FAA "Restricted like" License which I can obtain symply by giving a short english language check in recognized FAA offices in Italy. Any Idea about this "restricted" option? Whom should I ask to get more info?

I don't know what an FAA "restricted" license is, but I guess it may be a foreign-based license issued under 14 CFR 61.75; see this answer for more information.

It's an FAA private license that's linked to your Italian one and it gives you all the privileges of a regular FAA private license, including operating N-reg aircraft anywhere in the world. But note that if your Italian license has more restrictions than the FAA one (e.g. no night flying) then you still have to obey those restrictions.

The process to get one is described here but I believe you have to physically visit an FAA office in the USA to get it.


Could future airplanes use rockets to recover plane and/or prevent crash landings? With the advanced rocket technology that SpaceX uses in the Falcon for re-entry and controlled descent, could the same technology be used in commercial aircraft? I'm asking about using it in emergencies like engine failures, stalls, loss of pitch control etc. in order to prevent a crash, or at least minimize the damage and reduce the fatality rate.

For fuel storage, the rockets could burn what's available in the tank without any need for an auxiliary tank dedicated to them.

I know that there is a similar technology used in the military: the C-130 Rocket Assisted Takeoff (JATO). But it's only used for short runway landings and take-offs.

EDIT:

The question is specific to thrusters that could recover the airplane from  unrecoverable situations (dive, bank, upside down,...). Question Why are movable rocket thrusters not used in airplanes? answer is generic and not detailed for such use case.

They could but there really is no reason to do so. Most aircraft glide just fine if they lose all engine thrust and are perfectly controllable in such situations (provided they can actuate the control surfaces). As for the situation you propose there are a few issues with it,

First off, rocket fuel is not jet fuel and can't just be burned in the same way.

The JATO engines are a bit different than the rocket assist that Space-X is using (although the principal is similar). For takeoff the Jato engines help add to the totally available thrust and allow the plane to get off the ground in a shorter distance. The rockets are (I believe) single use and provide a very quick burst of thrust.

As for the loss of control this depends on what causes the loss of control and how the rockets are affixed to the airframe. In the case of the Alaska airlines flight you reference the jack screw forced the tail into a position and prevented it from moving, this put the plane in an unrecoverable dive that would only be exacerbated by rocket engines unless you could control the angle at which they were pointed. Again in the outlined case you would need to be able to counteract the full surface deflection of a control surface. In essence you would need the kind of thrust control that space ships have used over the years.

Now to answer the broad question of "could rocket thrusters be used to bring a disabled aircraft down softly" the short answer is yes. They could be added for such a task but the toll they take on the plane would make it a hard sell to the airlines. In short the use case is small and the added weight cuts into the profitability of the flight (you need a lot of jet fuel and heavy thrusters to pull this off). On top of all that the system would have to be built and certified for use, a task that should not be taken lightly. In the end you get a safety system that has a small use case and a big impact on the aircraft. Perhaps the best thing airplanes can learn from space craft landings is the use of parachutes.

The last and somewhat debatable point (common to all safety systems) is when do you actually turn it on? What is your definition of unrecoverable (yes this varies)? Cirrus teaches people to basically always pull the chute in an emergency. Are you going to teach pilots to to always trigger this rocket decent system even if the plane may be recoverable? What if you run out of fuel before getting to the ground and now you are to low to recover from the rocket assisted decent?


Can bird strikes be dealt with by throttling down engines to idle? From a related question, I got to thinking about bird strikes again and the fact that they are sucked into the engine at great velocity.

If a pilot had 5 to 10 seconds of warning time, would throttling down the engines to idle be a good idea? Would it save those engines even if a bird hit the intake fan? This would of course be a temporary measure, intended to prevent the birds being sucked in harder or faster than otherwise. Once the birds passed, the engines would be throttled up again.

I'm thinking about large commercial jets like the Airbus A320.

EDIT: one concern I have is that at high throttle, the engine will suck in objects from a wider area than just the engine inlet. Is this assumption true?

A goose can easily be flying at 40mph and a commercial jet will be landing at about 150mph, giving a closing speed of about 190mph (305km/h). That means that, to spot a goose with 10s warning, you would need to spot it when it's more than half a mile away.

Not only would you have to spot that tiny speck of a goose half a mile away during the busiest phase of the whole flight, but you'd also have to immediately work out whether the tiny speck was even coming towards you. If it was, you'd have to estimate whether it would even hit the plane.

Then, supposing you'd immediately and accurately recognized the danger to your plane from that tiny speck, you'd have to immediately reduce power to the engines, while being very careful not to hit the ground the bad way.

In short, regardless of how much better the outcome of a bird hitting an idling engine would be than one hitting an engine under power, it's just not feasible to try to power down the engines. Once you take into account the false alarms, which would vastly outnumber the actual bird strikes, trying to power down the engines would probably be far more dangerous than just hitting the bird.


Is it possible to calculate lateral acceleration from pitch/yaw/roll rates? Can you calculate an aircraft's lateral acceleration from the aircraft's yaw, pitch and roll rates? If so, how?

This is to implement a digital slip indicator.

No. Yaw, pitch and roll rates only describe the change in the orientation or attitude of the object relative to a frame of reference. Is it changing orientation to point further clockwise? or counter clockwise, or upwards/downwards, etc.

Any acceleration is a measure of the change in the velocity of the object relative to the frame of reference in question. An object can be rotating to change it's orientation or attitude in any way completely independently of it's velocity through that frame of reference or how that velocity might be changing.


Can current fighter jets perform a deadstick landing? If a current modern day fighter were to have a total engine failure over a suitable landing site or airport, can the pilot safely perform a dead stick landing? If so, are they routinely given training for the procedure?

Here's an F-16 doing it.

Apparently there is an emergency APU that runs on hydrazine to power the hydraulics instead of a RAT.  They mention it at the end because they have to call out fire trucks because of the hydrazine hazard.

I used to have an F-104 flight manual, which I must have loaned out to someone and never saw again.  It could actually glide fairly well, with an L/D of about 4 or 5 to one at about 190 kt if I recall.  It had a section on gliding engine out.


What is the lifespan of commercial airframes (in general)? You always hear that the Boeing 737 have been flying for so many years and the Airbus A310 for that many years, but of course that include new airframes being produced all the time. What is the lifespan of one airframe on average in general use by airlines, before it is scrapped?

Metal fatigue is normally the limiting factor of an aircraft's lifespan. Fatigue cracks build up every flight (at start & landing), called a cycle. Therefore the design life of an aircraft is typically given in cycles.

Metal fatigue cracks, source: Boeing

For long haul aircraft, that make relatively few cycles, the design lifespan is in the order of 40 000 cycles. For short haul the number is higher, sometimes up to 111 000 cycles.

For example, the Boeing 747 has a design number of cycles of 35 000, the MD-80 has 110 000.

An aircraft should be operable for about 30 years before reaching the cycle limit. The lifetime can be stretched by special inspection and maintenance programs.


Would climbing onto fighters placed on display cause damage? The question is about any more or less modern plane similar to Su-27 in its fuselage design. It's not uncommon to install retired planes as monuments. Here's one example in Eastern Europe (image from this post on some local Reddit equivalent):

The shot shows a retired Su-27 which had its engines and electronics removed and was installed on a pedestal somewhere in a city. It's not uncommon for locals to climb both such pedestals and the planes - youngsters do this for lulz and also to make photos of themselves. Standing and sitting on wings or behind the canopy are typical examples.

I guess this is not very good for the airplane fuselage. Not only will paint get damaged but also the fuselage may be mechanically damaged - mostly metal sheets might be bent.

I know that it's possible to accidentally fall from a plane, especially when it's wet because or rain or melting snow, but let's ignore that in this question. I'm only asking about what's good or bad for the plane.

How bad for such a plane is climbing it by an unprepared person (who doesn't know where exactly it's safe to step)?

If you intended to make the aircraft airworthy again, yes, it can cause major damage. But, for use as a static art display, it would be no different than any other display really. Most likely all the moving parts have been fixed so they don't move so making it airworthy again is pretty much impossible.


Why do aircraft take this strange route into JFK? Looking at FlightRadar24, I see there is a long line of aircraft heading into JFK, but that they seem to all be starting to head straight at it from over Pennsylvania (as can be seen in the image below, and at https://flightaware.com/live/flight/JBU1042/history/20170621/2301Z/KSAV/KJFK.
It's not just JBU1042, though (which looks like it's circling too?). A bunch of aircraft seem to be doing this. And not just ones coming from the western U.S. Many are coming from the south too.
It doesn't appear that way when I plug the route that FlightAware displays into something like SkyVector.

Stormy change of arrival plans

Originally, the flight was planned to use the CAMRN4 STAR coming in from SIE VOR south-southwest of KJFK.  However, there is a strong band of storms just south of that VOR on the FlightAware map, blocking that route.  So, the flight got a reroute to pick up the LENDY6 STAR instead, getting vectored northwesterly to around FQM (Williamsport) VOR first and then back onto the course to pick up the STAR in order to provide sufficient space to descend to FL230 by JENNO.


Is the transition altitude around Denali still 18 kft AMSL, and, if so, how is this handled? In North America, the transition altitude - the altitude below which everyone reports their altitude in feet above mean sea level (AMSL) and sets their altimeter to whatever the weather dictates that the local altimeter setting be, and above which everyone reports their altitude as a flight level (FL)1 and sets their altimeter to 29.93 inches of mercury (inHg) - is 18 kilofeet AMSL, this altitude having been chosen so as to put the transition altitude well above the elevation of every airport on the continent.

However, although the highest airports in North America are well below 18 kft, the highest land in the continent is not; Denali, the highest mountain in North America, reaches up all the way to 20.31 kft AMSL.  It’s also not particularly inaccessible, and happens to sit in one of the most aviation-centric - indeed, in great measure, aviation-dependent - regions of North America, fairly close to said region’s major city and said city’s major airport.

Given that one’s ability to maintain terrain clearance (read “avoid flying into the side of the mountain”) depends on one’s height above the ground, which depends on one’s actual altitude, on the ground’s actual altitude, and on the difference between the two,2 how is the situation of a mountain poking its nose above transition altitude handled?  Is there a higher-transition-altitude bubble surrounding Denali?  Are pilots required to set one baroaltimeter to 29.93 inHg (for flight-level purposes) and a second baroaltimeter to the local altimeter setting (for terrain-clearance purposes)?  Do they simply have to watch their radar altimeters like hawks anytime they’re flying near Denali’s peak?  Something else?

1: One flight level is roughly equal to one hectofoot; it does not equal exactly one hectofoot unless the sea-level barometric pressure happens to be 29.93 inHg and the atmospheric profile happens to match the International Standard Atmosphere (ISA).

2: And given that, while terrain clearance is always a high priority, it’s even more so where even the gentlest crash would be unsurvivable for anyone but a well-acclimated mountaineer, due to Denali being inconsiderate enough to locate its peak in what is, for non-acclimated persons, the Death Zone.

The US AIP (ENR 1.7 Altimeter Setting Procedures) is pretty clear about the question being asked, and provides an example.

Minimum altitude comes first.

The lowest usable FL is raised, and not just around Denali (Mount McKinley) in Alaska where the ground juts above 18,000 feet, i.e., the lowest FL can be higher than FL 180 in the contiguous United States.

Let's say the area is not marked yellow and is therefore not congested, and no natural reserves or special conditions exist. In this case the minimum safe altitude is 500 feet AGL.

Let's say ground is 20,300 feet as you present, it means the minimum safe altitude is now 20,800 feet (based on local altimeter setting and accounting for cold temperatures). This is what the AIP says in all caps:

  1.2 EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW TEMPERATURES AND PRESSURES.

Now that we've established 20,800 plus correction for temperature (assume none for the sake of explanation), the lowest FL now depends on that 20,800 plus a correction based on pressure via table ENR 1.7-2 (uploaded as image).

  2.1.3 (...) the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL ENR 1.7-2.

(Bold emphasis mine.)

If it's a rather low-pressure day, then the correction can be upwards of 2,500 feet. Assume that for emphasis. Our total is now 23,300 feet, which makes the lowest usable FL, FL 235 (or as directed by ATC as stated in 91.159 for VFR, which the AIP mentions).

The example the AIP gives:

  The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.42 inches of mercury. The lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude plus 500 feet).

This answer is based on the USA AIP at the time of writing this, it is generic, and it is not about a specific location, so do not use for flight.


Can a private pilot receive recurring training while waiting for a special-issuance medical approval? In the last year, I have had to undergo bypass surgery. I am fully recovered now (and past the mandatory 6-month waiting period), but have just been apprised by my AME that my 3rd class medical deferral is likely to take a month or two for the FAA to process.

I am also in a partnership, and our insurance requires annual recurring training. Normally, all four of us (partners) take the recurring training at the same time, and get a discounted rate as a result.

As fate would have it, this year's recurring training will fall into the time I am waiting to hear back from the FAA. Since the flight portion of the recurring training (essentially an IPC) is conducted under the supervision of a CFII, can I complete it without a current medical certificate?

My understanding is that your medical is currently invalid (because you had bypass surgery and need to get your medical reinstated), so you currently cannot exercise the privileges of your certificate - i.e. you can't act as pilot in command of a flight (or any other required crew member in a role for which a medical is required).

As Casey pointed out, you've got an out that allows you to still take training (and even to fly generally): Someone else with a valid medical and a certificate appropriate to the category & class of aircraft needs to act as the (legal, FAR 91.3) pilot in command while you're operating the controls.

If your CFII is willing to accept that role (and the attendant liability) while giving you your training you can go up with them and complete the recurrent training your insurance company requires, but this is something you need to discuss with your CFI prior to going up with them.


Why does lift curve slope increase when Fowler Flaps are deployed? Below is an image for a Fowler Flap:

Notice that as the flap is deployed, both the camber and the area increase. This change in wing shape modifies the lift curve as follows:

(source: Development of a Fowler Flap System for High Performance General Aviation - Wentz (NASA))

This behavior can be isolated and viewed in the simpler figure below:

Note that the lift curve slope has increased.

Here is my analysis:
If Fowler Flap is deployed, the planform area increases.
If the planform area increases, the aspect ratio decreases (since wing span is unchanged).
If AR decreases, then the lift curve slope should decrease (see How does the aspect ratio of a wing impact its lift? for details).

So where in my analysis do I go wrong? I know the lift curve slope should increase, then why does my analysis indicate otherwise?

Note that your plots show lift coefficient over angle of attack. This lift coefficient is referenced to a reference area which must be defined somehow. It is customary to use the projection of the clean wing area in the x-y plane for all flap settings in order to keep coefficients comparable. The lift curve slope increases because the real wing area increases while the reference area remains unchanged. The dashed line in the lower plot shows the isolated effect of camber increase while the upper solid line combines both effects (area and camber increase).


Are warbirds allowed to use afterburners? Suppose a civilian buys a military jet, for example an T-38. Would he be allowed to use afterburners?

No, no restrictions on using an afterburner. There are a few F104 Starfighters on the civil registration that must use the afterburner to take off.

However, most countries forbid supersonic flight by non military operated aircraft in their airspace. The only time Concorde went supersonic was over the ocean.


What are these markings under the passenger door? Is there anybody who can explain what the markings just below the passenger door edge do?

They appear to be alignment marks for the passenger loading bridge (PLB) and air stairs. Some airlines choose mark their aircraft and bridges, in order to prevent damage to the aircraft door due to misalignment of the bridge or air stairs.

On the PLB, there will be a corresponding mark on the floor, at the edge.

The two lower marks would be used for air stair alignment (i.e., when a PLB is not available).

Note the marks at the edge of the bridge in this photo (the position of the alignment marks are not standardized and are carrier specific):


Why do helicopters have skids? Why can't they just land on the helicopter's floor? If it's for stability, they could still use skids which are shorter than what is commonly used.

Image source

Larger helicopters have landing wheels, and as we can see on the picture above, they are placed as far outward as practically possible in order to provide stability in the landing. Touch down one wheel on an sideways inclined slope, lower the collective, and the helicopter will align itself onto the slope with:

The location of the Centre of Gravity (CoG) always in between the wheels. The higher the CoG, the wider the landing gear impact points must be.
The weight being supported on clearly defined and structurally reinforced points. Even if the bottom of the helicopter is perfectly flat, the landing ground may not be, and to support all of the weight at any random point at the bottom requires a lot of relatively useless weight for the re-inforcement.
The behaviour after first wheel touchdown being clearly defined. The wheel is a point of impact upon the landing plane, mathematically much more defined than a plane of impact upon a landing plane if the latter is curved.

All the above is for wheels, which allow the helicopter to taxi on-ground. Skids perform all of the above functions apart from the on-ground taxiing, and are cheaper and lighter than retractable landing wheels. Of course, the other downside is that they have more drag.

One would not want to land this helicopter without the skids!


How many angle-of-attack sensors does the Airbus A 320 NEO have? Are there 3 x AOA-sensors on all the Airbus family planes? In the Airbus flight manual there is one AOA-sensor called 'stand-by'. What does that mean?

Are all 3 AOA-sensors heated and can the actuall temperature of these sensors be monitored live by the pilots during flight?

I think there are three angle of attack sensors in all Airbus planes using fly-by-wire systems. The first two, which are arranged symmetrically near the nose are the main ones and the third one in the aft.

The angle of attack data is not typically displayed to the pilot. The computer, on the other hand displays only stall warning.

The angle of attack sensors are heated and active at  all times. The data from the sensors is received by the computer, compared (voted on) and displayed in the cabin.  The computer needs two values to agree for display.In case one fails or no two agree, the stall warning is lost.

The main use of the angle of attack sensors is their use by the computers in calculating the characteristic speeds and for providing stall warning. I don't think the pilots can monitor. let alone control the heating of the AoA sensors.


Is there any good database for aircraft technical specifications? Due to using them in my application, I am looking for a database(or guide) of aircraft technical specifications. Is there any source for it(preferably free).

One word for the definitive authority on this subject: Jane's.  They'll get you anything you want to know - if the price is right.

Another good source, especially for military aircraft is The Military Analysis Network, run the Federation of American Scientists (FAS).  Great information database and it's free, too!

As with so many other subjects, Wikipedia remains an excellent source of information on aircraft.  It is not a recommended source to cite for writing technical publications of your own, but is a excellent internet portal to additional information and resources e.g. books, periodical articles, etc.


Can you get cheaper insurance with more experience? I was exploring renters insurance on the AOPA insurance website and noticed that while you get a 10% discount for having no claims, there seems to be no difference in price between a private pilot with 50 hours and an ATP pilot with 5000.  Why is it not the case that more experience yields cheaper insurance?  Is this normal for aviation insurance?

It's not at all clear that more total flight hours correlates to lower risk. Data shows that pilots with <200 hours of total flight time are at the lowest risk of having a serious accident.

One can speculate the reasons for this, but the truth is, aviation accidents are still very rare occurrences. This makes gathering and analyzing data difficult. The data is very noisy due to the low sampling rate.

That said, statistical data points to a low rate of accidents until somewhere between 100 and 250 hours. Then it increases steeply, reaching its highest point around 500 hours for non-instrument rated pilots, and 800 hours for IR.

Annualized non-IR GA accident rates (median TFH=250.5)*

Figure 11. Annualized IR GA accident rates (median TFH=823.5)*

Statistics have long shown a "hump" in accident rates at the middle zone of experience.

From there it descends slowly over time but never reaches the low rates of new pilots. Another thing that occurs is that the number of pilots at a particular level of experience decreases extremely rapidly after a few hundred hours. It decreases to the point where the number of pilots with high hours is very small. The sample rate is then so small that it's close to impossible to come up with an accurate risk assessment.

Additionally, sometimes insurance rates are obviously driven by factors other than risk. Most aircraft insurance will give a discount for instrument rating. But if you look at the above graphs, IR pilots are at a higher risk than non-IR. They can fly in IMC, which is higher risk, but they have cheaper insurance. This is clearly a marketing decision. If one insurer gives you that discount where others don't it will attract more customers.

* Source: Predicting Accident Rates From General Aviation Pilot Total Flight Hours,William R. Knecht
Civil Aerospace Medical Institute, Federal Aviation Administration


How to do a Rapid Descent in a jet with inoperative spoilers? So, let us assume you are in a simulator for your favorite jet plane, and the instructor gives you a LOFT (Line-Oriented Flight Training) scenario that goes as follows:

A normal takeoff and climb to cruise altitude, and perhaps thirty minutes of normal cruise flight
The non-recoverable loss of speedbrake (flight spoiler) functionality due to a mechanical jam, or some other malfunction such as the loss of all SECs in a FBW Airbus.
Another 30 minutes of time to deal with the first malfunction: run the checklists, plan and initiate a diversion if needed.
You lose normal cabin pressure for some reason, and have to conduct a rapid descent to get back to 10,000' and complete your diversion.

How do you execute the descent, considering that the rapid descent procedures for most jets rely on using the speedbrakes to achieve a high descent rate without overspeeding the aircraft?  To use the 737 QRH checklist for Emergency Descent as an example:

  EMERGENCY DESCENT . . . . . . . . . . . . . . . . . . . . . .Announce

  The captain will advise the cabin crew, on the PA system,
  of impending rapid descent. The first officer will advise
  ATC and obtain the area altimeter setting.

  ENGINE START switches . . . . . . . . . . . . . . . . . . . . . . . . CONT

  THRUST levers . . . . . . . . . . . . . . . . . . . . . . . . . . . . CLOSE

  Reduce thrust to minimum or as needed for anti-ice.

  SPEED BRAKE . . . . . . . . . . . . . . . . . . . . . . . . . FLIGHT DETENT

  DESCENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Initiate

  Target speed . . . . . . . . . . . . . . . . . . . . . . . . . . . Mmo/Vmo

  If structural integrity is in doubt, limit speed as much as
  possible and avoid high maneuvering loads.

  Level-off altitude . . . . . . . . . . . . . . . . . . . Lowest safe altitude
                                            or 10,000 feet, whichever is higher

The answer is actually quite simple.

FOLLOW THE MANUFACTURES PROCEDURES.

In some aircraft (I don't have a 737 flight manual handy) there is an altitude limitation if the spoilers are inop for just this scenario.  When the aircraft was certified, they needed to demonstrate an emergency descent (in the allowable time) without them, and that might not be possible from the certified ceiling. If that's the case in the 737, then descend to the max altitude with spoilers inop (assuming that you started above it).

If there is no altitude limitation and no special spoiler-inoperative emergency descent procedure (I have never seen one of those, but there's nothing to say that a manufacturer couldn't have one) then you simply fly the regular procedure without spoilers.  The manufacturer had to demonstrate this during certification and it must have been acceptable at the time.

As rbp says, most of the time you fly Mmo/Vmo at idle power after you don the oxygen mask.  IF the flight manual/checklist says that you can put the gear down (some do) then you may, but I wouldn't otherwise.  I certainly wouldn't slow to gear speed (unless the manufacturer says to....  Unlikely in a jet like this) because you are decreasing the drag by slowing, and taking time at altitude to do so.


If I were to build a J3 cub twice the size of the original using the same CG would it fly? How big of an engine would I need, if the original is 65hp?
What would be the weight of the up-scaled plane? Simply twice the
weight?
What would be the stall speed?
How fast would it go?  75mph?
Is there a software program to test aircraft design?

Unfortunately a simple scale-up would fall victim to the square-cube law: all dimensions times two means the volume and weight are 2x2x2 = 8 times the original, while the wing area is 2x2 = 4 times the original. It is the wing area that keeps the aeroplane up. It is the reason why birds cannot get any larger than they already are.

Someone in the 1920s analysed this and predicted that the maximum size for aeroplanes would be about that of the DC-3, but improved technology has proved that prediction wrong. A B777 has a way higher wing loading than a DC-3 of course. The square-cube law is not a law, it can be defeated by engineering. Point is that linearly amplifying all dimensions does not work.

  How big of an engine would I need, if the original is 65hp?

Minimal: 8 x 65hp = 520 hp. We need a serious amount of extra thrust.

From this answer:

$$D =  C_{D0} \cdot \frac{1}{2}  \rho  V^2 \cdot S + \frac{2  W^2}{\pi  A e \cdot  \rho  V^2 \cdot S} \cdot $$

$C_{D0}$ would stay the same if everything is scaled up
S = wing area = 4 times as high
W is 8 times as high.

$D_2 = D_1 \cdot (4 + 8^2/4) = D_1 \cdot 20$

So you need 20 times as much thrust at the same speed.

Increase V and the first factor increases, second factor decreases. There are of course many optimisations possible (weight saving, wing loading increase etc), but that goes beyond the scope of the question.

  What would be the weight of the up-scaled plane? Simply twice the weight?

Eight times that of the original.

  What would be the stall speed?

Higher than that of the original because the wing loading is higher. Depends on the flaps & slats installed.

  How fast would it go? 75mph?

Yeah it might..but it would probably need to go faster as @xxavier mentions.

  Is there a software program to test aircraft design?

FlightGear is an open source flight simulator that allows you to link your own flight dynamics model into it.


What date and time definition is used for the second leg of a flight? Let's take flight BA0015, from London to Syndney, via Singapore.
This is data from FlightStats.

The first leg has this data:

  "carrierFsCode":"BA",
  "flightNumber":"15",
  "departureAirportFsCode":"LHR",
  "arrivalAirportFsCode":"SIN",
  "departureDate":{
    "dateUtc":"2019-08-17T20:30:00Z",
    "dateLocal":"2019-08-17T21:30:00"
  },
  "arrivalDate":{
    "dateUtc":"2019-08-18T09:45:00Z",
    "dateLocal":"2019-08-18T17:45:00"
  },

It takes off from London, on 2019-08-17 at 20:30 UTC
and lands in Singapore, on 2019-08-18 at 09:45 UTC

but then the second leg:

  "carrierFsCode":"BA",
  "flightNumber":"15",
  "departureAirportFsCode":"SIN",
  "arrivalAirportFsCode":"SYD",
  "departureDate":{
    "dateUtc":"2019-08-17T11:25:00Z",
    "dateLocal":"2019-08-17T19:25:00"
  },
  "arrivalDate":{
    "dateUtc":"2019-08-17T19:10:00Z",
    "dateLocal":"2019-08-18T05:10:00"
  },

It takes off, in the past, from Singapore on 2019-08-17 at 11:25 and lands in Sydney on 2019-08-17 at 19:10.

Is is possible that the day doesn't get incremented because the second leg still belongs to the same flight code?

You are looking at two different planes.

This is the flight from London to Singapore, which departed on 17th of August, on flightaware.com:

The dates and times are local, meaning the flight departed 2019-08-17T21:11:00Z (UTC) and landed 2019-08-18T10:23:00Z.

Before this flight even took off, another 777 departed Singapore headed for Sydney:

The flight departed 2019-08-17T11:56:00Z and landed 2019-08-17T19:09:00Z.

The plane which landed in Singapore on the 18th of August also continued to Sydney later that day:


Does the glide slope flag detect percent of modulation (80%)? Please advise if the glideslope flag validates that the received frequency is being modulated at nominal 80% or 40% for a localizer.

I wanted to post this as a comment, but it's a bit long.
From ICAO Chicago Convention annex 10, vol 1:

2.3 Alarm conditions for ILS airborne equipment
2.3.1 Ideally, a receiver alarm system such as a visual flag should warn a pilot
of any unacceptable malfunctioning conditions which might
arise within either the ground or airborne equipments. The extent to
which such an ideal may be satisfied is specified below.
...
2.3.3 The alarm system should indicate to the pilot and to any other airborne system which may be utilizing the localizer and glide path
data, the existence of any of the following conditions:
...
b) the percentage modulation of either the 90 Hz or 150 Hz signal
reduction to zero with the other maintained at its normal 20 per cent
and 40 per cent modulation respectively for the localizer and glide
path;
Note.— It is expected that the localizer alarm occur when either the
90 Hz or 150 Hz modulation is reduced to 10 per cent with the other
maintained at its normal 20 per cent. It is expected that the glide
path alarm occur when either the 90 Hz or 150 Hz modulation is reduced
to 20 per cent with the other maintained at its normal 40 per cent.

Glide path receiving equipment operational standards are detailed in RTCA DO-192 and EUROCAE ED-47B


Can a controller clear you through an SFRA? A pilot I know (lets call him Joe) recently had a minor run in with the FAA after he skirted the edge of the Washington DC Special Flight Rules Area (SFRA) while returning to Leesburg (JYO).  Here are the details:

Joe was returning from KOMH.
Joe was trying to get on the ground quickly to avoid a wall of convective sigmet from the west, moving east quickly and threatening to engulf Leesburg and his landing in a small GA plane.
Joe was in contact with the Leesburg Tower Note: Leesburg is NOT usually a towered airport.  However Leesburg has been a test site for mobile air traffic control tower and that tower was operational when this occurred.
Joe was instructed by the tower to "Proceed directly to Leesburg".  He immediately turned directly towards Leesburg (right turn shown on chart), received permission to land and landed.
Joe was provided with a number to call by the tower.
Joe was interviewed by someone in the FAA, told he had skirted the SFRA, received a warning and the case was closed with no further action.

Though the case was closed it was still the FAA's position that a violation of the SFRA rules had occurred.

What I find confusing about this case is that, if ATC instructed Joe to "proceed directly to Leesburg", doesn't that mean they cleared him through the portion of the SFRA he will need to fly through to comply with their instruction?

Research done for this question

I have taken the SFRA training required to fly VFR in the SFRA and have again reviewed this material after hearing of Joe's incident.  I've found nothing in that material that conclusively answers this question.
I have reviewed this question/answers about flying in the Washington DC SFRA.
I have reviewed this general question about flying in SFRAs.
I have reviewed the NOTAM associated with Ingress/Egress to/from Leesburg airport.
I have reviewed already asked questions and did not find this specific question having already been asked.

An AOPA article on this has a chart of the Leesburg Maneuvering Area (LMA) which, as you said, doesn't require a DC SFRA flight plan (you do need one for pattern work, though):

If you compare the area in the chart to the flight path in the image in your question, it looks to me like Joe entered the SFRA southwest of JYO and then entered the south edge of the LMA. I suspect the FAA is saying that he should have flown further north, then entered the LMA from the west. In other words, he cut through a corner of the SFRA. I've marked that in red here:

The LMA NOTAM says:

  PILOTS MUST ENTER THE LMA VIA THE MOST DIRECT ROUTE AND AVOID ENTERING
  REST OF THE DC SFRA

By cutting the corner, Joe entered a part of the SFRA that was outside the LMA, hence the call with the FAA. That's my interpretation, at least, although I'm surprised that the FAA didn't spell out to him very clearly what the issue was.

And to answer the question in the title, ATC cannot clear a pilot to do something he isn't authorized to do. ATC could clear a student pilot into class B airspace, but if the student doesn't have a logbook endorsement for that airspace then he still isn't allowed to enter (barring emergencies). "Proceed direct Leesburg" doesn't mean "fly in a straight line to Leesburg regardless of other considerations", the pilot still has to get there legally.


Why does the zeppelin NT tail use 3 fins? I don't if I use the correct vocabulary. By "fin", I mean the control surface at the tail of the airship. As a control surface, it contains a moving part.

As airship used to have at least 4 fins at the tail: an elevator (1 fin on the right side, another one the left side) and a rudder (one fin on the bottom and one on the top, I highly suspect both are useful to be sure at least one is in undisturbed airflow while cruising at any positive or negative angle of attack). This seems reasonable to ensure both redundancy and simplicity (not mixing pitch and yaw controls). Yet, aircraft with V-tail prove that we can reduce the number of mobile control surfaces located on the tail, but it does not seems to be reasonnable in all cases.

For the Zeppelin NT, only 3 fins are used. I clearly imagine the vertical one is used for yaw control only. The 2 others are shaped as an inverted V, thus I imagine they mix both yaw and pitch control as V-tail does.

What is the rational behind the choice of number of control surfaces? Why do the Zeppelin NT uses 3 of them?

Wikipedia provides few reasons I fail to understand:

it states less fins allow weight gains. for me, less fins implies bigger fins to provide the same authority, thus mitigating this point
it states the loss of one fin can be compensated by the 2 other. For me, the loss of one fin would provide a rolling moment when trying to compensate, and there is no other surfaces to mitigate it (on an aircraft, coupling is possible because it has wings)

The likely reason would be ground clearance.

While it is hard to guess the reason for a relatively minor design decision like this, it seems that using four fins would complicate the tail design. Specifically, the airship would need to have a lower gondola or an X configuration for the tail, which may or may not impose a weight penalty on the tail structure.

As you note V control surfaces tend to be a net weight penalty once you factor in the higher loads placed on fewer surfaces. However, when the choice is between four small surfaces and three larger ones, the latter is likely to be more efficient.

You also note that the loss of one lateral surface would create a rolling moment, which is true, but this is the one situation where an actual "pendulum effect" (sigh) can be observed: the buoyant lift of the airship is directed against gravity, so it has a stable equilibrium point with the gondola pointed down, which simplifies the control problem.

Without knowing the critical load cases for that tail, it is hard to give a definitive answer, but this stands out as a plausible one.


How are AOA sensors tested after working on them? If a maintenance or replacement has been done for an AOA sensor, would the mechanic be able to test if the sensor now works correctly? Can such a sensor be tested on the ground?

If there is no reading from the AOA sensor available while in the flight, and there is no alert that redundant sensors disagree, I cannot imagine how even pilots could help during the next flight.

At the manufacturing facility, regulatory requirements mean you need a specialized wind tunnel to perform testing. The big part of the testing revolves around aerodynamics and icing. Testing is done at cold temperature and high altitude conditions and involves, among other things, subjecting the device to solid ice particles and supercooled liquid water droplets in high concentrations.

On the flight line and local maintenance facilities, AOA's are usually tested along with the rest of the pitot-static system with RVSM (Reduced Visual Separation Minimums) test sets such as this one.

This all applies to commercial airliners and private jets, as well as military aircraft. Most general aviation propeller-driven aircraft do not have AOA sensors or indicators, but you can buy third party systems and install them if you wish.


In the Cl vs Cd graph, Why the drag coefficient decreases initially with the small increment in lift coefficient? Cd = Cdo + KCl2

Total drag = Parasite drag + Induced drag

When  Cl = 0 , Cd = Cdo

Increasing in Cl introduces another component of drag (Induced Drag) and therefore the total drag Cd should be increased. But, Why in this plot Cd decreases initially with the small increment in Cl?

Think about it this way.  Assume a cambered airfoil, not a symmetrical airfoil.  Assume the wing has zero twist and zero incidence.  To place the wing at the zero-lift angle-of-attack (zero CL), the fuselage and wing will have to fly at a somewhat nose-down pitch attitude relative to the airflow.  This does not yield the lowest possible drag coefficient, regardless of whether we are looking at the wing, the fuselage, or the whole aircraft.

Bear in mind that in this part of the flight envelope (near the zero-lift angle-of-attack), drag is dominated by profile drag, not induced drag.

Related:  Is drag coefficient lowest at zero angle of attack?

Now imagine using the same data to generate a graph of CD versus airspeed, assuming Lift=Weight.  What would that look like?  Also what would a graph of Drag versus airspeed look like?