Do all passenger aircraft have a nickel cadmium battery? In the event of both the engines failing in a double engine aircraft, does the aircraft a have nickel cadmium battery to provide propulsion to make a successful landing?

No, they don't.  At least, not for propulsion.

Jet engines burn oil and can't be powered by batteries.
Even if we forget that, jet engines produce enormous amounts of power and no battery technology can store anything close to that much. Certainly not NiCad, which isn't even suitable for powering your phone or laptop.
Airliners are only allowed to operate on routes where, if one engine fails, the plane can still reach an airport with plenty of safety margin. A second engine failing independently is so unlikely that I wouldn't be surprised if it had never happened on a modern plane.
Even if the second engine fails, airliners can still glide for tens of miles to find somewhere to land (e.g., the Gimli Glider and Air Transat flight 236).
In reality, the only way that all your engines are going to fail is if there's a common cause. Running out of fuel is the most obvious, and is in the news at the moment. The other one that comes up from time to time is flying through volcanic ash clouds. The solution to all of these problems is management, not technology. Airlines are very careful to put enough fuel on board for every flight, and regulations require there to be not just enough fuel to reach the destination but also extra to allow for delays and diversions. Likewise, they're very careful not to fly their planes through conditions that will break them.  Another possible common cause is a bird strike, though it's unusual for that to affect more than one engine. However, bird strikes almost always happen close to the ground and it's usually possible to get back to the airport.

So, even if batteries for backup propulsion were possible (which they aren't), they wouldn't be useful. They'd be required on such a minuscule number of flights (surely less than one in a million) that you'd probably kill and injure more people with battery fires than you'd rescue from engine failures.

And that's before you consider the huge amounts of extra weight you'd have to carry around in the form of batteries and electric engines.  Extra weight means you need more power, need to burn more fuel and have less space for passengers and cargo.

Note that planes do carry batteries to power some electrical systems. They also have a thing called a ram air turbine (RAT), which is a little windmill that drops down and provides power for the electrical and hydraulic systems in case of engine failure.


Why don't combat aircraft have rear-facing *laser* weapons? This question is similar, but only talks about guns. I don't mean a laser that is going to destroy the plane, but why aren't rear-mounted, rear-firing lasers that can blind a pilot a thing? Most of the concerns about weight and relative velocities from the other question disappear with a lighter-weight laser that is firing light.

Such weapons are not used by countries that abide by the Geneva Convention:

  It is prohibited to employ laser weapons specifically designed, as their sole combat function or as one of their combat functions, to cause permanent blindness to unenhanced vision, that is to the naked eye or to the eye with corrective eyesight devices.

For the U.S.A., page 45 of Navy Shipboard Lasers for Surface, Air, and Missile Defense says:


What is the total wing surface area of the A320? I was trying to calculate the total area of an A320, and in this link I found the wing area and tail area. But here I read that the wing area is just the 2D projection of the wing. So if I wanted to calculate the total surface of the wing I should consider the wing area (122.4 m) and multiply by two, which would be an approximation.

By doing this, and considering the fuselage to be a cylinder, I get a total area of 847 square meters for an Airbus A320, which is close to the 900 square meters reference I found in this article.

So, is it correct to say that the surface area of a wing is approximately the wing area multiplied by two?

As I'm not sure whether you were only looking for the methodological answers provided earlier, this is a more detailed answer to the calculation problem you were trying to solve in the first place, providing an answer to the question posed in the title of your question. It is based on the A320 Airport and Maintenance Planning Manual, which provides approximate dimensions of the aircraft.

For the wings and wing-like surfaces, details are as follows:

Wing top surface: $99.7 m^2$ (excluding wingtips devices)
Wingtip devices (normal, non-sharklet A320, both sides): $1.8m^2$
Horizontal tailplane top surface: $27 m^2$
Vertical tailplane (both sides): $43 m^2$

Counting horizontal wing and tailplane surfaces twice (upper and lower), that yields a total of $298.2m^2$. This does not properly account for thickness (using the factor $1.07$ mentioned by Daniel yields $319.074 m^2$) and movables (high-lift devices, control surfaces, etc).

For the fuselage and nacelles, only top surfaces are listed. Modelling these as simple cylinders therefore seems the best way to go. Based on a fuselage diameter of $4.14 m$ and a length of $37.57m$, that yields a surface area of about $488m^2$. Of course, the fuselage is not strictly cylindrical, but I'm assuming here that is offset by the fact that I'm not explicitly including the front and rear surfaces (of a closed cylinder). Nacelle dimensions are not specified (probably as that somewhat depends on engine type), but based on drawings, I'm estimating a length of $5 m$ and a diameter of $2.5m$, resulting in a total surface (again: cylindrical approximation) of $78.5 m^2$ for the two engines. For the pylons, lets add another $5m^2$ per side.

In total, that adds up to $872 m^2$ (or $902.874 m^2$ with the 1.07 thickness approximation). That corresponds nicely with the $900m^2$ you found online - and also shows your own calculation wasn't too far off (some 3%).


Did the DC-9 ever use RATO in revenue service? While reading the NTSB report on the crash of ALM Flight 980, I came across this interesting tidbit:

  ...  The fuel charts used for the study made allowance for a higher consumption rate than the standard DC-9-30 series aircraft.  This was the result of a fuel audit by the Douglas Aircraft Company on ONA aircraft and was due to jet assisted takeoff fairings installed on those aircraft.  [Pages 7-8 of the paper report; pages 12-13 of the PDF file of the report.  ONA = Overseas National Airways, who operated the flight on behalf of ALM.  Emphasis added.]

Did these DC-9s ever actually use RATO during revenue operations?

EDIT: "Revenue operations" here means "any flight conducted for monetary compensation".

The JATO/RATO retrofits were used only by Overseas National (later National Airways) when operating out of hot/high airfields for military contracts. They were never used as passenger flights, but you could say that these were "revenue" flights as they were done for monetary compensation.

I can't find any history on ALM980, only that it was registered as N935F and operated by Overseas National as a passenger flight. N935F does not show any history in the FAA registry other than it being reserved. It is possible that this was a converted cargo aircraft.

Here is a picture of an Overseas National flight using the JATO rockets:

Source: UT Dallas

It should be noted that the DC9 was not the only passenger type aircraft to get the "JATO" treatment. Boeing modified the 727 to operate out of short fields by adding JATO rockets. Aviation Week did an article featuring the DC-9 JATO modifications:

Source: Aviation Week (article paywall)

The magazine cover linked above shows a DC-9 take-off (an Overseas National flight).


Are monoplane struts under tension, doubling the stress limit of the spar? Are monoplane struts under tension, doubling the stress limit of the spar?

If monoplane struts are under tension, they would be pulling the wing down,
which would put the bottom of the spar under compression, at least between the strut and wing root where the load is the highest.

However, the aerodynamic load when flying is upwards, trying to bend the wing up, so the bottom of spar would have to go from 1/4 max compression ultimate stress, through zero and all the way to 1/4 max tension ultimate stress to reach 1/4 max flying stress limit. Assuming both limits are the same and the safety factor, n is +/-4, hence 1/4 max ultimate stress.

That would roughly double the amount of stress the spar is allowed to  carry.

Sort of like prestressed concrete.

Is this correct?, at least conceptually?

The spar on a strut braced wing effectively moves the bending stress out from the wing root to the strut attachment.  In other words, a strut braced wing is a cantilever wing from the strut attachment out, and will have the normal bottom cap in tension and upper cap in compression at that point.

Between the strut and fuselage, it's more complicated.  The spar along that section is like an inverted hammock being effectively supported at both ends.  It wants to bow up. At the root fitting, there are shear forces which depend on how far out the strut attachment is.  The forces are inboard and up if the strut attachment is closer to the tip, and inboard and down if the strut attachment is closer to the root (a Twin Otter's wing's root fittings are under inboard and downward shear because most of the wing is outboard of the strut attachment).  There is a 'sweet spot' point where you can attach the strut to the wing spar and the loads on the root fitting are straight-in compression with no vertical shear.

This is why you will see astonishingly small bolts attaching the wing root to the fuselage on some airplanes (as in, half inch, or 5/8 inch diameter bolts at the strut attachments, and quarter inch bolts at the wing roots); if the strut picks up the wing at the right location, the loads at the root are almost all very moderate inboard shear loads.

As far as spar bending goes, the most highly stressed location is the cap strip above and below the outer strut attachment and that is where the spar has to be strongest in bending, with tension loads on the bottom and compression loads on top, in normal upright flight.


What materials make up most of the weight of an aircraft? What are the material types account for most of the empty weight of an airplane (Boeing 747), besides aluminum or its alloys used in fuselage?

Besides aluminum alloys, most of an airplane's weight is steel, titanium, and composites. Here is a chart comparing the 747 and 777 structural weight, with a breakdown typical for similar airliners:

Source

Steel is used in components where strength or part life is most important, like landing gear and other important structural elements, as well as many fasteners. Titanium is used where weight and high temperature are important, such as around the engines. Composites are traditionally used for many less-structural applications such as panels and fairings, but may also be used in the empennage (tail surfaces) or on the interior as floor beams. Other materials will include alloys like nickel, as well as specialty materials like rubber, plastic, and fabric, which are used mostly in the cabin.

New aircraft like the 787 and A350 use composites much more extensively, replacing much of the aluminum. Titanium is also used more in these aircraft because it does not tend to corrode in contact with carbon fiber composite like aluminum does.

Source

Federico pointed out that the two graphics differ in the amount of aluminum on the 777 (70% on the first vs. 50% on the second), because the first chart shows only structural weight, while the second is overall.


What is the purpose of the glazed round area atop the Ju 87 Stuka G? I haven't been able to find any pictures of this feature aside from models, nor any reference to what it is.

I'm going to make a complete guess and suggest it's a searchlight for night-fighting to illuminate the underside of bombers.

Italeri Ju 87 G-2

Hobby Craft Ju 87 G-1

Hasegawa Ju 87 G-1

(Source) Ju 88 model.

Not just the 87, some Ju 88 models also had it, with no access to it in-flight. As mentioned, it's an antenna. The antenna is part of the EZ6 direction-finding unit.

The glass is not for viewing. It's the construction method to keep the installation as thin as possible. The sense antenna is painted on the glass.

  The PRE/6 ferrite antenna for the EZ/6 was extremely flat so that it could be mounted in a 10 cm deep "tub". The lid of the tub was a plexiglass cover with a star shaped sense antenna sprayed on. The plexiglass cover would sit flush with the aircraft skin so that there would be no air resistance from the antenna (unlike the earlier loops used with the EZ 2). (Source)

(Source)

(Source) The EZ6 unit with its different components put together by a serious hobbyist. In the glass box you can see the rotating antenna, and the thin sense antenna on top.


Is it unusual for an aircraft to flash its landing lights in sequence? Walking this evening I looked up and saw a plane which I assumed was on approach to Liverpool John Lennon. When I looked carefully I could see that as well as the anti-collision strobes it was flashing two lights in sequence. I assumed they were the landing lights as they were very bright and positioned towards the wing tips. However, they were clearly flashing one at a time, left then right.

I've never seen this before, is it normal and does it signify anything?

The flashing lights help to make the airplane more noticeable. They have also been suggested to reduce the risk of bird strikes. This is offered as an option or add-on for many types of aircraft.

In addition to helping the pilots to see ahead of them when on the ground in dark conditions, landing lights help to make planes more visible to others. The FAA requires landing lights to be on for commercial flights under 10,000 feet, and encourages pilots to turn them on at least when near an airport or in low visibility. Flashing the lights makes them less likely to be confused for something like a star at night.


Where can I find the flight path data for the Russian spy plane that flew over the US on 9 Aug 2017? Multiple news outlets are reporting that a Russian spy plane, a Tupolev Tu-154M, flew past various locations in the U.S.

Politico is reporting that the plane was seen on a flight-tracking website in Washington D.C., Bedminster, NJ, and Dayton, OH.

Is there a way (maybe on the flight-tracking website Politico mentions) that I can look up historical data on this flight to see when it was in each place and what its flight path was?

It's on flightradar24.com blog:

  RF-85655’s Open Skies flights over the United States since the beginning of 2017

Here you can see more details over Washington.

Two of the recent routes are here. One of them flying as low as 3,800 feet.


Is there a list of all airways? Is there a public and current (AIRAC-cycle 1509)  list of all worldwide victor- and jetairways (including the pertaining intersections)? I'm not searching for a graphical map of the airways, but more for a text file listing all items.

Sorry for the poor tagging, I couldn't find any suitable tag (there isn't one for "airway" or "intersections").

Airway data is published in each country by their respective regulatory authority. This is published as a part of AIP(Aeronautical Information Publication).

This page contains links to some of the sites having electronic AIPs. The relevant data can be found in Part 2- Enroute(ENR). For example, this gives the details for UK airways.

I don't think there is a worldwide list as the airways of each country is controlled by the respective government through regulatory body and this information is updated periodically (in cycles of 28 or 56 days).


What are the reasons behind this pusher propeller configuration? Can a pusher configuration sort of like this have its propeller close to the tail (if it were one of those split tails like on a A-10)?

Why did they put the propeller above the cabin and not in the tail?

Yeah good question. They need it to be out of the way of the spray, so high up, and they position it on top of something that looks like a vertical stabiliser, so why not use the vertical stabiliser, I presume is what you mean. Like this one, but then in a pusher comfiguration

My answer would be that that would position the centre of gravity too far back. The engine is heavy, it's best if it is mounted close to the centre of the plane. The combination of pusher prop and tail mounting would make for a configuration that wants to tip backwards. Placing the wings further back is awkward, it's a small plane and leaves little moment arm for the tail surfaces to work effectively.


What mechanism or design aspect prevents jet blast from escaping the front of a Ramjet or scram jet? Enormous air pressures result in the thrust of a Scram jet or ramjet engine, more so the pressure required to drive the aircraft forward at supersonic and hypersonic speeds.  Given that these engines are designed to function without a  fan which sucks air and at those speeds would prevent air from coming in too fast or going back where at came from post being heated.  What mechanism or design aspect prevents jet blast from escaping out the front of a Ramjet or scram jet, resulting in lost thrust and or perhaps negative thrust given that explosive pressure would drive air in any direction that would allow it to escape.

I'm also considering that the air entering the engine does not have that much pressure to prevent this cause if jet blast did exit out at the front, scram jets and ram jets would not be able to produce thrust

Pressure is the greatest just before the combustor in both jet and ramjet engines - therefore air would have against the pressure differential for this to happen (turbine only has therefore an indirect role)


Where can I find info or techniques on calculating the influence of a duct on the thrust and efficiency of a propeller? I am trying to run numbers on a concept VTOL fixed-wing aircraft, and I am looking at having fans or props positioned inside the wings, (which would therefore make the wings relatively thick for a fixed-wing aircraft). Naturally this would see the fans/props enjoying the benefits of a duct, but with minimal downsides such as the need to add weight/material as the wing is already going to be there.

I am using Blade Element Theory to trial propeller dimensions/numbers to see what kind of sizes I would need to achieve the VTOL (if I can minimize the radius of the VTOL fans then I can the wings thinner or with a smaller chord), but I don't know of any way to estimate the influence that the duct would have on the final thrust/power values.

If there is a rule of thumb for this, or a database, or some equation or anything, that would be a great help!

There is info on power computations for ducted propellers in J. Gordon Leishman, Principles of Helicopter Aerodynamics. The beneficial contribution of the duct can be found in section 6.10.1 Fan-in-fin Design, and in section 3.3.10 Prandtl's Tip-Loss Function. In short:

If the duct is designed to prevent wake contraction, extra thrust is generated and less power is required to drive the propeller. The effects are computed with momentum theory, which is a simplification. For thrust T and power P, with $T_D$ and $P_D$ = ducted, $T_U$ and $P_U$ = unducted, $a_w$ = ratio of outlet/propeller area.:

$$\frac{T_D}{T_U} = \frac{1}{2a_w} \tag{1}$$
$$\frac{P_D}{P_U} = \frac{1}{\sqrt{2a_w}} \tag{2}$$

Tip loss. The duct prevents circulation loss at the tip if the clearance between tip and duct is small enough. Prandtl has been working on this in 1919 using momentum theory, an example for an untwisted blade is depicted below.

There is also this masters thesis, with measurements of small ducted rotors with low tip speeds.


Does the term track guidance apply only to conventional procedures? In ICAO's 8160 "Aircraft Operations" manual there is a term called "track guidance", for which I can not find any clear definition.

A few examples:

  Departure Procedures 3.1

  3.1.1 There are two basic types of departure route: straight and turning.

  3.1.2 Track guidance shall be provided:

  a) withing 20.0 km (10.8 NM) from the departure end of the runway (DER) for straight departures; and

  b) within 10.0 km (5.4 NM) after completion of turns for turning departures.

  3.1.3 Surveillance radar may be used to provide track guidance.

..

  3.2.2 Types of Straight departure

  b) straight departure with track guidance:

  1) facility ahead or behind; and

  2) offset (track parallel/track offset/track crossing).

..

  1.6 TRACK GUIDANCE

  1.6.1 Track guidance should normally be provided for all phases of flight through the arrival, initial, intermediate,
  final and missed approach segments. When track guidance is provided, the appropriate segment shall lie within the
  established coverage of the navigation facility on which the track guidance is based.

Judging from the context in which track guidance is mentioned, I would say that it refers to conventional navaid-based procedures, where the aircraft needs navaid facilities in order to keep on track.

All things considered, does the term track guidance have any meaning for RNAV/RNP procedures? Are RNAV/RNP procedures considered procedures without track guidance?

Any clarification would be appreciated.

ICAO Doc 8168 "Procedures for Air Navigation Services - Aircraft Operations - Volume I - Flight Procedures" states under 2.1.3 Types of SID:

  Track guidance may be provided by a suitably located facility (VOR or NDB) or by RNAV.

To answer your question: RNAV/RNP SIDs are considered procedures with track guidance.

The better your tracking is the smaller the protection area required (especially for turn construction). So flying inbound or outbound a navaid or having RNAV capabilities greatly increases your precision und therefore allows for procedure construction with relaxed climb gradients in areas with difficult obstacle situation.


Why can't you just set the altimeter to field elevation? Why do you have to put the current altimeter setting in the kollsman window? Why can't you just set it to field elevation when you're on the ground? I understand why you would have to do it in the air.

Good question. The reason is things fail.
If you set the altimeter to show field elevation, but the altimeter is faulty, you could end up flying differently from the rest, leading to loss of separation or worse.
But if you set the QNH, and then the reading is within 75 feet of the field elevation, you're good to go.

3. Altimeter Setting Procedures
3.1.3 If the difference from the known field elevation and the altitude read from the altimeter is plus or minus 75 feet or greater, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction. (USA AIP ENR 1.7)


May an emergency aircraft land on a closed runway? Let's say that you're on approach to a remote airport to refuel (for arguments sake, Funafuti International Airport, Tuvalu), with 1-2 hours of fuel remaining in your aircraft. Contacting ATC to request permission to land, they tell you that the only runway is closed for an unspecified reason and will not reopen for ~2 hours.

The nearest airport is 295nm away, so you agree to hold, hoping the runway is soon reopened. After waiting a little while, the runway is still not reopened, so you declare an emergency.

Is the airport required to reopen the runway? Does the fact that the nation is not a part of The Chicago Convention change anything?*

Supposing ATC refuse to reopen the runway, is the pilot permitted to land anyway?

Let's presume the aircraft is not certified for ditching.

Searching Google, I found that at least one emergency aircraft has landed on a closed runway.

* 2020 update: in Nov 2017 Tuvalu became ICAO's 192nd member state (icao.int).

Preflight Planning

  Let's say that you're on approach to a remote airport to refuel, with
  1-2 hours of fuel remaining in your aircraft. ...

  The nearest airport is 3-4 hours away...

First of all, it looks as if the pilot has already broken several regulations in order to end up in this situation.  He either didn't file an alternate, or if he did then he didn't carry enough fuel to get to it (and additional reserves as required).  If the runway is closed for an extended period of time, there would likely have been a NOTAM stating that the runway would be closed.  All of this sounds like poor preflight planning, and probably the 'ole careless and reckless thrown in for good measure.

The Problem

  Contacting ATC to request permission to land, they tell you that the only runway is
  closed for an unspecified reason and will not reopen for ~2 hours.

Okay, we need to figure out what the "unspecified reason" is because this can change things dramatically.

Is it for endangered migrating turtles?

Is it for construction?

In any case, he needs to use all available resources.  Talk to ATC and explain the situation. Ask them how they can help clear the runway so that we can land.  He should tell them that an emergency is imminent without their assistance.  They will do everything that they can to help you, including coordinating with people on the ground to get out of the way.  Be creative.  Did you know that there is no regulation (in the US) requiring you to land on an actual runway?  Ask them if you can land on the taxiway.  For that matter, land in the grass beside the runway.  It would be better than going for a swim or crashing after running out of gas!

  Let's presume the airport is surrounded by ocean, and the aircraft is not certified for ditching.

Ummm, who cares if it is certified for ditching?  If you are over water, it doesn't matter.  If you are over land, it doesn't matter.  If you have a choice, pick land (even if it's off-airport).

  Supposing ATC refuse to reopen the runway, is the pilot permitted to land anyway?

Well, they wouldn't really do this unless there was a really good reason, mainly meaning that it would be dangerous for the aircraft to land there (like in the second picture above).  In this case, we can't really ignore ATC and land anyway until:

Declaring the Emergency

  ...so you choose to hold, hoping the runway is soon reopened. After
  waiting a while, the runway is still not reopened, so you declare an
  emergency.

At least our poor pilot did this right.  Rather than blindly continuing to hold, he declared an emergency.

Getting on the Ground

Now we have a lot more latitude and options, and ATC is better able to help us because they can pretty much do whatever it takes to save the aircraft.  This can change a "Sorry, there is nothing I can do." into a "Cleared to land any runway or taxiway." all on its own. They may throw in a "Landing at your own risk." if there are still hazards, but that's just to clear them of liability.  Even if they still refuse, you can simply advise them of what you will be doing and go ahead and do it.  Keep in mind however, that it may be better to scope out an off-airport landing site if the danger is so great that ATC won't allow you to land even under such dire circumstances.

Remember, the pilot is the final authority for safe operation of the aircraft.  Get on the ground and sort it out after the fact. (Poor turtles....)

In the end, just do what you can to stay alive, and preferably use the airplane again.  There are going to be tons of questions and probably major legal battles.  Someone will probably go after the pilot's certificate.  On the bright side, he will be around to actually see it happen though.


What is the reason why dihedrals are used when building a plane? How does dihedral affect flight characteristics of a plane? I know it increases stability, however is it the only reason why it is implemented? If so, aren’t there more effective methods to stabilize the plane?

Because when an airplane rolls to a bank angle, the airplane begins to slip toward the lowered wing.  If the wings are mounted with a dihedral, this causes two things:  the inside wing will experience an increased angle of attack, and the outside wing experiences a decrease in angle of attack.  This combination causes an imbalance in the lift forces on the wings resulting in a rolling moment about the center of mass and returning the airplane to a wings level condition.
Dihedral angle is much more common on low wing airplanes, since the location of the CG above the CL creates a static unstable roll condition when the airplane is banked.  Dihedral angle can be used on high wing aircraft as well to enhance roll stability along with the pendulum effect of the CG below the CL e.g. a Cessna 172 still employs a slight dihedral angle to its wings.
In the age of digital flight control systems dihedral angles to wings are not absolutely necessary, though for light airplanes and gliders it’s far more elegant to design the airplane with a slight natural static and dynamic stability in all three axes.  Aircraft designers also work to ensure that airplanes have more authoritative directional stability than lateral stability from a dihedral angle to prevent unwanted Dutch rolling tendencies.


How can we operate wing bending tests? I understand why we do wing bending tests. Now I want to learn more about the test. How is it performed?

I mean in what conditions, with what equipment and what are we expecting after we perform this test?

First you need a wing that is representative of a flightworthy article.

Next, you need to collect the significant load cases. The most obvious one is that of maximum lift, but sometimes wings have failed even when they were strong enough to support the maximum bending moment. Torsion from aileron deflections and flutter are important, as is the in-plane bending moment from lift at high angle of attack.

Normally, the wing is placed in a rig and computer-controlled hydraulic jacks are used to pull on it, but this can be made much simpler. Below you see the ultimate strength test on the MIT Dedalus wing: Discarded soda bottles were collected, filled with a pre-calculated amount of water and hung from the main spar while the center of the inverted wing was lifted up by a forklift.

Load test on Dedalus wing. Yes, the guy with the Daedalus shirt is Mark Drela. Own work.

Besides the maximum strength tests, fatigue tests are of equal importance. Here a statistical load distribution (frequent low loads, infrequent high loads) is applied to the wing over thousands of hours. These tests stretch into the early operational phase of a design, such that the structure is cleared for more hours as the test progresses.


Why is there a maximum landing weight limit? I was told that if the aircraft's weight exceeds the maximum landing weight, it could cause a structural damage during landing, but why?

My understanding is that with a heavier aircraft, I need fly with a faster approach speed; but if the runway is long enough, I could land with more lateral energy with minimized vertical energy without causing any structure damage, is that right?

At the instant of touchdown, any vertical kinetic energy the plane possesses must be absorbed by the landing gear, which thereafter must bear the entire weight of the plane. That kinetic energy depends on the sink rate and the plane's weight.

The sink rate ideally will be zero at wheels down in a perfect world, but the gear is designed to accomodate a certain range of sink rates and plane weights via compression of its spring struts and shocks. If either the sink rate or the plane's weight causes the maximum compression range to be exceeded, the suspension will run out of travel and bottom out hard and the resulting shock can damage the attachment points of the gear to the fuselage or the gear itself- or even cause the gear to collapse.

The maximum gross weight at takeoff (full tanks, maximum payload) is set by engine power and wing lift when configured for takeoff and the expectation is that by the time the plane has reached its destination it will have burned off enough fuel to be under the allowable gross weight at landing (which is set by the design of the landing gear as explained above). This means that the plane's designers can minimize the weight of the landing gear by not having to strengthen it enough to routinely accomodate a landing at maximum gross weight at takeoff.

And that, in turn, means that if a fully-loaded plane has to make a quick return to the line for any reason, it must dump fuel first.


Does lateral stability decrease during transonic flight? If so, why? I haven't got much more to explain other than the title of the question itself. This question was written down in my notes that I took while studying Principles of Flight for my EASA commercial license.

Ideally it would not. With increasing Mach number the lift curve slope of the vertical tail goes up proportional to the Prandtl-Glauert factor $\frac{1}{\sqrt{1-Ma^2}}$ while the destabilising fuselage as a slender body is mainly unaffected by Mach effects. But that is only theory which assumes a perfectly rigid body.

In reality, elasticity decreases the effectivity of the vertical tail. Higher loads lead to higher deformations which in turn decrease the loads and with them the stabilising moments. What happens in detail depends on the structure: A light aircraft will experience decreasing directional stability as dynamic pressure increases.

At supersonic speed, fin effectivity is further reduced by the now again decreasing lift curve slope as Mach increases further. Now even the rigid aircraft experiences decreasing directional stability and elasticity will reduce stability further.

Directional stability over Mach, from Ray Whitford's Fundamentals of Fighter Design


What are the abandoned technologies in aviation industry And technologies has come to end of to their life cycle  Especially technologies that developed for aviation. This could be a manufacturing technology or an engine rather than avionics or sensors.

The biggest one that comes to mind for me is Cathode Ray Tube technology for cockpit displays.  Rockwell Collins (called something else now) has ceased production of CRT displays and users of Proline CRT Primary Flight Displays, Secondary Flight Displays and EICAS systems are in a scramble to replace the CRTs when they wear out.  There are a number of companies offering LCD units to upgrade CRT units under STC and this is happening across the airline industry.

Another one could be navaids like Non Directional Beacons that are starting to go the way of the A/N Range and the related receivers, and eventually other analog navigation equipment.

Mechanically, not that much.  Light aircraft are still built today with magnetos that use points and condensers like tractors from the 40s.  They are simple and work.  The business is very conservative, a vital self-preservation characteristic in such a life-or-death environment, and things that are simple, work, and are easy to fix and easy to diagnose, and let you know when they are degrading, are hard to replace.


How do you choose CAT II vs. CATIII ILS? When should we use CAT II or III during landing in bad weather? What we have in our country the standard shows 800m and RVR of 550 m for the ILS, which category procedure should be used if the visibility below or above?

On the other hand nothing tells us if it is CAT II or CAT III inside the charts except at the large airports.

And regarding the DH during the CAT III is it always 50 RA ?

If both are available for the runway you're using, it's often easiest just to set up the Cat III, because you have the least risk that way of a random cloud between the Cat II DA and the Cat III DA interfering with your approach.

That said, there may be reasons to prefer the slightly higher mins of the Cat II, and those are typically specific to your equipment and your procedures.  Perhaps crosswinds, either steady or gusts, make the Cat II more desirable, or if your equipment has a limitation on "XYZ must be operative for Cat III approaches" and that happens to be deferred on MEL for you, then the Cat II is the better (only) option.  But generally, if both are available, setting up for the Cat III is usually preferred unless some specific factor drives you to the Cat II.

Separate from this, some runways only have a Cat II approach and not a Cat III available.  This is typically driven by runway lighting differences, although other equipment may drive this as well (specifically, a backup Glideslope transmitter, IIRC).

As much as I hesitate to answer "always," I can't think of any Cat IIIA DA that's other than 50' RA, and I can't think of a reason it would be, since you're over the runway itself by that point.  Cat II DA's can be slightly above or below 100' RA since the terrain you're over when you reach 100' above the TDZ (on glideslope) may be slightly above or below the runway elevation, but that's a Cat II thing, not a Cat IIIA deal.


Even after years of research, why are planes unable to keep passengers alive in case of a fiery crash? Aviation dates back to the 18th century, and since then tremendous research has been put into making aircraft safer and more efficient. Though these efforts have resulted in better aircraft, why are they still not strong enough to keep the passengers alive in case of a fiery crash?

In recent times, especially, it's a rare incident that a plane crashed and even some of its passengers survived. So, why has this issue not yet been resolved?

Short answer

The kinetic energy involved in a plane crash is inhumanely high.

Slightly longer answer

We can built bombs which will go through concrete roofs and ceilings of a bunker, counting the number of floors they crash through while descending so they can explode at the level where the bad guys sit and not where the widows and orphans are kept. We could equally well build a plane with this kind of strength, so the fuselage stays intact even when it crashes into a mountainside. That's not the problem.

Limits of the human body

The problem will be that rescuers will only find dead bodies inside. The human body was "designed" to endure things like running into a tree, but not being hurled around at Mach 0.85 and then being stopped almost instantly. Thanks to years of research we now have quite a good idea where the limits are. Martin Eiband collected a lot of data on this, if you want to know more, google for "Eiband diagrams". If you want the full picture, read the Army Aircraft Crash Survival Guide. It comes in five volumes, and volume 1 covers the design criteria. The Eiband diagram below is lifted from this source.

Note the time scale: A deceleration with 40 g can only be tolerated for a duration of 0.1 seconds; if the deceleration takes more than 1 second, the limit is only 10 g. Now let's see what deceleration distance is required to stop a human with an average deceleration of 10 g. The energy $E$ of a body of the mass $m$ increases with the square of speed $v$:
$$E = m\cdot\frac{v^2}{2} = m\cdot a\cdot s$$
At $a = 98.0665$ m/s², the stopping distance $s$ from an initial speed $v$ is $\frac{v^2}{2\cdot a}$:

Car crash at 30 m/s: This needs 4.6 m and good restraints, but is generally survivable.
Free fall at 60 m/s terminal velocity: 18.4 m. A few humans have indeed survived this by falling into soft ground like a snow-covered conifer forest. This is also the typical approach speed of airliners, and the 18.4 m is the fuselage ahead of you. That is the reason why a crash is more survivable for the occupants in the rear seat rows.
Propeller airplane flies into mountain (120 m/s): 73.42 m. This kind of crumple zone is simply not available, and nobody has survived such an impact.
Airliner flies into a mountain (240 m/s): 293.7 m. To make such a crash survivable, every airliner would need to carry a 300 m boom of stiff material around which would be needed as a crumple zone in an accident. Just think what kind of aft fuselage and tail this would require.
And to top it: Astronaut on a space walk collides with a satellite in an opposing orbit (16,000 m/s): 1305.23 km. Note that I had to switch units to keep the number manageable.

Please consider lower deceleration limits for elderly and untrained persons; the limit in the Eiband diagram was established using healthy young pilots (and hogs, chimps and corpses for the higher limits. A lot of blood was spilled to arrive at these numbers).

The problem is not the aircraft structure, it is the fact that humans like to go fast but are not built to stop quickly.


Why should you not turn in the direction of an inoperative engine? In two-engine aircraft with wing-mounted engines when one engine quits the aircraft will have a natural tendency to turn to the dead engine. So if you need to turn, it seems logical it should be easier to turn that way.

However in the discussion here is a comment:

  By dinger on Monday, Aug 11th 2014 13:42Z:
  SOP in a twin is NOT to turn in the direction of the failed engine. Right engine failed and they turned Right.

Is it really standard procedure, and if it is, what is the reason behind it?

The AvHerald comment is correct, you generally do not want to turn towards the dead engine. The aircraft will tend to turn (both yaw and bank) towards the dead engine due to asymmetric thrust, allowing it to do so at low speed will make it difficult to end the turn, possibly to the point where you lose control. If you turn away from the dead engine, you'll have a tougher time getting into the turn, but the live engine will help you get out of it. That said, attempting low altitude turns with an engine out seems like a bad idea, you should concentrate on going straight and maintaining optimum airspeed to make sure you get some altitude

There are a couple of mnemonics when dealing with engine failures, like "dead foot, dead engine" (determining which engine failed) and "raise the dead" (keep bank towards the live engine)

I can only speculate as to why they chose to go right, either they wanted to avoid the populated area, or they were already unable to control the turn. The high density altitude would certainly affect the OEI performance as well, perhaps luring them into losing airspeed below Vmc.

Disclaimer: I'm not multi engine rated, and have no first hand experience. This is just what I've read.


Can I do an alternator check at idle in a C152? I have one question about alternator check in Cessna 152. During engine run-up we should check whether alternator is working or not, so we switch it off and then the warning light should illuminate. But we do it on pretty high RPM (1700). Would this warning light illuminate if I switch off the alternator also on lower RPM i.e. idle?

The problem is the light might come on on its own at idle, which is normal.
The warning light comes on when the system voltage is below 12.5 volts on a 14 volt system, and 25 volts on a 28 volt system. Automotive alternators (which most Lycomimg powered light aircraft have) need to spin at at least 2000 rpm to make adequate charging current to maintain system voltage under load.
The belt drive on the engine that overdrives the alternator relative to engine rpm may or may not maintain this rpm with the engine idling, so the bus voltage of the system may be just over 12.5/25 vdc or just below it, depending on the loads on the system (like radios and lights, etc.) and the alternator's condition.
The 152's POH specifically warns you that the light may come on after extended idle periods with electrical loads on the system, so this is to be expected, and it means that a test done with the switch can't be trusted to give a valid result (the light could be on because of "normal" idle low voltage, not because you switched it off).
Doing it at runup rpm just makes sure the alternator is spinning well above this marginal operating condition so it's producing normal charging current when you switch it off.


What Is a "Fully Stalled" takeoff? Reading this article (on the Boeing 737 MAX saga; irrelevant), I found the following excerpt:

  Boeing ultimately accepted Davies recommendation and, we understand,
  halted production of the 707 for 10-11 months, whilst a new fin was
  designed and installed.  All existing 707s were then retrospectively
  modified.  A ventral fin was also installed on some variants to
  prevent ‘fully stalled take-offs’ which the Comet was also susceptible
  to.

I tried to search, but I found nothing on "fully stalled take-offs", whether about the Comet or any other aircraft.

So, what is a "fully-stalled take-off"? Are some aerodynamic designs (or aircraft models) more susceptible to it?

A "fully-stalled take-off" is an attempted takeoff where the elevator has too much authority. This can cause the aircraft to rotate to an excessive nose up attitude, which can exceed the stall angle of attack, before takeoff speed is attained.
Ventral fins were added to some B707s to improve yaw control, and prevent over-rotation. Over rotation which could lead to a "full stall at takeoff".
https://en.wikipedia.org/wiki/Boeing_707

The British Air Registration Board refused to give the aircraft a certificate of airworthiness, citing insufficient yaw control, excessive rudder forces, and the ability to over-rotate on takeoff, stalling the wing on the ground (a fault of the de Havilland Comet 1). Boeing responded by adding 40 inches to the vertical stabilizer, applying full instead of partial rudder boost, and fitting an underfin to prevent over-rotation.


What punishment will I get from CASA if I fly in a cloud while a student pilot? I am a student pilot in Australia. Someone said he saw me fly into a cloud at 1700 feet above ground level for five seconds. But I am sure I did not fly in the cloud. I have no instrument rating, nor a pilot license yet. I am grounded and waiting for further investigation.

What punishment will I get from CASA if it rules I flew through a cloud?

I have had a fair bit of experience with CASA, including a close friend who worked for them for a number of years. They have a bad reputation these days but I'm not sure it's entirely deserved. I obviously cannot predict exactly what they would do but I hope to give you some advice.

First things first, I would believe your version of events much more than someone on the ground - slant visibility and all that.

The regulations (CAR 172) effectively states that if you fly into cloud the penalty is 50 penalty points - I think each penalty point is currently 110 dollars so the answer to your question is you could be fined $5500. However, CASA uses discretion as to when to enforce this fine - and the Department of Public Prosecutions gets the final say. If it is not a good use of their time, or if it doesn't really serve the public interest, they won't bother with it. A student pilot who accidentally flew into cloud? I would be very very surprised if they followed it up - particularly because as a student you are under the care of an instructor, who shouldn't let you fly solo if there was cloud in the immediate vicinity.

I don't think this would apply to your situation, but a more common course of action for CASA is the "fit and proper person" catch-all. Basically it means CASA must be comfortable that you will be responsible with the powers your licence gives you. CASA uses this power to remove the licences of people who frankly, shouldn't have one. It tends to be reserved for people who break the law either intentionally or through gross negligence. Also for example if you are charged with a non aviation offence, like fraud, CASA might take your licence claiming you are not a fit and proper person to hold one.

Other items in CASA's arsenal include temporary licence suspension, forced re-training or re-sitting of an exam or flight test. There is also a 'demerit points' system if you are a repeat offender.

Some general advice now (remembering that I'm not a lawyer) - don't lie. According to my friend (who should know), CASA is more concerned about your attitude than anything. They want to see people taking responsibility for their mistakes and accepting the consequences. This certainly doesn't assure you immunity but you might find that this reduces a licence suspension from 24 months to 6 months. If you are particularly worried, remember that you don't need to tell CASA anything if you are the person under investigation - like in other legal matters, you have the right to remain silent. Before any action is taken against you you will receive a "show cause" notice, which tells you what they are planning to do and invites you to tell them why they should not do that. If you don't like what they propose I'd advise upon getting a lawyer to respond on your behalf.

But all pilots make mistakes and only 'temporary pilots' pretend otherwise. If you really did fly into that cloud, thoroughly debrief with your instructor and turn it into a learning opportunity. If CASA does come knocking then you can talk about what you have learned and that you are a better pilot for it.

But, in the circumstances you've described, especially as a student pilot, if CASA comes down on you I would be questioning where my taxes are going. What you are going through must be intimidating but don't be fearful - if your goal is to get into the airlines you will quite likely experience this sort of situation again. As long as your intent was never to break the rules or put anyone in danger, you honestly should be fine.


Why doesn't the autobrakes and ground spoilers deploy on the A320 if the speed is below 72 knots? I really can't understand the reason as to why Airbus has put a speed restriction of 72 knots for the activation of autobrakes and deployment of ground spoilers. Could someone explain why this has been imposed?

Not a definite answer: I imagine someone backtracking on a long 4000 m runway might actually taxi at around 71kts?

If armed, MAX braking will occur once the system detects that ground spoilers are deployed.
From the FCOM, ground spoilers:

  If the ground spoilers are armed and the speed exceeds 72 kt, the ground spoilers will automatically extend as soon as both thrust levers are reset to idle.

Because of Airbus SOPs by the time you enter the active rwy, your autobrakes are on MAX and spoilers armed. Once armed&active, they will deploy the second the ADIRS will sense any deceleration. With thr lvl retarded to idle, the plane now thinks you are actually rejecting a take off, not knowing you are slowing down at the end of the runway for the 180deg turn to lineup. All hell will break loose as your plane insta-handbrakes on the spot with the activation of spoilers+max brakes.

Incidentally, my company used to have a 30kts speed limit for backtrack which has been recently removed.


Being told to monitor a frequency, when are you allowed to transmit on that frequency? Changing frequency, how long do you have to wait before you are allowed to transmit if you have been told to monitor the frequency. Is there a specific time limit? In what aviation manual can the answer be found?

First a note on phraseology. In the context of your question, I believe the FAA uses monitor like most of the world uses stand by. I'll be covering the general procedure - if you are in FAA land, replace stand by in my answer with monitor.

If told to monitor a frequency, you are not expected to transmit on that frequency. Monitor is used when a pilot is requested to listen on a frequency to retrieve information from a broadacst. For example, I might tell you to Monitor ATIS 122.750. I then expect you to tune 122.750, listen to the ATIS transmission, and then return back to my frequency afterwards.

If told to stand by on a frequency, you are expected to tune that frequency and wait for air traffic control to contact you there. For example, Standby for Tower on 118.1. When standing by on a frequency, air traffic control is supposed to contact you, not the other way around. However, if you do not get contacted within a reasonable amount of time, you should go ahead and transmit. There is no fixed rule for how long you have to wait - use common sense - just like you would if told to stand by on your current frequency. For example, if you are waiting in line for departure and there are 3 aircraft ahead of you, don't expect the tower to call you before you are next in line for departure. On the other hand, if you are holding short of the runway and there appears to be no other traffic around preventing you from taking off, if the tower hasn't called you within a minute or so, please do try to call them instead. Maybe the controller is busy dealing with other stuff, or maybe they have simply forgotten about you. It does happen, although not often. You could also accidentally have tuned the wrong frequency, and might only realise this when no one replies to a call made by you. It goes without saying that, if you have a safety related message, you transmit it without delay.


What is the method to find the resonance frequency of a PowerFlow exhaust? I asked a related question about PowerFlow Exhaust systems, which produced another question for me: How do they go about finding the resonance frequency so they know how long to make the exhaust pipe?

I guess I'm interested in the technical aspects of it. What formula do they use? Do they have special equipment?

I think it's interesting that they now offer a "short stack" for some mods:

  “Classic” or Short Stack? The Classic is our original design from
  1999. It is the “ideal” tuned length for this engine. Everybody loves the performance, only a mother (or an engineer) could love the looks.
  So, we came up with a compromise: The Short Stack is virtually
  indistinguishable from the stock Cessna exhaust and will give you
  about 85% - 90% of the performance gain of the Classic in a much more
  attractive installation.

Is this one of those instances of the 80-20 rule (approximately)? It seems interesting that a similar length gives you 85-90% of the gains. I'm guess then, that it's not just about the length of the new exhaust, but also the width?

I think there is a misconception with how these systems work.  These are 4-to-1 or 6-to-1 header systems, as used on cars, and the primary feature of a header system is that the pipes between the exhaust ports and collector are exactly the same length, which is why, on cars, they snake this way and that, because the port closest to the collector has to have a pipe that is just as long as the port farthest away.

The objective is the make sure the exhaust pulse from each cylinder reaches the collector at an evenly spaced interval to minimize interference between the pulses, and also to have the low pressure behind each pulse help pull the next pressure pulse into the collector (they are sometimes called "extractor" exhaust systems because the pulse timing creates a kind of suction effect).

The timing of each pressure pulse's arrival at the collector is the key factor.  To the extent that tuning goes on, it's to make tiny adjustments in pipe length to account for small variations in pulse timing due to different pipe contours - a pipe with more bends may need to be shortened a tiny amount to because its pulse arrives a bit late because of its more convoluted path.  In other words the tuning is the ensure the pulses are arriving at the collector evenly spaced.

From a performance standpoint, longer equal-length pipes are better than shorter equal-length pipes because the inertia of the flow stream is higher and this helps evacuate the cylinders better, enhancing the whole suction effect, kind of the inverse of the slight supercharging effect of long intake runners on the intake side.

The short stack Powerflow system gets 85-90% of the gain because it's still a 4-1 header, which is the primary feature; it's just that with shorter pipes there is a bit of efficiency loss from the shorter flow run compared to long pipes. You also save a couple pounds with less piping.


Do flaps have a maximum speed limit but spoilers don't? Why? This related question on Aviation SE describes how in most aircraft spoilers (flight spoiler mode not ground spoilers) are deploy-able at any speed But flaps on the other hand mostly have a V_FE above which they cannot be deployed.

I assume the speed limit imposed on using flaps is due to structural reasons? I am just wondering why spoilers do not have a similar limit? Looking at a spoiler photo in the deployed position it looks like the spoiler mechanism ought to be under a lot more stress than the flaps. Am I wrong here?

Or are spoilers reinforced to a degree out of need that is not felt necessary for flaps? i.e. There can be a legitimate need to deploy flight-spoilers at a speed higher than for flaps.

Spoilers are used to shed energy when the aircraft is too fast or too high. That means they need to be used when the aircraft is flying fast and are designed accordingly.

On the other hand flaps are used to increase lift and when flying fast, there is plenty of lift, so there is no point in using flaps at high speed and therefore they are not designed for it.

Additionally flaps on large aircraft are more complicated mechanically. Spoilers only rotate around their forward edge, but flaps on large aircraft extend to the rear in addition to rotating downward.


Do any regional turboprop aircraft have leading edge high lift devices? I'm currently working on an aircraft design project for a regional turboprop. I wanted to know if there are any regional TP's with leading-edge devices. It seems that a lot of them have deicing boots on the leading edge that make high lift devices difficult to install. Also maybe TPs just don't need the additional lift.

Neither the Dash 8 nor ATR families (which cover the bulk of regional TP market) do.
Your last two sentence pretty much summarizes it.  Some of the additional lift potential of a slatted wing is from the camber increase when the slat is extended, but most is from the ability of the wing to operate well beyond the normal stalling AOA of around 15-16 degrees, into the low-mid 20s.  You have to design your airframe to operate at very high pitch angles to exploit this.  I'm not sure that boots are a factor because there isn't really anything stopping a designer from installing boots on a slat (it's effectively a rubber air mattress glued on the LE with contact cement).
Jets use slats to improve safety margins at reference (approach) speeds (also lowering the reference speeds somewhat) with their more highly loaded, high speed wings (over 100 lbs/sqf, with supercritical airfoils that have poor stall characteristics - the 737 is in the 140 lb/sqf range). Turboprops operate with somewhat lower wing loadings (under 100 lbs/sqf - the Q400 is about 97 lb/sqf) and more gentle-stalling airfoils, and don't really need slats to achieve the mission objectives cost effectively.
None of the DeHavilland Canada products, even those designed for extreme STOL like the Caribou/Buffalo or Dash 7, have used slat systems.  It was preferred to use fixed leading edge cuffs (drooping LEs) and elaborate double slotted flaps (full span in some cases) to get the desired lift coefficients at "normal" approach deck angles.
The regional airliners like the Dash 8 are not designed for STOL operation in the first place (the Dash 8 is a redo of the STOL Dash 7, which was a marketing mistake, developed around a "stub runway" concept based on MLS that never materialized), so there even less of a need for them.


What is the significance of stall speed Irrespective of the stall speed, the only reason aircraft stalls is when its control surface (elevator, aileron) exceeds critical AOA. Then what is the significance of the stall speed?

Does the aircraft stall at the "stall speed" even if its AOA is less than critical?

There is not one stall speed, instead there are several, depending on aircraft configuration and flight conditions. They are used to determine speeds for the safe operation of the aircraft or for limit speeds.

For example: The stall speed in landing configuration and level flight with the actual weight is used for determining the approach speed. By multiplying with a safety factor of 1.3, you arrive at the recommended approach speed. Or: The stall speed in take-off configuration is used for determining the rotation speed. And so on.

Another example: The speed range in which it must be safe to deploy flaps is limited by 1.4 times stall speed of the clean aircraft, or 1.8 times the stall speed in landing configuration, whichever is greater. Or: The maneuvering speed v$_A$ is calculated by multiplying the stall speed of the clean aircraft with the square root of the maximum allowable load factor. And so on.

If you wonder what a stall is, maybe it helps to read the answers to this question. It is not flow separation or exceeding a specific angle on the control surfaces that determines the stall condition, but the reversal of the lift curve slope over angle of attack: Stall is when more angle of attack will no longer increase lift, so it is the angle of attack of the whole aircraft that counts. Physically, this means that large-scale flow separation appears over part of the wing. A good design will still have fully working control surfaces at this point.


How common are regulations limiting when the FO can be PF? I was reading that in India regulators have specified certain airports at which the FO is not allowed to take off or land.

  The Pilot-in-Command shall not authorise a Co-pilot to effect supervised take-off or landing when TODA/LDA is  marginal. A margin of at least 1000 feet must be ensured.  Critical airfields,  [Mangalore, Port Blair, Leh, Agartala, Lengpui, Shimla & Kullu airports]  are specifically excluded for supervised take off and landing.

As a matter of fact they have several limits in place where 'supervised takeoffs and landings' are not allowed and the PIC must fly the plane.

In this unique to India or are limits on FO's common?

All the airports listed in the AIC are quite tricky to takeoff/land in. For example, the Lengpui airport runway is a classic example of tabletop runway.

Source: www.misual.com

The Leh runway is surrounded by steep hills.

Source: geolocation.ws

The kullu airport runway has a river in both its ends.

Source: www.airportsindia.org.in

This is the reason for these restrictions. I'm not aware of any such regulatory restrictions in case of other countries. However, the operating airlines can have their own rules about (Category C) airports.


What is pressure refuelling and how does it work? In this answer to this question, "pressure refueling" was given as a reason to not worry about fuel vapors escaping when refueling larger aircraft.

What is pressure refueling and how does it work?

With a pressure fueling system, rather than fueling through holes in the tops of the wings, a fuel truck or fuel hydrant is used to push fuel into the airplane through a fitting at about 50 psi. The fuel fitting is generally in the right wing. A fueling control panel allows the fueler to fill the tanks to the amount called out by the flight plan.

These large aircraft still have an overwing fueling port for those times when the pressure fueling system is inoperative.

There is nothing to prevent fuel vapors from escaping to atmosphere.


Does the sterile cockpit rule mean flight attendants could not inform the pilots if a passenger is in the lavatory while on final? Context:
I've seen this interesting question just the other day, asking what would pilots do after receiving this information while on final:
either go around or continue with the landing.

However many others mentioned the Sterile cockpit rule under 10,000ft. Meaning that they could not discuss this issue.
So what would this mean exactly? The flight attendants could inform the pilots but they can’t respond?? Or the FA’s just can’t/won’t inform the pilots about the situation at all?

"Sterile Cockpit" refers to the concept that pilots should not discuss anything not related to the flight during certain phases of flight (often defined as below FL100).

A passenger occupying the lavatory while the flight is on approach and passengers are supposed to be in their seats definitely does have an impact on the flight, so it does not fall under the "sterile cockpit"rule.


What is this wire going from mid-fuselage to the tail on this 737? In  this question the following image is shown, with the question what the two bulges at the aft fuselage are.

However, I'm wondering what this wire is that starts somewhere midway the fuselage, and goes to the vertical stabilizer.
Is it also radar related?

Image Source

It is the HF antenna. It was mostly used in older aircraft, which used them in trans-oceanic flights or in areas without UHF or VHF communications facilities.Modern 737s (and other aircraft in general) have done away with them. On later versions it was fitted in the leading edge of the Vertical fin of B737-200s, as shown in the figure below.

Image from b737.org.uk

Note that these aircraft were delivered in 1982-83. Newer 737s don't have these.


Is it okay to use magnetized tools on airplanes? Let's say you stored your tools on a magnetic organizer in your toolbox, such as the one pictured.

Now you go and use a socket or wrench that is slightly magnetized. Have you done wrong?

The answer: In most cases, no. There are a few exceptions.

The most common exception is when adjusting any sort of magnetic compass system. Flux valves, whiskey compasses, etc... are usually supposed to be adjusted with tools made of brass.

Even then, your typical A&P will adjust the whiskey compass, for example, wait for the thing to stop spinning like crazy, and then make a slight adjustment back the other way. He'll repeat that process until he gets it right. I'm not saying it's right, but it's what most of them do.

The only other scenario I can think of would be bringing those tools too close to a CRT-based display system. Magnetized tools can make the display go a little wacky.

There may be others, but that's all I can think of off the top of my head.


For an FAA commercial license, can I use EASA/ICAO 2nd class medical? I have an FAA 3rd class and EASA 2nd class medical. On my EASA 2nd class medical it says "This medical certificate complies with ICAO standards".

To obtain an FAA commercial pilot license, will I need to re-take the 2nd class medical by FAA standards with a different AME or does the FAA accept an "ICAO compliant" medical?

You can "obtain" an FAA Commercial Pilot Certificate with a Third Class Medical Certificate; the Second Class Medical Certificate is only required to be able to "exercise the privileges" of a Commercial Pilot.


How do pilots of small planes know where to move people? On a recent flight from Washington, DC to Toronto, the pilot gave very specific directions to rebalance the plane (a Dash-8 IIRC), something like "someone needs to move to seat 11E."

How did he know that putting someone in seat 11E would balance the plane properly? Does the plane weigh itself or is there a computer model somewhere that tells him what to do? Or would an experienced pilot be able to look at the seat chart and know what to do?

Depending on the airline, and the size of the aircraft, the Weight and Balance calculations for each flight are either done by the pilots, or by the Flight Operations, Load Department.

If it is done by the pilots, they will receive the loading information which contains the weight and location of all passengers, cargo, and fuel. They then calculate manually, or by using software, the exact center of gravity.

If done by the Load Department, the center of gravity information will be communicated by radio or ACARS datalink.

It is not unusual to require a last minute seating or cargo change in order to ensure the aircraft is within the proper center of gravity limits for takeoff.


What does STERILE RUNWAY mean? When a pilot receives the message : THE RUNWAY IS STERILE, what is the meaning?

A "sterile runway" indicates that all traffic has been cleared from the runway.

This can include both traffic currently active on the runway as well as potentially other inbound traffic on approach, or even elsewhere on the tarmac - if it is deemed disruptive.

A sterile runway is often used in emergency situations, such as when an airplane is coming in for an emergency landing. For the safety of both that plane as well as the others, the runway is cleared of any traffic.

That said, this term is not exclusive to emergencies. For example, the FAA requires 250 feet on either side of a runway to be clear of airplanes when a Commercial Airplane lands. An airport that is smaller and has tight taxiways might have an aircraft taxiing closer than that. To be within FAA compliance, they would need to ensure that the runway was sterile and that there were no active planes taxiing within that 250 feet at the time of the commercial plane landing. An example of this rule being observed includes this notice posted by Friedman Memorial Airport in Hailey, ID, where they adjusted their procedures in response to FAA determinations in order to accommodate commercial traffic and maintain a "sterile runway".


How does flying a helicopter compare with flying a fixed wing plane? There has been a lot of interest in the ease of flying a helicopter recently, and questions have arisen on how easy it is to control a helicopter in less than ideal circumstances such as low visibility.

What are the main difficulties in controlling a helicopter, compared to controlling a fixed wing aeroplane in similar circumstances?

Controlling a helicopter is trickier than controlling a fixed wing aeroplane, because:

a helicopter can hover, in which case it is unstable.
the helicopter rotor is horizontal,
the rotor has long blades, applying a lot of torque to the fuselage.

The hover. in a conventional helicopter with a single rotor on top, the hover is an unstable situation requiring constant correcting inputs with the cyclic stick, as shown in the above clip. The guest pilot on the left seat has a successful go at controlling the cyclic stick (the stick at the centre), while the impressively capable right seat pilot controls the pedals and the collective stick. The helicopter wants to fall away to the sides and forwards/aft all the time, from disturbances caused by the wake interference with the ground, wind etc. Random accelerations need to be corrected by providing control corrections, based on what the pilot sees in relation with the environment. Peripheral vision is pretty crucial in this, since it rapidly and accurately picks up movements. Four degrees-of-freedom need to be actively controlled: pitch, roll, yaw, vertical speed.

That is all to hover at constant altitude. Now hover and climb:

increase collective (more eying power), which results in more torque;
so compensate with the pedal input for the tail rotor, which increases sideways thrust;
which must be compensated by lateral cyclic. Three control inputs for one degree-of-freedom!

Horizontal rotor. Once the helicopter picks up speed, its vertical and horizontal stabiliser keep the nose more or less aligned into the airflow - more at higher speed, but the neutral angle also changes with speed. The picture above is also shown in this answer, and depicts what happens with relative blade speed in forward flight. The blade going forward creates more lift than the blade going aft (which is compensated by increasing the Angle of Attack (AoA) of the retreating blade), but there is an area of reverse flow to the side of the rotor axis which results in a sideways tilt of the rotor. Which must be compensated by a cyclic stick left input, more stick at higher speed.
Rotor torque. Installed engine power equates to torque applied to the rotor, times the rotor rotational speed. A propeller is smaller than a rotor and rotates faster: the propeller requires less torque application for a given engine power setting, and the ailerons at the end of the long fixed wings can easily compensate for that. The helicopter rotor requires a lot of torque, which requires a tail rotor to compensate. The faster the helicopter flies, the more torque, the more force required from the tail rotor - which thrusts sideways, creating a drift in the flight path, which must be trimmed by the lateral stick. Even with a vertical stabiliser, tail rotor thrust is a function of airspeed.

Humans can learn to control helicopters of course, but there is a lot more to it than controlling a fixed wing aeroplane. Our brains rely on inner ear and visual inputs for detecting accelerations. More brain stimulation by input detection via multiple channels, results in more precise control. Plus experience of course.But take away an input, like the peripheral vision when flying in reduced visibility, and the control of a helicopter becomes much harder.

In larger helicopters such as S76s, pilot control is helped by a Stability Augmentation System (SAS), an Inner Loop system with no feedback on the flight controls; and an Automatic Flight Control System (AFCS) which changes the trim point of the flight controls as a function of flight path and target point. But these systems are not flight critical: if they fail, the pilot must take over, and must be able to fully control the helicopter by manual input. From visual and inner ear cues, and this always remains harder than in the auto stabilising fixed wing.

Fixed wing aeroplanes can and must be designed to be aerodynamically stable. Helicopters cannot.


Why does a de Havilland Comet have radiation symbols on the instrument panel? This image of a De Havilland Comet cockpit shows a radiation hazard symbol on each of the three centre panels, next to the (to me unknown) blue, green, and grey indicators. Why are they there?
I can't find any resources to shed light on it, but then again, I'm no aviator myself so I probably looked in the wrong places.

Source: Wikipedia

Instrument markings back then were painted with luminescent paint containing radium, so they would glow in the dark continuously without having to be energized by a light source.
You can buy luminous watches today with the same feature that use tiny vials containing tritium gas for each marking (much less radioactive than radium and produces negligible amounts of radioactivity, so the watch can't hurt you). These also glow 24/7, but tritium has a shorter half-life and they lose about half their brightness every 7 years or so.
The radium paint was mostly a danger to the women who painted the instrument faces, who would lick the brush tips to get a fine point and could potentially ingest enough radium to develop radiation poisoning.
The radioactivity warning signs may be perhaps a little bit of overkill since the actual radioactivity emissions aren't that high and most of the risk is from ingesting radium directly; it's advised to avoid opening an instrument and poking around without appropriate precautions (If people knew the amount of radiation they're exposed to just being out in the natural environment, they'd probably want to move into a lead house.  I recall a story about a 3 Mile Island Nuclear employee who was setting off the radiation detectors at work while coming IN to work, not while leaving.  From some kind of radiation exposure while off work.) .


What does "speed checked" mean? Here is a Scenario:

PF: Flaps one.
PM: Speed Checked, Flaps one.

What does that mean and why do they have to say "speed checked" every time?

Because there is a maximum deployment speed for things like flaps, gear extension (and sometimes retraction). Exceeding these speeds and using those devices can cause damage to control surfaces or other equipment. The speed check is there so that when the PF calls for flaps, the PM remembers to check the speed before performing the action.


Why do they tell you to tie your lifevest on the left side during safety briefings? This might be UK specific, given I have only been flying internally recently.

During the safety briefings, when describing how to inflate the lifevest the instructions are always:

"Wrap the lines around you and tie them using a double knot on your lefthand side".

I have asked the crew and none of them can tell me why and I cannot find any reason online there is an emphasis on the lefthand side.

After many questions to the various crew members/ground crew on the next few flights I did, I have managed to get an answer that makes sense.

You are told to tie the life vest on the left side for two reasons:

The inflate toggle hangs on the right side of the lifevest and if you were trying to tie the knot below it then you might pull it by accident, inflating the lifevest inside the plane.
If you tie the knot with a bow, instead of the double knot, then if the bow was on the right side there is a chance that you will get it tangled in the pull toggle while moving around the plane, inflating it before you exit.


Why do pilots normally fly by CAS rather than TAS? I think I know the answer to this question, but I want to confirm. Airline pilots normally fly a CAS/Mach profile, which means that above the CAS/Mach transition altitude they fly by Mach, and below it they fly by CAS (calibrated airspeed). Why do they fly by CAS rather than TAS (true airspeed)? My guess is that they use CAS because that is what their airspeed instruments show naturally, whereas TAS requires computation to determine. Is that right? Or is it also because it helps to slow down in descent and speed up in climb (since a constant CAS in climb corresponds to an increasing TAS)? Thanks.

The reason is somewhat deeper than just the indication. Up until Mach-related effects come into play, CAS is what the airplane 'feels' as speed, and consequently, is all the pilot needs to know. The TAS, or even better, ground speed, is needed primarily for navigation, but not for flying per se.

All the most important aerodynamic quantities (lift, drag, moments, etc.) are proportional to dynamic pressure:

$$q = \frac{\rho V^2}{2}$$

where V is TAS. So apart from TAS, they all depend on the ambient air density $\rho$. CAS is, essentially, TAS 'corrected' for the factor $\sqrt {\rho / 2}$, i.e. the effect of density. Instead of two parameters now you need one. But it doesn't mean CAS is calculated from TAS; rather the opposite: it is easier to naturally measure CAS (or q), and hence it is displayed (becoming IAS). To get TAS, you need to measure $\rho$, which is normally done via known pressure altitude (i.e. static pressure) and temperature.

If you climb at a constant CAS, your TAS will increase as the air becomes thinner ($\rho$ falls), but most importantly, the airplane handling qualities remain largely the same. But at some point, you'll hit the Mach limit and will have to obey it. Technically, the transition is governed by Mach itself rather than the altitude.


What legal actions would be taken against a drone pilot if they flew a drone into class B airspace? Let's say someone has bought one of those quad copter camera UAVs and wasn't completely aware of FAA rules regarding airspace and where they can fly their drone, etc.  Then let's say that they flew it into the airspace of a Class B airport by accident.1  What would happen?

I assume the airport, if they knew about it, would shut down the airspace until the drone had cleared (kind of like that recent fire.)  But after that, what happens?  Does the FAA have a way to figure out who was flying the drone?  And even if they do figure it out, is their an FAA based penalty for flying into controlled airspace with an unauthorized drone?

1  Here's an example of a Class B airport, for anyone who isn't sure what that is: Lambert-St. Louis International.  So, basically, a really big airport.

For those who seem to think this question is a dupe of "Is it legal to fly a Quadrocopter/Drone into an Airport?  What will happen?":  The only fragments that kind of deal with my question are as thus:

From dvnrrs regarding legal action:

  Were you to actually fly it over the fence, you could be charged with
  trespassing. If you had any intent to cause harm, other charges might
  apply (up to attempted murder if you were trying to bring down an
  airliner).

From ratchet freak regarding what the airport might do:

  When the tower sees unauthorized entry of a drone they will clear the
  airspace and send security to catch the guy controlling it.

Both of these comments lack detail, and that's probably because both of those snippets are from answers that were mostly dealing with the safety aspects of the question rather than with the legal aspects (eg., could a drone actually cause damage).  I am after the legal aspect, and I am looking for more detail than "It's not legal, you may get charged" plus "they'll send security."

CGCampbell did start down that road by listing a statute that contains more detailed rules about what you can do, but it doesn't say what will happen if you don't follow said rules.

I want to know what laws the FAA will use to prosecute, how they will track down the perpertrator and, lastly, what evidence they will need to either fine or charge them with a crime.

The answer is somewhat unclear, as there is limited (but emerging) case law and regulation on the subject and the outcome would depend heavily on the details of the airspace violation. However, there are some principles that we can use to shape our understanding:

The FAA has jurisdiction over anything flying, including UAS

This was most recently restated in the NTSB decision in FAA v. Pirker. Combine the definition in FAR 91.1: "Aircraft means a device that is used or intended to be used for flight in the air" with the Applicability section in FAR 91.1(a):

  ...[T]his part prescribes rules governing the operation of aircraft (other than moored balloons, kites, unmanned rockets, and unmanned free balloons, which are governed by part 101 of this chapter, and ultralight vehicles operated in accordance with part 103 of this chapter) within the United States including the waters within 3 nautical miles of the U.S. coast.

We see then, that anything flying is regulated - either under part 91, 101, or 103. Since UAS are not regulated under part 101 or 103, they fall under Part 91. Finally, the FAA has authority to impose a civil penalty for violations of the FARs under 49 U.S. Code § 46301. Note that an FAA certificate is not necessary for the FAA to assess a civil penalty: in the Pirker case, the FAA assessed its penalty on a Swiss citizen who did not hold a FAA certificate.

Finding the operator of a UAS is difficult

Finding the operator of a UAS is much harder than finding the operator of a piloted aircraft. The FAA was able to find and assess a penalty to Raphael Pirker because he posted a video he made flying the UAS online. In most situations where UAS were where they shouldn't have been, the operators either were never found or turned themselves in:

UAS crash-landed on the White House grounds
UAS interrupts wildfire-fighting activity - a reward was offered for identifying the operator
UAS reported by pilots on approach to KJFK

Basically, if you're flying a UAS far enough away from yourself that a police officer can't see the remote in your hands, you're unlikely to be found if just your UAS is detected. UAS for personal use do not currently require registration or external markings. A recent research project proposes a tracking system - but would require "affixing radio-frequency detection tags to [UAS]."

Actual enforcement would vary

After all of that, what would happen if you did violate controlled or restricted airspace and were caught? This area is pretty fuzzy, and would depend strongly on a number of criteria, including severity and intent. The FAA's toolbox for airspace violations is broad, but is generally tuned toward certificated pilots. A certificated pilot who accidentally entered class B airspace might get anything from a slap on the wrist, to a required training session with an instructor, to a short-term suspension of their certificate, based on the situation: an overwhelmed or confused pilot would get more leeway than one who just wasn't paying attention or was reckless in their navigation.

Suspension and required training are not really in the FAA's toolbox for folks without a certificate, so you likely would expect some sort of monetary fine. The FAA is limited to $10,000 in its fines, so you could not be charged more than that, and could well see something less.

Prevention is the best cure

Some UAS manufacturers are building in safety restictions on their aircraft to prevent them from flying near airports (note that, at least in the linked situation, the restricted areas are less restrictive than B airspace, so would not necessarily keep you out of that airspace.)

The FAA is also working on a mobile app which should easily allow UAS operators to determine areas to avoid. This is relevant to the enforcement situation: if you enter airspace without clearance, but the app said it was OK, that would be a mitigating factor. More to the point, the app would provide information about the status of nearby airspace, allowing you to avoid violation, and the associated penalty, altogether.


When a Tower and the Approach Control closes who takes over the airspace? When a Tower and the Approach Control closes, who takes over the airspace?
and also what about When a Tower, within an Approach Control, closes, who takes over the airspace?

All airspace is ultimately owned by some Center (US: ARTCC, ICAO: ACC). Some is delegated to Approach facilities (US: TRACON), and Approach can sub-delegate part of their airspace to a Tower. And Center can delegate airspace directly to Towers without an overlying Approach.
If any facility closes, it’s associated airspace automatically reverts back to whoever had delegated it.


Is aspect ratio efficiency related to the momentum added to the air? Many years ago I was taught that for generating lift or propulsion, lower losses were associated with imparting less momentum change to a larger volume of air, and higher losses were seen when imparting larger momentum changes to a smaller volume of air - and that the losses were due to friction (i.e., the air's viscosity).

The propulsive example of this is the commonly-cited turbojet-versus-high bypass turbofan engine comparison, and I had been under the impression that the airfoil example was high aspect ratio versus low aspect ratio wing designs. However, having just learned that this is in fact incorrect (it has to do with induced drag instead), is there in fact any connection at all between the imparted momentum change/volume of air argument and airfoil aspect ratio?

You're almost there. The difference lies in the kinetic energy required to move a certain mass of air, not in the air's viscosity. Copying freely from one of my earlier answers:

  The force to keep the object aloft is $$F=m_{object}g$$ The force generated by downwards momentum transfer is $$F=\dot{m}v$$ with $\dot{m}$ indicating mass flow (kilogram per second) of the air (not the mass of the object). The energy flow (power) required to  impart this momentum on the airflow is $$ P=\frac{1}{2}\dot{m}v^2$$ Here we can draw an important conclusion. The power requirement is arbitrarily small, by increasing the mass flow and decreasing the downwards velocity.

The power requirement expresses itself in the form of induced drag. By affecting a large mass of air (a long wingspan), you reduce the power requirement, which manifests itself in the form of lower induced drag.


Can transition on an airfoil occur without prior separation? i.e. can transition occur without the presence of a laminar separation bubble? If yes, what would this look like in a pressure distribution?

Here, a clear laminar separation bubble can be seen.

The Reynolds number has been increased by a factor of 10. The transition point is still visible, but it is not so clear anymore whether there is a separation bubble. Is there a possible scenario where a laminar boundary layer transitions without first separating?

EDIT: I'm not asking about how to force transition in XFoil or in real life (using turbulators), but rather if "pure" transition can occur naturally on an airfoil.

can transition occur without the presence of a laminar separation
  bubble?

In short YES.
To elaborate, there are usually 3 mechanisms causing the boundary layer to go turbulent.

Natural transition
Forced transition
Bypass transition

Because the question is about the Natural transition; this is usually caused by sufficiently amplified Tollmien–Schlichting waves. The reason why you see a transition followed by separation in low Reynolds number flow is because in the separated flow, the amplification factor is around 4 times as much as for the attached flow.

  If yes, what would this look like in a pressure distribution?

  Here, a clear laminar separation bubble can be seen. The Reynolds
  number has been increased by a factor of 10. The transition point is
  still visible, but it is not so clear anymore whether there is a
  separation bubble. Is there a possible scenario where a laminar
  boundary layer transitions without first separating?

As indicated above, it is possible for a boundary layer flow to make the transition to turbulent without separation.
N-factor growth rate scales as $\sqrt{RE_\infty}$ for attached laminar flow and for sufficiently enough $RE$ you can easily see the transition undergoing without any separation.

Also, it's best to inspect the boundary layer parameters through the VPLO menu which gives a clear indication on whether the flow is actually attached or separated.

A very good reference for this is Prof Mark Drela's Flight Vehicle Aerodynamics. - Chapter 4.


What lead time is required to change an already submitted flight plan? A government agency of an EU country commissions a private company with a chartered flight from Europe to Africa/Asia/... (e.g. to involuntarily take people to a specific country).

Then it wants to change the time of departure with short notice. How long before the changed departure time do they need to file the new flight plan and what are some obstacles for changing it at the last minute?

The flight plan can be changed by the minute, as long as the two governments and the two airports and the airline agree on this.

As it seems like the government pays the airline to just take off whenever they are allowed to, take the airline out of the equation.
Likewise for the airports, since they are either state-operated or have a concession which states that they have to assist in lawful deportations, emergencies or state affairs.

This keeps two governments in, which have to agree on a time and the who-pays-whom-how-much for the deportation to really take place. In fact, I think this is the only reason why the plane is not yet airborne - because of two governments playing their diplomacy games.


How is it possible for supersonic aircraft to push air out of the way? If the force (pressure) can be transferred/propagated only at the speed of sound, how are supersonic aircraft able to move air out of their way/do work on the air? It seems that they must transfer the force faster than the speed of sound or else they would intersect with the air. Does the speed of sound increase at the shock to allow it?

The idea that force can only be transferred/propagated at the speed of sound is a simplification.  It's a very reasonable simplification, which works in a very wide array of cases, but it's a simplification.

A fundamental concept in gas mechanics is the "mean free path length."  This is how far a molecule can travel on average before it collides with another.  This is on the order of hundreds of nanometers for pressures you'd see at reasonable altitudes (68 nm at STP).  If you are looking at processes that occur at scales much larger than this, it is very reasonable to model their behaviors as transmitting things at the speed of sound, because these statistical effects lead to a very natural expected speed of propagation.  It can be seen as a result of the central limit theorem applied to a very large number of molecular collisions where the drift velocity (average movement of particles) is relatively smooth.

As you push things, this nice model breaks down.  As you approach the sound barrier, you get pressure gradients that get shorter and shorter.  Near the speed of sound, these gradients start to get into the single-digit-multiples of this mean path length.  At this point, we can no longer assume that we can just model everything with the nice clean statistical models we had earlier.

What actually ends up happening in most situations is that you get a "shock."  Far from the flying body, the air transmits force at the speed of sound with respect to the unmoving air (or slowly moving, if there's a wind).  Right up against the flying body, the air transmits force at the speed of sound with respect to the flying body.  At "reasonable" speeds, there's a smooth shift from one to the other, where the pressure gradients aren't so steep that we can't call them pressure gradients.

At supersonic speeds, that smooths shift doesn't happen.  Instead, we end up with a thin shock region (on the order of 1 μm or thinner) where it's not reasonable to model the air as having a "pressure."  It acts more like a bunch of billiard balls being flung along.  This region, a few mean free path lengths wide, is where the usual rules we're used to, break down enough to resolve the physics.  On each side of the shock, "normal" speed-of-sound behaviors occur.  Inside the shock, it's messy.


How Military to Civil Aircraft Transition is made by National Authorities? As we know military aircrafts are not regulated by civil aviation authority organisations,namely FAA, CAA and etc. Is there any possible way to transform a military aircraft( like helicopters) to civil ones? And if it is, where are these conditions mentioned?

In the US, ex-military aircraft must be issued a special airworthiness certificate before they can be flown as civilian aircraft. Some aircraft may be denied the certificate outright, but most can be flown this way. The hard part is maintaining them, the cost of operating them, and finding qualified people to crew them, including a qualified flight instructor and even a Designated Examiner if the FAA requires a type rating to fly it.

In some very rare cases, it is possible to get a standard or utility airworthiness certificate, but the aircraft must have been previously approved that way before you can do that.

In the majority of cases, the aircraft is given a special airworthiness certificate under the Experimental category, Exhibition subcategory. This means the aircraft can only be operated to exhibit the aircraft’s flight capabilities, performance, or unusual characteristics for air shows, motion pictures, television, and similar productions, and for the maintenance of exhibition flight proficiency. To fly it, you must meet certain experience requirements and in some cases you need to obtain a pilot type rating for that aircraft.

In other cases, if you want to race the aircraft in air races, you can get a special airworthiness certificate for that purpose, to operate the aircraft in air races, practice for air races, and to fly to and from racing events.

To get the airworthiness certificate in the first place you have to submit a maintenance plan for the aircraft to the FAA (often derived from maintenance plans others have drafted) and have it approved by the FAA, and the aircraft must be subject to regular inspections as per the plan.

Source: FAA Experimental Airworthiness Certificates home page

In yet other cases, some military cargo aircraft are converted for missions such as firefighting. Those are exceptional cases, almost unique to the US in that they are not required to comply with many federal aviation regulations.

  The Federal Aviation Administration (FAA) has abrogated any
  responsibility to ensure the continued airworthiness of "public-use"
  aircraft, including ex-military aircraft converted to firefighting air
  tankers. Although these aircraft are awarded FAA type certificates,
  the associated certification processes do not require testing and
  inspection to ensure that the aircraft are airworthy to perform their
  intended missions. Source

For other countries I suggest you list the ones you are interested in. I only know the rules for the US/FAA.


What is the feasibility of solar powered drones? What is the feasibility of solar powered drones? From my understanding, solar panels are becoming more and more efficient, in terms of weight vs energy captured, and in terms of surface area vs energy captured.

What I am interested in is whether solar panels could be placed on different sized drones (small civilian quadcopter and MQ-9 Reaper), and provide enough solar power to keep the aircraft aloft indefinitely (under a theoretical condition of an "endless sunny day").

If this is not possible, are there are realistic changes that can be made to make it possible, while still keeping the core functionality of these two different drones intact?

For the purposes of this question, instead of focusing on a 100% solar powered drone, we can assume that fuel can be carried for takeoff/climb to loitering altitude, at which point is can switch to 100% loiter.

Let's take the MQ-9 as an example...

The MQ-9 is powered by a 900 HP Honeywell TPE331-10GD engine. In order to generate 900 HP, it will take 2100 kVA (assuming 3-phase electric AC motor). 2100 kVA is about 1680 kW. That is 1.68 million watts.

Now, the sun delivers (if you could get 100% of the energy sent to Earth) about 4.8kW per square meter (depends on Latitude, which can range from about 6kW to less than 1kW). This would mean you need about 350 square meters of 100% efficient solar panels.

The problem is that the current "world record" for solar efficiency is 46%, let's round up to 50% for easy math sake. So we have to double our solar panel area to 700 square meters.

Now the wingspan of an MQ-9 is 66 feet (about 20 meters), and a length of 36 feet (11 meters). If the MQ-9 was box shaped, it has a surface area of 220 square meters. Lets assume that 20% of that is real "upward facing" surface area so in reality, if you covered the MQ-9 with the most efficient solar panels in the world, you would only generate (220 * .2 * (4.8 * .46)) about 97kW or about 6% of the power actually required to run the aircraft.

This is also assuming that the extra weight of the batteries/wiring/charge controllers don't make the aircraft heavier or require a larger power plant, and assuming that the 3-ph 2100kVA motor is the same weight as the TPE331-10GD.

This gets even worse for quad-copters because they just don't have the surface area, and don't benefit from aerodynamic lift to reduce the amount of power required to fly.

Sources:
USAF MQ-9 Facts
Wikipedia Solar Cell Efficiency