Are the vortex generators on the Cessna Caravan necessary and sufficient for safety of flight? I've been doing some reading about the C208B (Cessna Caravan) in prep for my new job flying cargo; I came across this article which (as far as I can tell) is a transcription of litigation surrounding the caravan crash in Dillingham, AK 2001*.

In it there is an interesting discussion of the design integrity of the Caravan's horizontal tail: specifically, are the installed tail vortex generators necessary (or even sufficient) to maintain safe, level, and controlled flight within a reasonable envelope of flight conditions?

Cessna 208 Caravan tail with vortex generators right ahead of the hinge line (picture source)

I'm no aerodynamicist, so please correct any mistakes in my reasoning:

When center of pressure, $C_p$, is forward of the center of gravity, $C_g$, then there necessitates an up-force on the horizontal tail in order to maintain level flight.

From the POH (pdf pg. 29) the aft $C_g$ limit of a stock 208B is 40.33% MAC aft the leading edge; whereas $C_p$ is typically located about 25% MAC aft the leading edge. This would presumably generate a significant nose-up moment and demand proportional upforce on the horizontal tail surface. At higher AOAs the $C_p$ would be even farther forward of the $C_g$ and, as the above article discusses - in specific such flight configurations its possible that the tail is actually in the downwash of the wings, which could disturb airflow across the tail and therein fail to meet such demands.

So, the questions I have are:

(1) How typical is it that aft-$C_g$ limits are behind the $C_p$, such as in the 208? Wouldn't this imply longitudinal static instability?

(2) How do engineers typically design tails to maintain necessary up-forces in such cases? Are vortex generators viable solutions or are they, perhaps, after-thought band-aids to remedy an inadequately designed tail? Does anyone have more published references about the caravan's tail specfically?

You need to keep several things apart:

The center of pressure is where all vertical aerodynamic forces can be summed up. It does not have a fixed location, but its location changes with angle of attack.
The aerodynamic center (or neutral point) is indeed at about 25% of the chord of a wing. Its location is fixed as long as the wing planform does not change.
The neutral point of the whole airplane is the weighted mean of the local neutral points of all horizontal surfaces. This is the most important neutral point for longitudinal stability.

In flight you will shift the center of pressure such that it is at the same longitudinal station as the center of gravity. You do this by regulating the tail lift or downforce with the help of the elevator.

  When center of pressure, $\text{c}_p$, is forward of the center of gravity, $\text{c}_g$, then [this] necessitates an up-force on the horizontal tail in order to maintain level flight.

This is true if you speak of the wing's center of pressure. In a conventional configuration like the Cessna 208 the wing's center of pressure is ahead of the airplane's center of pressure.

  How typical is it that aft-$\text{c}_g$ limits are behind the $\text{c}_p$, such as in the 208?

This is quite normal. No need for concern as long as the center of gravity is still ahead of the airplane's neutral point.

  Wouldn't this imply longitudinal static instability?

No. Stability is reduced compared to a more forward center of gravity location, but still positive.

  How do engineers typically design tails to maintain necessary up-forces in such cases?

The biggest concern of engineers in such a case are too light control forces. The force feedback loop is important for avoiding control deflections which could overstress the structure. Since the tail is in the downwash of the wing and carries less load per area than the wing, it is very, very unlikely that it stalls first. Without knowing the details, I am quite sure that the vortex generators were not added to cure a deficiency which occurs with a rear center of gravity location. Now I did some background reading and think that the vortex generators are needed to avoid early separation on the elevator in case of low-speed flight with flaps retracted and a rear center of gravity.

  Are vortex generators […] after-thought band-aids to remedy an inadequately designed tail?

Yes, they are. They normally cure one corner of the envelope where something went out of whack during flight testing or after a modification (say, an increase in gross weight or in the permissible center of gravity range).

  Does anyone have more published references about the Caravan's tail specifically?

Sorry, not me.

Now I have read the accident report and the linked transcript about the Caravan crash on October 10 in Dillingham, AK, in 2001. These are my remarks:

The witness in the transcript is very knowledgeable and I agree with all what he said.
His statement about the tail load change upon flap retraction seems spot on: Even though the accident report is inconclusive about the flap position, it seems likely that retracting the flaps coupled with insufficient lift on the tail made the aircraft pitch up and stall.
All the accident report has to say about de-icing focuses on the wing, but since lift-off speed seemed normal, I don't think ice on the wings was a problem. But what about ice on the tail? There is no mention of that.
Vortex generators on the upper side of the tail are unusual and a "red herring" about late fixes for insufficient pitch down control authority. Add some ice on the upper surface of the tail, and the fix will not be sufficient and cause an uncontrollable pitch-up with a rear center of gravity location.
That the pilot seemed to have pulled on the yoke on the way down did not help, either. Lift loss on the wing would cause a nose-down pitch change, but the aircraft remained stalled throughout its descent. Yes, it needs some effort to pitch down when the flight path is already 40° down, but this is the only way to avoid a crash. Remember AF447.

My conclusion from all this is that the Cessna 208 Caravan is stable and safe to fly as long as the tail is in pristine condition. Any contamination of ice and even de-icing fluid will compromise its ability to produce the required lift in low-speed flight with flaps retracted and a rear center of gravity.


What happens to Vyse and Vxse above the OEI ceiling? An EASA ATPL question/answer insists, counterintuitively, that a twin which loses an engine and must drift down will find its least rate of sink at Vxse. I've been trying to understand this, but haven't come up with an answer yet. Do Vy and Vx even exist as concepts when above the ceiling? Does "best rate of climb" become instead "least rate of descent"? Does "greatest distance between the power-required and power-available curves" become "least distance" between same? Would the speeds of Vx and Vy (with or without SE) change when going from below ceiling to above while at the same altitude and mass?
One way to determine Vx and Vy is to look at a ROC vs Speed chart, where ROC will be an inverted parabola. The peak of the parabola is Vy, the tangent of it to the origin is Vx. If the whole curve is shifted below the x-axis, however, the tangent will fall on the far side of the Vy peak. So I would expect Vx > Vy when above the ceiling. But that still doesn't help understand why EASA would say least sink is at Vxse.

Vx increases with altitude while Vy decreases with altitude.  When they coincide, you have reach the airplane’s absolute ceiling, which it can no longer climb and can only maintain altitude at that specific calibrated airspeed.  Vx and Vy do exist above you maximum single engine ceiling, however, should an engine fail above the maximum single engine ceiling the airplane will begin to descend in order to maintain a given airspeed.  Pitching the airplane for blue line, Vyse, and maintaining the correct bank angle in the direction of the good engine (3°-5° typ.) will result in the minimum descent rate during drift down.
I have no idea why EASA would state that the airplane should be flown at Vxse above the maximum single engine altitude to minimize drift down; every source of information I’ve ever encountered indicates your airspeed should be maintained at Vyse, in addition to maintaining proper coordinated flight with OEI, in order to minimize your drift down rate.  This makes sense, since the airplane will have the most power available to it for climbing and will result in the least loss of altitude over time in this configuration vis a vis conservation of energy.


How should flaps be used during an emergency landing (PPL)? I'm still doing my PPL, but one question has puzzled me recently. During an internal exam before my first solo nav on a C150, the examiner pulled the throttle, so I would do a simulated emergency landing. I chose a field right to me, and with a good gliding speed and without any flaps did a 180 turn (base+final) and found myself flying straight into power lines before the field. We had to go around.

The examiner showed me how it should be done. After the base turn, he deployed flaps 10 and then on final 20. We flew way above the power lines. Now I'm not sure all the altitudes and turning points were the same, but he meant this to show me that I should have used flaps.

My understanding had always been that the use of flaps during emergency landing should be restricted to the final leg from the point that you are certain you will make the airfield. My instructor (not the examiner) had the same view and could not explain it to me.

Any idea?

Specifically in your case you may have lost more altitude in the turn than your instructor or your turns were less coordinated etc. A wide variety of other variables that are hard to note with out accurate course data could have led to your outcome. You would have had to enter the maneuver at the same place/altitude to get similar results and the wind would have an effect here as well. Did you have a head wind or tail wind? Its worth covering some general procedures for this maneuver.

When turning under glide its important to watch your airspeed and know how the plane will sink in a turn since you sacrifice some lift for the turn.

Remember flaps allow you to make a steeper approach with out gaining airspeed but what that also means is your ground speed is effected (slowed).

When it comes to the emergency landing procedure you are free to use flaps at your discretion and as mentioned its generally taught to use them once you are sure you have the field made. If the field is long enough you should shoot for a mid point incase you come up a bit short. Likewise if you are high you can go full flaps and slip it to lose altitude fast (if your instructor has not shown you how to slip a plane to lose altitude I would advise asking him to do so).


Is there a standard definition of the "stall" phenomena? In the process of increasing angle of attack of a wing, lift is increasing to a maxima at a certain AOA, then decreasing as this AOA is exceeded.  This seems to be the current agreed upon definition of the stall point.

Yet the sharp pitch down and transition of the center of lift to 50% chord does not occur until the top lift pocket completely collapses.  Is this the true definition of "stall"?

There is also pre-stall buffeting as high AOA turbulence begins to affect the wing trailing edge.

So, are there really 3 parts to this that a pilot must understand, rather than just one?

The scientific definition of a stall is as you state in the first line, however in the cockpit that definition isn't actually that useful. Practically there's 2 parts of stalls a pilot needs to be aware of because they are things a pilot experiences, and impact the flying characteristics of the airplane:

Stall buffet: this isn't a stall, but it happens when you're very close to one. Recognizing a buffet may lead to a pilot decreasing AoA before entering a fully developed stall
Flow separation: when the flow separates from the top of the wings it may do so on one wing before the other, causing a wing drop, aileron input may work opposite of intent, and you of course have a high rate of descent. This can feel mild or violent depending on the airplane's characteristics and the speed at which the stall happens

Whether the definition of a stall is when the airflow separates or a change of the center of lift is academic, what a pilots needs to know is how to recognize the condition and recover.


Can I go into clouds if I got IFR clearance? (uncontrolled airport) Just to make sure if I am right before I get in trouble.

If I am taking off from an uncontrolled airport for IFR flight, assuming I received IFR clearance on the ground, can I enter into the clouds before I contact approach (or center) after takeoff? Even when the Golf starts at the surface or 700 ft?

Would clearance give me a heading and altitude that I would want to follow right after takeoff?

For typical uncontrolled airports which have instrument approaches, they are usually in Class G airspace from the surface to 700’ AGL.  In this airspace, yes, you can - at least from a regulatory perspective fly in IMC without ATC clearance, provided your remain clear of controlled airspace.

In order to fly a cleared IFR flight plan, you would have to depart into uncontrolled airspace prior to climbing into the Class E shelf above the airport.  Typically, IFR departure clearances from uncontrolled airports contain a time window in which you must be airborne and in contact with a regional departure control or ARTCC by eg “Clearance void 30 minutes from now”.  Basically, in doing so, ATC have given you a block of airspace which you have use of for a small time window in order to get airborne and join ARTCC on your cleared flight plan.  Once you have departed the uncontrolled airport on CTAF, you would immediately go over to your assigned departure control frequency and report in.  The controllers would thence take over and direct your flight.

A typical IFR clearance from a untowered airport would be something like as follows:

  Mooney Three One Two Seven Zulu, Atlanta Approach, Cleared IFR From Cartersville Victor Papa Charlie To Charleston Charlie Hotel Sierra Direct NELLO Then As Filed.  Climb And Maintain 5000, Expect 11000 Five Minutes After Departure, Atlanta Departure Control Is On 121.00, Squawk 2477. Clearance Void 30 Minutes From Now.

From the moment you copy the instructions and the controller acknowledges so with a “READBACK CORRECT”, you now have thirty minutes to get airborne, on a course direct to the NELLO fix and contact Atlanta Departure on 121.00 or the clearance becomes invalid and you must request a new clearance to do so.  If there are low clouds between you and the time you switch over to departure, so be it, you’re going into IMC.  Usually, though, it’s best once airborne and cleaned up, to immediately go over to departure control as quickly as you can.

Another example of IFR flight in Class G is flying an instrument approach into an uncontrolled airport.  Once you descend below the Class E shelf, the controller hands you off to CTAF and you continue on in uncontrolled airspace.  If you’re flying an ILS approach down to minimums, that may well be another 400-500 ft descending uncontrolled in IMC before you either break out and land or go missed, thence rejoin center to report a missed approach.


What happens to the cabin pressure if all the packs (ACMs) stop working? I have gone through the cabin pressurization article in Wikipedia. It says there will be some number of Air Cycle Machines (ACMs, or packs) to supply pressurized air into the cabin.

What happens if all the ACM machines stop working? Does the flight have enough oxygen on board supply all passengers until they can safely land the flight somewhere?

If all ACM failed, an alarm would sound in the cockpit. And two messages would appear on the ECAM/EICAS¹. One indicating that the ACMs failed and the other indicating that cabin altitude² is rising above desired level.

Pilots would execute corresponding checklist, and the one for rising cabin altitude calls for rapid descent to 10,000 ft. That is the altitude where humans can breathe safely.

If the problem is a hole in the fuselage, the oxygen masks would drop to allow breathing until the aircraft descends, and emergency descent (with full spoilers) would be done. But if it is just failed pressurization, the outflow valve would close and the pressure would only decrease slowly, so the aircraft would most likely descend before the masks would be needed and they would not release. Even normal rate of descent is usually sufficient in such case.

If you're flying above mountains, there are additional rules where to fly to avoid any terrain rising above 10,000 ft, which will be part of the flight plan. For example flying along the mountains of Peru, you'd need to deviate toward the ocean during such descent.

Now at lower altitude, the aircraft flies slower while burning more fuel, so it probably won't be able to make it to the original destination any more. However when planning flight and calculating the required fuel this is taken into account, so the aircraft has enough fuel that it can make it to one of suitably selected diversion airfields if it needs to descend to 10,000 ft at any point along the route.

¹ Modern aircraft have a display on which information about any problems and abnormal conditions is printed, which is called ECAM by Airbus and EICAS by Boeing. Older aircraft had large panels of warning lights.

² Pressure in the cabin is usually given as altitude at which that pressure occurs in standard atmosphere. Normal cabin altitude is 7,000–8,000 ft, 10,000 ft is maximum safe altitude (with generous safety margin). The oxygen masks fall out if the cabin altitude exceeds 14,000 ft.


How does one become an airline pilot in Australia? I am 19 years old and from a young age I have always wanted to become a pilot at an airline. Becoming one is my biggest dream, to me an airline pilot is the most coolest jobs - I get goosebumps just by thinking about me in that position.

I have finished my high school education and hope to go to Australia to study in order to become a pilot. However, most of the university programs don't give me the feeling that I can become a pilot. Most of what is learned and practiced is in small planes, even the career opportunities don't mention that one will become an airline pilot.

Does anyone know the best and fastest way to make my dream come true? What should I do?

The only course that has got me attracted to is the one that's in Swinburne University, but then again, small planes. I am fine with flying small planes, but where does that big jump to airplanes happen, if it's not taught in university. Where do I learn it? Does anyone know the procedure or the best place to train to become a pilot?

Also, I'm not an Australian so I will be an international student. I have checked for jobs in my home country airlines, they require a minimum of 5000 hours of flight, does this mean normal plane flight hours? Where will I get these hours covered from? Of course this many hours aren't offered in university.

I am Australian and I hope I can help.

Firstly, you don't need to fly through a university. There are hundreds of private flight schools here which are of great quality, and very few of them are associated with a university. More and more international airlines are sending cadet pilots to our schools. Most of them offer theory courses in addition to the flying, or you can always self study.

Having said that, the university courses are great and airlines like people with a degree (but it's not mandatory to have one). You'll be paying more though, it's a trade-off.

Your comment about small planes - All flying is initially in light aircraft - even if you got an airline cadetship you start small. No single course in Australia or anywhere will let your first flight be in a jet.

Speaking of airline cadetships - these are definitely the best and quickest way to get into an airline cockpit but the positions are highly competitive (and maybe not open to international students, I don't know). But you can still get an airline job without a cadetship...

The normal progression is this - you can technically get a flying job after 150-200 hours of private training and flying, having achieved your commercial pilot licence (CPL). I say 'technically' because if you only have minimum hours and minimum qualifications you have to look very hard for a job. Most people straight away get an instructor rating, so they can get a job teaching people to fly. Other early jobs include sightseeing flights, parachute flights, and mail flights to remote towns.

Your training will have been in light aircraft, but fortunately Australia has possibly the largest light aircraft industry in the world, so there's lots of jobs in these planes. It's highly unlikely that your first job will be in anything bigger than a 6 seater plane.

If you get extra qualifications (such as multi-engine endorsement, and instrument rating), you can get more jobs, and gradually fly bigger aircraft.

After you have your CPL, the next big step is the Air Transport Pilot Licence (ATPL). Regional airlines here generally require as a minimum, 1000 flying hours in light aircraft, the multi engine rating, the instrument rating, and passes in the ATPL theory exams - note that you don't necessarily need the ATPL licence itself. It will take most people a couple of years until they get this experience.

If you have that much experience flying 6 to 8 seat aircraft, it's not actually a huge leap to the 20 seat planes of the regional airlines, and the airlines will train you for this step. And once you've flown those for long enough any airline can train you to their jet.

Of course if you want to go back home after the training it will be different. But no school here can train anyone to go straight to an airline. You've got to do the small things first. Think of it as character building!

I'm sorry but I'm a bit rushed for time but I hope I've helped a bit. Australia is one of the best places in the world to train in my opinion (but I might be biased!). Please ask if you have any more questions. And good luck.


May a non-instrument pilot legally file IFR as long as he amends and doesn’t accept an instrument clearance? Say a pilot plans to make a cross country flight along Victor airways, but under VFR. Depending on workload and communication effectiveness along the way, our hero may provide the planned fairly complex route to the FAA at least three times.

Flight Service Station
Tower, if departing from such an airport
Approach/Departure Control or ARTCC

This seems like needless frequency congestion. Our hero happens across a suggestion from an ATC specialist that appears to solve the problem for everyone, emphasis in the original.

  If you file an IFR flight plan through any system, I will receive it. Make it clear to the first controller you contact that you want to fly your filed route as VFR and give the altitude. It is a very simple amendment I will be happy to make. The only difference on my flight strip is the altitude block that says 60 for IFR or VFR/65 for VFR, and a V in your data block on my radar scope. If you do this, do not accept an IFR clearance. If a controller says cleared to any fix, that is an IFR clearance. Using this method, everything on my end will work smoothly whether you depart a towered or nontowered airport, and this would be the easiest method for the controllers involved. CAVEAT: I do not know if it is legal for a non-IFR rated pilot to file an IFR flight plan even if you don’t accept an IFR clearance.

May a pilot lacking an instrument rating or currency legally do so?

My instrument students, when they get more advanced, file FP with their name, and in remarks say "Training xxxxxx CFII"

That way it is clear.  The FAA office investigated one student, dug up his past flight plans, and determined that the way he filed was acceptable.  The FAA told me they had reports of him going actual prior to his receiving a rating. I gather if the remark wasn't filed, there would have been more questions.

Having said all that, my recommendation is that you file a VFR flight plan for flight following, and then advise ATC upon check-in.  If you are leaving a controlled airport, normally you will advise clearance or ground prior to departure, and then when switching from the tower to departure you will check in with the phrase, "flight following."  On a workload permitting basis you will receive the same handling as IFR flights, in general, and there will be no question as to your intent.

Addendum #1
Just because there is no regulation prohibiting behavior, does not mean that an agency such as the FAA will not prosecute.  In fact, FAA issued a LOI in 2008, on just this topic, stating that if a pilot were to file an IFR flight plan, without holding an IFR pilot certificate, they would consider that intent to violate 61.3.

Now, whether that resulted in certificate action or caused a letter to be filed in your folder, I cannot say, but I would not want my certificate jeopardized by the poor judgment of filing IFR when one was not authorized to conduct IFR operations.

The 2008 LOI comes a decade or so after the incident I reported above with a student.

I don't see how any responsible person can advocate filing a flight plan which is premised upon false and misleading information. Furthermore, while there is not a specific regulation dealing with this particular lack of judgment, the FAA has made clear, with this specific matter, that would consider such a flight plan filing to be a intent to violate 91.3.  Not something that I can see advocating for anyone to do...

Addendum #2


Can hydrogen gas be an alternative fuel source for a gas turbine engine? I want to do research for an alternative fuel source for a commercial aircraft that will reduce the cost for flights.

Will the hydrogen gas be ignited in the combustion chamber and produce enough thrust to power the aircraft for flying? As for the problem that people had stated before for storing the gasses in a high pressure gas tank, can an increase in the aircraft weight be reduced by replacing the tank with usage of a hydrogen generator instead of a whole bulky tank that will weigh more?

By using a generator which converts water into hydrogen gas, we can just use water as our main fuel source which is more cost efficient?

In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy. If carried out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.

Yes, and it has been demonstrated 30 years ago on the Tupolev 155. This is/was a hydrogen-powered version of the Russian Tu-154B tri-jet. Only one has been built and has since been retired after demonstrating the use of liquid hydrogen in 5 experimental flights. In total, the Tu-155 performed about 100 flights with several fuels, among them hydrogen and liquefied natural gas.

Tu-155 (top), cut-out (center) and layout (bottom). Source: https://www.aviaru.net/pr/?id=11633

Hydrogen is actually better for a jet engine: It is gaseous at normal temperature, so there is no delay for the evaporation step to happen as there is for liquid fuels before mixing and combustion can start. Also, hydrogen burns in a wide variety of mixing ratios with oxygen, so flame-outs are much less likely, making a smaller combustion chamber possible.

The downsides, of course, are storage and the small molecular size of hydrogen. It is very hard to contain, and it needs big volumes to store a given amount of chemical energy. Pressurized storage at 200 bars holds only 18 kg/m³ or 45 times less than kerosene. With 142 MJ/kg, hydrogen holds three times as much chemical energy than kerosene, but then the volumetric efficiency of kerosene is still a factor of 13.3 better.

Cryogenic storage swaps pressure for low temperature: Below 33 K and above 13 bar, hydrogen becomes a liquid and storage density increases to 30 kg/m³. Still, cryogenic hydrogen storage needs 4 times the volume of the same amount of energy stored as kerosene, plus the isolation and the energy to cool it down and compress it.


Why is there a helicopter visibility reduction restriction for this approach? For the RNAV (GPS) RWY 17 approach at KSMN, why does it say "Helicopter visibility reduction below 3/4 SM NA."?

Looking at the LNAV, there is a 5 mile visibility requirement and if we cut that in half per AIM 10-1-2, wouldn't there be a 2 1/2 mile visibility requirement? If so, then a helicopter couldn't fly the approach with 3/4 mile visibility anyway.

Helicopters may operate under special VFR when in flight visibility is less than 1 SM.  Therefore, the approach can be flown at lower visibility than that published on the approach plate.  However, the note must be published to further restrict these types of operations:  "The procedure is annotated with 'Visibility Reduction by Helicopters NA.' This annotation means that there are penetrations of the final approach obstacle identification surface (OIS) and that the 14 CFR Section 97.3 visibility reduction rule does not apply and you must take precaution to avoid any obstacles in the visual segment."1


How is engine management being done without auto throttle during a VNAV approach? I am wondering how the engine power is controlled if Auto-pilot in a coupled approach mode with VNAV capability, and the aircraft is not equipped with Auto-Throttle?

Is the pilot continuously adjusting thrust levers (or power levers in the case of turbo prop) to maintain appropriate angle of attack, as the speed, decent rate and heading is controlled by the auto-pilot...?!!

Or...there is no such thing as auto-pilot with VNAV capability and no Auto-Throttle.

Is the pilot continuously adjusting thrust levers (or power levers in the case of turbo prop) to maintain appropriate angle of attack, as the speed, decent rate and heading is controlled by the auto-pilot...?!!

In 747-100/200 operations at the two 747 carriers I flew for, if you chose to have the autopilot fly a coupled ILS approach and didn't have auto-throttle capability, the autopilot tracked the glideslope by changing the pitch of the aircraft.

Personally, I never did this because I preferred to hand fly all approaches, but I watched others do it when I was the non-flying pilot, and they did indeed continually adjust the thrust levers as necessary to get the desired speed, just as we did when hand flying the approach.

In the 747 classics this wasn't hard. Initially you set the thrust levers to a fuel flow of 5,000 lbs/hr per engine, maybe a little less if very light, maybe a little more if heavy. Often that initial setting would give you the speed you wanted within a few knots and no further adjustment of the thrust levers would be necessary until starting into the flare. If it didn't, you adjusted as necessary, and with experience it took very little jockeying of the thrust levers.


Why does an airspeed indicator not simply read TAS? If the airspeed indicator has inputs regarding air density (static port) and temperature. Why isn't Indicated airspeed from the indicator simply the same as True airspeed

Well, indicated airspeed is the most important speed a pilot needs to know, so it makes sense to have that in the most obvious position.

IAS can be thought of as the pressure that the wings are experiencing - therefore all of the stall, manoeuvring, flap extension, and landing speeds etc are based off indicated airspeed.

True airspeed is really only important for flight planning and navigation. Given that more and more aircraft have GPS nowadays to give the groundspeed, the need for the pilot to know the TAS in flight is further reduced.


How do test pilots determine an aircraft's capabilities? How do test pilots figure out an aircraft's capabilities (service ceiling, cruising altitude, etc.)? Do they simply take the plane to its limits to get that information?

Short:  The pilot has a well-defined range of discretion within progressively increasing bounds very tightly set as part of a structured development program aimed at eliciting maximum information regarding actual versus theoretical flight envelope in all conceivable modes and situations.

Longer: A test pilot's main task is to carry out a progressive program of tests aimed at investigating the correspondence between designed and actual capabilities, and identification of any unexpected variations from nominal which occur along the way. The designers know everything they consider the aircraft should do and shouldn't do and should be able to do long before it first flies. The job of the test pilots is to find out the places where the designers are wrong by carrying out an extremely well planned program while applying her or his skill to come back alive when the designers and/or manufacturers get it wrong.

The pilot will usually have input into the process of deciding the rate which the program is carried out, how large progressive steps are and attempting to anticipate in advance what is likely to be viable or problematic. Their feedback along the way will help inform decisions about modifying program objectives or rate of implementation or lead to modification to the aircraft. A highly skilled test pilot is a member of a synergistic team and while program decisions are the responsibility of the 'project manager' (or equivalent) it is likely that what the pilot says and wants will be close to indistinguishable from what actually is done.

However, the actual program on a given occasion is not the pilot's responsibility and limits on what are attempted or implemented are well-defined in advance. The pilot will be given discretion in advance regarding ranges of parameters which may be investigated, e.g., closeness of initial approach to Mach 1, rate of ascent or g forces applicable in various modes*. And the safety of the aircraft first and the pilot second (in many cases) is left to the pilot's discretion BUT subject to substantial guidance, e.g., 'bringing the aircraft back intact' is often going to provide far more information when a problem occurs so a pilot may make every effort to do so in situations where an ejection may otherwise be preferred. A  pilot who ejects more than most may or may not be criticized for any given decision but is less likely to be given as many chances in the future to risk their life even more than before. However, a pilot who crashes fatally while trying to save an out of control situation will in the future only take part as the subject in training videos and in establishing "see if you could have handled this one" simulation scenarios.

Striking the right balance between these two 'modes' is what makes a great test pilot. But they still don't just get to go out and "see what it can do".

A well judged ejection occasion - genuine.

From here

English Electric Lightning early testing.  Prior ejection would very likely have been justifiable but is unlikely to have been considered. The pilot (George Aird) survived with relatively minor fractures - in part caused by descending through the roof of a glasshouse :-). Pilot was on final for an emergency landing as a fire was attacking the rudder actuator. At 100 feet altitude and 10 seconds from touchdown, Murphy won, the actuator support burned through and the plane pitched nose down. Go!

Glasshouse with pilot and ejector seat entry points (side by side) ringed in red. Canopy caused damage at far end.

Photographers are lucky to get one opportunity like this in a lifetime.
I'm still waiting :-)

What do I know?: Layman. Engineer. Non pilot. Read too many test-pilot autobiographies. The ones you read tend to be from those who are good and didn't die.

I was recently rererereading Roland Beaumont's "Phoenix into Ashes" - an  account from the leading test-pilot of many (most) of the early British jet aircraft. He recounted the loss of a test aircraft while pulling 9g (as I recall) in a situation which should have been inside the design envelope. The pilot died. The observer lived to provide 'human black box' functionality. It was established that fuel transfer protocols were not correctly followed, leading to over-stressing part of the airframe and thus fatal loss of control, i.e., "just" mishandling fuel tank sequencing may kill you. Nowadays computers will tend to stop this sort of error. But Murphy has many alternatives.


Is drag the only aerodynamic force that acts upon an asymmetric body (a glider in particular) travelling through the air? It is known that for a symmetric body, like a sphere falling through the air, the only aerodynamic force that acts upon it is the drag that points always in the opposite direction relative to its speed.

Now, an asymmetric body traveling through the air, I guess, will have a drag that makes an angle different from 180 degree with its speed.

As a glider is just an asymmetric body, it means that the only aerodynamic force acting upon it is the drag that is conventionally decomposed in what we call Lift and Drag.

Is it true that Lift and Drag applied to gliders or airplanes are just a convention, two forces that in reality do not exist, and there is only a True Drag that makes an angle of a certain value with the speed of the aircraft?

You are right, lift and drag are just convention. But the aerodynamic forces are usually grouped into one total force named "total aerodynamic force" ("résultante aérodynamique" in french).

As explain in how it flies:

  drag is a component of the aerodynamic force, namely the projection onto the direction parallel to the relative wind.
  lift is another component of the aerodynamic force, namely the projection onto the two directions perpendicular to the relative wind.

This convention applies to any aircraft.

For further reading, see the section about forces of how it flies.


What are the reasons for the end of the dogfight era? A lot of sources mention that a lot of modern era (5.Gen) fighter jet designs such as the F-35 are focussing on qualities and properties such as stealth, long-range attacks, and versatility just to name a few. It is often brought up that the era of dogfights is over. What was the reason for this shift of desirable qualities in air defense and combat?

Primarily this was driven by the invention of the guided missile which can be deployed far outside the range of bullets (making them a standoff weapon) and which can actively track the target autonomously ("shoot and forget").
With these two features, it no longer became necessary to maneuver close enough to another plane to shoot it down with bullets and to accurately track the target aircraft with one's own aircraft while doing so- which made WWI and WWII dogfighting skills unnecessary.


Is it possible to have an afterburner in a turbofan with bypass ratio greater than 1:1? Is it possible to have an afterburner in a turbofan with a bypass ratio greater than 1:1? At 1.05:1, say, can it have an afterburner?

fooot has pointed out the GE F101 (powers the B-1 Lancer) has a BPR (bypass ratio) of >2:1 and comes with an afterburner.

Other examples are the Turbo-Union RB199 (1.1:1) and the Volvo RM8 (0.97:1). Generally, anything below 4:1 is a low BPR, and bypass air is needed for cooling. Pure turbojets resort to other means of cooling, such as the ram air inlet on a J75, or they run inefficiently at low combustion temperatures.

I can't speak for the inner workings of a classified military engine, but an ideal supersonic engine would be a variable cycle engine (VCE). Operating with bypass in the subsonic regime, and without in the supersonic high-power demand regime.

Such theoretical (to the public at least) examples from the 90's are shown (and linked) below. Do note the idea behind the VCE is the elimination of the wasteful and noisy afterburner.

So, it's an either/or situation. A dual operation of afterburner and high bypass in a non-VCE would not have benefits.

(Flight—PDF) Image shows how a VCE routes the bypass air in both modes of operation. Other variations by RR and Snecma can be seen here. Note the novel design of the Snecma with the free-turbine—extra-fan. VCE's would definitely be heavier and more complex engines.

Related: Why do military turbofan engines use a low bypass ratio?


How does envelope protection work in Airbus vs. Boeing aircraft? I keep reading this thing that "On Airbus the computer overrules the pilot, not on Boeing". Smells like urban myth or at least gross oversimplification. How does envelope protection work differently in Airbus and Boeing aircraft?

Boeing: as far as I understand it, it's possible to apply force on the column to override the autopilot and have zero limitations, i.e. you're free to (try to) fly the aircraft in whatever attitude you want.

Airbus: if all the systems are failure-free there's no way to override the autopilot, i.e. you cannot fly in any attitude you want.

Is my understanding/comparison accurate?

It would be really lovely if someone with factual knowledge on the matter elaborate more, maybe explaining the different laws and when they can be / are activated.

Related post What are the main differences piloting Boeing vs. Airbus aircraft? does not answer the question.

There are two sorts of "autopilots", and it is important to make a distinction between the two. One is for the behaviour of the aircraft around its Centre of Gravity (CoG), the other one is for defining the path of the CoG.

The Inner Loop autopilot: behaviour around CoG, or the aircraft attitude control. This autopilot should not be called autopilot since its characteristics are actually shielded from the pilot: it does its job without showing what it is doing. It only moves the control surfaces (elevator, ailerons) or helicopter swash plate, and not the flying controls (stick, wheel). To the pilot, the system is invisible and just results in a stable platform, in the case of the A320 and B777 a platform with envelope protection that does not allow for the aeroplane to enter a situation that would be dangerous. In helicopters this system is referred to as Stability Augmentation System (SAS).
The Outer Loop autopilot: defining the path of the CoG, which is ultimately what pilots are paid for :). They are in control of the flight path and administer this control by deflecting the flying controls (stick, pedals). The pilot can delegate this control to a feedback system that provides inputs to the same flying controls, via an actuator with a slip clutch. With an old fashioned mechanical linkage, the actuator moves the whole circuit from stick to surfaces (or swash plate in a helicopter); the pilot can override this input by applying a force greater than the override force of the slip clutch.

So normally, by design, the Outer Loop autopilot shows the pilots what it is doing by moving the flight path input devices for them, in a way that is transparent and can be intuitively overridden by applying a high force. It is this bit where the A320 has a different philosophy: the input stick never moves other than by the hand of the pilot. This main distinguishing feature between the two types of autopilot does not exist in an A320. It does in a B777, which also has an inner loop autopilot plus envelope protection, and an outer loop autopilot which actually does move the column/yoke. So the B777 is functionally identical to the A320 in every meaningful way, plus retains the feature of moving flying controls. It has no mechanical linkage, and it uses two separate sets of actuators for this function: one set for the surfaces, and one for the flying controls.

This is the situation on both Airbus > A320 and Boeing > 777, when all systems are functioning. These aircraft are all dynamically stable: aerodynamics take care of bringing back the aircraft attitude to a neutral position. The F16 for instance is aerodynamically unstable in order to provide high manoeuvrability: it always wants to pitch and roll already, and the flight control computers provide constant rapid inputs to elevators and ailerons to maintain attitude by active control. This is not the case with passenger aircraft, which do not need to be as manoeuvrable as a fighter jet and just need to bring the passengers home safely.

Therefore, both on the Airbuses & Boeings, the aircraft can fly without the Inner Loop autopilots as well, without any problems. The aircraft just lose their envelope protection and now it is possible to command them into potentially dangerous situations such as fully developed stalls. In both types, it is always possible to disconnect the outer loop autopilot. For the inner loop ones:

The B737 does not have one.
In the A320, it is not possible to disconnect the inner loop autopilots if all is functioning correctly, except by pulling circuit breakers for resetting the ELACs as this site indicates;
In the B777 there is a guarded switch that allows disconnection of the envelope protection as @Cpt Reynolds pointed out.

In the latter two types, the system disconnects features by itself if failures are detected, to allow control in degraded mode. Both manufacturers have implemented a direct mode as the lowest mode, where surface deflection is a direct function of stick deflection. There is however no direct mechanical cable connection to the surfaces, even the direct mode is still an electrical input, so technically still Fly By Wire.

Above picture is from Aircraft Systems Mechanical, electrical, and avionics subsystems integration Third Edition Ian Moir Allan Seabridge, and shows the top level comparison of the implementation of the flight control loops. The redundancy features are different, functionality is much the same: both implementations prevent excessive AoA that brings the aircraft into a stall, and excessive bank angle.

So although the functionality of modern Airbuses and Boeings is very similar, we do often see the myth that Airbus ranks the automated systems higher than the pilot. There is a difference in user interface implementation, in that Airbus has sticks that are not coupled with each other, and have no means to be driven other than by hand. At the time of implementation, this was a new feature that pilots were not accustomed to, as was the envelope protection system. New features can be poorly understood by inexperienced users, and the aviation world is conservative: when the DC3 introduced a 3/4 control wheel, pilots were protesting that they might be gripping the bit of the wheel that was missing and that they were used to.

Ultimately, the most relevant fact is that both Airbus and Boeing have excellent and very similar safety numbers, and that both manufacturers make aircraft that enable the pilots to bring the passengers home safely in 99.9999999 % of flights.


What is the purpose of the black paint below the cockpit windows? Today I saw the following picture of the B747-400 SuperTanker, which shows a black painted region below the cockpit windows.

I have also seen vintage pictures of other aircraft that also had this black painted region.

What is the purpose of the black paint? Is it for decorative purposes?

It reduces glare (specular reflection).

Football players paint black stripes under their eyes for the same reason.

The legend for the paint scheme is excerpted in the inset, upper right. Item #33 "Antiglare areas shall be painted aircraft black."

If you read this technical manual, which is publicly available, it describes in the body of the text the use of anti-glare paint.


What is the risk of leaving the MAN START PB 'ON' on the A320? During a manual start on a A320, the ENG MAN START P/B opens the start valve which then runs the starter motor. During the MAN START process, at some point, the start valve closes automatically. What are the risks associated with continuing to leave the MAN START P/B ON at this point?

Well, documentation is a bit vague about this, and I suppose a technician would know more about the different things the ENG MAN START bp is doing, but from a quick glance at the FCOM there is one major difference that I can spot:

  [The FADEC durring a manual start]  (...) Closes the engine start valve, and cuts off the ignition when N2 reaches 43 %.

Bold emphasis is mine, no other conditions except N2>43%.

Leaving the ENG MAN START pb on means FADEC keeps assuming we are trying a manual start, and will cut off ignition anytime the N2 is above 43%. No prob as long as engines are running, right?

But that means (I assume) when you then enter turbulent/stormy weather in flight and turn on  IGN/START to prevent a flameout, nothing happens as FADEC is happy that N2 is above 43% and you manual start has been ‘succesfull’.

I don’t really know if it will even attempt an auto restart when you get a flameout, as the N2 will probably keep windmilling for some time depending on your airspeed, so you could end up with a big problem.


Can this aircraft wreckage be identified? I live in Salinas (central coast California) and there’s an old plane wreck (located within the boundaries of Toro Park County Park in a remote section). On a hike yesterday, we located and took some pictures and I was seeking some help to identify the type of aircraft and then perhaps some more information (searching the CA plane crash registry came up empty). So looking for more details – firstly identifying the plane and then perhaps locating who/where/etc – there are no other known details.

I have some photos for reference (but would only let me add 1).

A plane should be able to be identified by the serial numbers of the various parts recovered at the crash scene.  The history of any part on a plane is documented extensively by the serial number.  The NTSB should be able to track down when the part was produced and who the part was delivered to.   As Tanner mentioned the NTSB should be contacted immediately and the wreckage should not be moved.  There are many plane crashes that have occurred many years in the past which have still not been found. You may have stumbled on a piece of history that could bring closure to a family who may have never known what happened to their loved one.


What navigation systems were used by the Space Shuttle during reentry? What equipment did the Space Shuttle orbiters carry to support navigation during reentry?
Were there any differences between the individual orbiters in this regard?

With the exception of GPS upgrades, I don't believe there were any substantial differences in navigational equipment between the various Space Shuttle Orbiters for most of their operational history.

Each space shuttle had:

3 Inertial Measurement Units
3 TACAN receivers* see GPS Upgrades
4 Barometric Altimeters
3 MSBLS Receivers
2 Radar Altimeters
2 Air Data Probes
Star Tracker (on-orbit)
Rendezvous Radar (on-orbit)

There is lots of information about the Shuttle's navigational equipment in the Shuttle Crew Operations Manual.

GPS Upgrades

In the early 1990s, it appeared that TACANs were going to be phased out, so NASA began evaluating GPS as a possible replacement. Space Shuttle Endeavor was the first to have a single GPS receiver installed in 1993, followed by the other shuttles beginning sometime around 1996.

By 2002, each shuttle had 1 GPS and 3 TACAN receivers. The plan was to remove the TACAN receivers and install two additional GPS receivers - resulting in a 3-string GPS system. However, after the loss of Columbia in 2003, it was decided that the Space Shuttle Program would be discontinued in 2010 (later extended to 2011), and therefore it wasn't worth upgrading all three remaining shuttles. Only Endeavor was upgraded. Discovery and Atlantis continued to fly with 3 TACANs and 1 GPS for the remainder of the program.

You can read a lot more about the history of the GPS system here: Operational Use of GPS Navigation for
Space Shuttle Entry.


Did factors specifically related to the concorde design influence the Air france flight 4590 crash? This youtube video partially attributes the retirement of all concorde fleets to the airfrance flight 4590 crash:

Were the factors in this accident due to the concorde's design? Specifically, the tires of the concorde?

In short: No, the crash of 4590 could happen to other aircraft in the same situation: i.e. a blown out tire, a tank fire, subsequent structural issues, and at least two engines (of 4) not running properly.

The official accident report summarizes the causes pretty well (and is worth a read)

  The accident was due to the following causes:

  • High-speed passage of
  a tyre over a part lost by an aircraft that had taken off five minutes
  earlier and the destruction of the tyre.

  • The ripping out of a large
  piece of tank in a complex process of transmission of the energy
  produced by the impact of a piece of tyre at another point on the
  tank, this transmission associating deformation of the tank skin and
  the movement of the fuel, with perhaps the contributory effect of
  other more minor shocks and /or a hydrodynamic pressure surge.

  • Ignition of the leaking fuel by an electric arc in the landing gear
  bay or through contact with the hot parts of the engine with forward
  propagation of the flame causing a very large fire under the
  aircraft’s wing and severe loss of thrust on engine 2 then engine 1.

None of these are related to the design or construction of the airframe itself. This podcast with one of the pilots also touches upon the accident and is perhaps one of the best primary sources of Concorde info.


What is a TAPS combustor and how does it differ from any other annular combustor? I found out that the GEnx engine uses a TAPS (Twin Annular Premixing Swirler) combustor. Is it a single or dual annular combustor ?

See this question.  It is a single annular combustor (one ring of full nozzles).  I think that "twin" refers to the fact that within a single fuel nozzle, there are two different fuel streams: the main stream and the pilot stream.  This video might also help explain it a bit.


Are there any aircraft powered by tip jets in the propeller blade? I am familiar with tip jet powered helicopter experiments. Is there any aircraft that use/used this mode and would it be useful to use a tip jet driven prop on aircraft instead of piston or turbine power?

There was a study done on the use of tipjets in civilian aircraft in the 1950's, if I can find the source I'll edit it into this answer. In any case, the conclusions from that study were as follows:

Because of the fundamental inefficiency of subsonic ramjet propulsion, a tipjet-powered propeller "engine" in the 100 to 150 horsepower-equivalent range was completely uncompetitive with piston power. The fuel flow rate required to run the tipjet prop was excessive.

The tipjet-prop plane had extremely limited range if carrying the same amount of fuel as the piston plane, and no useful load if it carried enough fuel to match the range of the piston plane.


Why is TO/GA button disabled in 777 with weight on wheels or below 2 ft RA? Recently a final report for accident of an Emirates B777-300 at Dubai on 3rd August 2016 was published. An important factor there was that when the pilot tried to recover from long landing by going around, the TO/GA buttons were inhibited because the wheels were on the ground and the pilots did not realize this.

Now at first sight, this seems like an unnecessary complication of the auto-thrust system that just adds a point the pilots need to learn. But I suppose the engineers did have some reason to add this bit of logic.

So what dangerous condition is inhibiting the TO/GA button when weight on wheels or radio altitude below 2 feet for 3 seconds supposed to prevent?

I would understand after reversers deployed, but before that going around is considered appropriate solution for long landings and bounces, both of which might trigger the inhibition.

Boeing's officially stated reason in the report is:

  ... to prevent pilots from inadvertently activating TO/GA mode at or after touchdown. Inadvertent activation of TO/GA could result, among other things, in the aircraft departing[1] the runway. ...

[1] As in runway overrun/excursion.

Which is in accordance with AC120-29A (also from the report), where:

  ... If an automatic go-around mode can be engaged at or after touchdown, it should be shown to be safe. The ability to initiate an automatic or flight director go-around at or after touchdown is not required or appropriate. ...

It's either Boeing did not want to, or could not, demonstrate it would be safe, hence the inhibition (nor is it a required system). Boeing does not elaborate on the "among other things".

Some remarks on the 777's TO/GA switch:

Applying full power on a go-around on a 777 after consuming most of the trip fuel is tricky because of how powerful the engines are. It would produce a big nose up.

For this reason, pressing TO/GA – unlike most planes – does not command full power, rather enough power for a 2,000 ft/min climb. Pressing TO/GA again, would command the full power (source: FCOM 6.4 Page 51).

So there's logic involved.

The closest Boeing patent I found was the Total energy based flight control system published in 1984, which does exactly that, among other things. A system for commanding elevator and/or thrust based on the plane's current and commanded energies for a target flight path. (Download the PDF for the logic diagram.)

For that to work, the plane needs to know its current flight path. If the wheels touch, and with the drag of the ground friction, which is unaccounted for in the logic, the system can't function as designed, or additional logic will need to be added and certified.

Note: The same inhibition, climb target logic, and a double-push for THR REF is also on the 747-400 (source: FCOM 4.10.17). The patent's date makes it plausible that the 747-400 was of the first to receive this system, and it was carried over to the 777.


How pressure fields change around wing? In the 3D case,downward turning is immediately above and below the wing is stronger than in 2D case,for the same lift.This is because more rapid dying off the pressure above and below the airfoil means the vertical pressure gradient near the wing surface is stronger than in 2D case.The airfoil pressure field dies out more rapidly ahead of the airfoil and behind,which results in less upward turning of the flow in this region.So in 3D case we have more downward turning above and below the wing and less upward turning ahead and behind.Vertical extent of pressure distribution in 3D is lower than in 2D for the same chord and lift per unit span.
Reducing vertical extent of pressure distribution means increase  pressure gradient close to the wing surface and a reduction far from the surface.

So in short, smaller vertical extent(small pressure field) of pressure=larger pressure gradient=larger force which drives downward accelaration.

Pressure gradient=delta pressure / distance (Pa/m)

Do you see some contradictory in explanation above or everything is logically?

because,when we increase AoA,vertical extent of pressure distribution is larger so pressure gradient is smaller and downward acceleration must be also smaller,but it isnt because with AoA rise downward turning(downwash) as well.

Can someone explain in detail what happend with pressure fields,pressure gradient in relation to upwash/downwash at 2D/3D wing and when change AoA...

Take a look at https://www.sciencedirect.com/topics/engineering/angle-of-attack  and  https://www.grc.nasa.gov/WWW/K-12/WindTunnel/Activities/lift_formula.html.
Lift graphs have a linear portion for small angle of attacks (and in 2D scenarios), but drag and airflow (laminar (non-shedding) vs. turbulent (shedding)) are both are non-linear, and both affect pressure and thus lift, and are important in real-life and 3D calculations.


What would the call sign of a non-standard aircraft carrying the US President be? So, it's well known that the government planes operated for the US President are called Air Force One; less well-known is that this is a call sign given to any Air Force aircraft that carry the president, and that if the President rides in an aircraft owned by another branch of the US Armed Forces, they would become known as Marines One, Navy One, or Army One, while any civil aircraft that carries the US President becomes Executive One.
What about aircraft that are exceptions to the normal FAA rules for aircraft licensing, however? If the US President was riding in (or piloting) a glider, ultralight, or autonomous drone taxi, what would their call signs be, if any?

This question is unanswerable in its present state. A glider is a certified and registered aircraft in the US. Ultralights are not register aircraft in the US. And, they are not typically in contact with ATC. Autonomous drone taxis have yet to receive FAA certification, airworthiness, and registration. So, the best manufacturers can do with drones at this time is to fly them unmanned or on test flights.
The bigger question is whether the Secret Service would allow the president to pilot any aircraft. Let alone fly in a non-certified, unregistered aircraft. After all, they do not allow the president to drive a car while he is in office.


Why might the crew switch off air conditioning in the cabin of a passenger jet during descent? I've noticed on some commercial flights (most recently today in an A319) that as the aircraft descends in preparation for landing, the air in the cabin gets hotter. I also noticed that this seems to be caused by the air conditioning being switched off entirely or to a lower flow rate.

Is there a reason for the cabin/flight crew to switch the air conditioning lower/off for landing?

Systems vary.

Transport aircraft and larger biz aircraft have airpacks handling the AC and at flight idle power settings there is less bleed air, and pressurization is a priority.  The packs simply do not have full power when in a flight idle descent on many aircraft.

The aircraft I routinely fly have separate radial compressor driven refrigerant systems, and have large heat loads from onboard electronics and laser systems.  They have to remain cool, and normally both AC systems are left on until just prior to engine shutoff after taxi.  If it is a hot environment, we use the APU and keep the AC on after engine shutdown.

Once again, the situation is different in different aircraft, and is dependent upon the cooling system, the power plants, the installations and the design and operational goals.

Addendum #1: Some aircraft APUs can provide bleed air for cabin pressurization, but this is not universal.  Also, some aircraft, such as the 787 do not use engine bleed air for pressurization, and rather use electrical compressors.  The flow of pressurized air is rather constant in flight.


How many aircraft and pilots can a part 135 cover? If you form an LLC for charter flights, can you hire an unlimited number of pilots to fly under the same part 135 certificate (assuming all are commercial pilots), or does each pilot and/or each aircraft (all aircraft owned by the LLC) need to have their own part 135 certificate?

There is no restriction on the number, type of aircraft, or pilots for a "Full 135 Operator."

There are 4 levels of 135 operators:

Single-Pilot Operator
Single Pilot-in-Command
Basic 135 Operator
Full 135 Operator *

Source: http://usac.com/faa/InstructionTwo.asp?Product=Full%20135%20Operator


What is the reference length in calculating the friction coefficient? I am building a ground effect vehicle and I want to calculate the parasite drag of it.
So I found this site to calculate the friction
But I don't know what the "reference length" is. Is it the chord, the wingspan or the mean aerodynamic chord? And in case that the reference length is the chord, is their a different way to measure it for an inverted delta wing?

Since the site computes atmospheric conditions, only the friction coefficient $c_f$ needs a reference length. This is the length of the object causing friction, measured along the direction of airflow. In case of a wing, it is its local chord, in case of a fuselage, it's the fuselage's length.

Each part of an airplane and each section of the wing will have a slightly different $c_f$, depending on local chord length. The highest contribution to friction drag is caused at the leading edge, when the boundary layer is thin and the speed gradient at the surface is steep. The farther downstream you look, the less each additional unit of length will add to friction drag, so the average over the full length (that is what $c_f$ stands for) will become smaller for longer bodies.

To compute total drag, you need to add the contribution of each part, multiplied with its surface area relative to the airplane's total area. Yes, for a wing the upper and the lower wetted surface area will each contribute their own drag component. Since airflow is faster on the upper surface of a lift-creating wing, you would need a correction factor to account for the speed difference between upper and lower side, but if the difference is small, you can just assume the same speed everywhere. To be precise: $c_f$ is the friction coefficient of a flat plate parallel to the airstream.


Why should lower limit of a Control Area (above 3000 ft) coincide with a VFR cruising level? In ICAO Annex 11 it is recommended:

When the lower limit of a Control Area is above 900 m (3000 ft) MSL it should coincide with a VFR cruising level

Why should it coincide with a VFR cruising level?

VFR cruising levels are always located between two IFR cruising levels. With half of the required radar separation minimum to each the IFR level above and below it.
For example:
required separation minimum: 1000ft
IFR lvl: 200
VFR lvl: 195
IFR lvl: 190
The sectors are split at VFR cruising levels to ensure, that both the upper and lower sector can use ALL of their IFR cruising levels while maintaining separation to aircraft that may fly at the highest/lowest level of the sector below/above.
Example:
Sector A is working from FL145-FL245
Sector B is working from FL245-FL355
Sector A may use every IFR cruising level within the sector (FL150, FL160, ..., FL240).
Sector B as well (FL250, FL260, ..., FL350).
As the difference between the highest and lowest level is always 1000ft, separation is ensured also between aircraft flying on top of each other in different sectors!


How should I interpret the difference in flight direction (azimuth) for great circle and rhumb line? I am trying to understand the image above. If you fly in the Northern Hemisphere going west. The image said it's 150 degree if you fly Great Circle while 270 degree flying rhumb line.

If you think about this on a map, how is it possible to be 150 degree? Shouldn't the degree be between north(360) and west(270) as you fly west? I'm so confused.

I have no idea where the image comes from, but it seems that you have spotted an inconsistency/copy-paste typo.

Let's remember that the heading is always measured w.r.t. the north, increases in a cloclwise fashion and indicated the direction the aircraft is flying to, as shown in the following figure:

(Image Source: www.allstar.fiu.edu)

On the Rhumb line both East and West directions are shown both with arrows ($\rightarrow \;\; \leftarrow$) and with the initial headings ($90^o \;\; 270^o$)

On the Great Circle lines only the westerly arrows appear and only the starting headings from the West are correct. These seems to have been copied to the East side, but they should have also been augmented by $180^o$, i.e. as you were expecting, you should have $330^o$ for the northern hemisphere (in lieu of the $150^o$) and $210^o$ instead of $30^o$ for the southern.


What RNP level is required for an RNP approach if the chart does not specify a value? RNP (Required Navigational Performance) approaches require the aircraft to monitor the navigation accuracy
within a specified limit, called RNP level:

There are several different levels of RNP. Examples of RNP levels used for approach include RNP 0.1, RNP 0.3, and RNP 1.0 (There are also RNP 4.0 and RNP 10.0 levels that apply in the en route environment). "A performance value of RNP 0.3, for example, assures that the aircraft has the capability of remaining within 0.3 of a nautical mile to the right or left side of the centerline 95 percent of the time."

[...]
The minima line will include a performance value, RNP 0.30 for example. GPS with WAAS can support RNP 0.3 and above.

(faa.gov on Satellite Navigation)
Usually the RNP level is given in the minima section of the approach chart (and according to the above FAA quote, this is always the case in the US).
There are however approach charts that do not include this. Consider for example the following 3 approaches into Innsbruck (LOWI), Austria. While the first example lists the RNP levels, the other two do not:

RNP Z RWY 26 (AR)

There are two different minima for the different specified RNP levels.

RNP E RWY 26 (LPV ONLY)

There is no RNP level specified, also not anywhere else on the chart (as far as I could find). Maybe this is because this approach is LPV only (requires EGNOS1)?

RNP Y RWY 08

Again, there is no RNP level specified and this time there are only LNAV minima. So which RNP level would apply here?

1 EGNOS is the European SBAS (Satellite Based Augmentation System), equivalent to the North American WAAS.

RNP approaches require a minimum value of 0.30, be it LNAV only, LNAV/ VNAV or RNP AR. Most of the approaches around the globe are RNP approaches. So, they do not put in the value because it remains the same at 0.30.
In RNP AR approaches they put in the value because these are approaches that require specially trained pilots and specially equipped aircraft. That is where the AR comes from. It stands for Authorization required. The RNP AR approaches have RNP values starting from 0.30 down to 0.10. The biggest difference between a typical RNP approach and an RNP AR approach is the lateral obstacle clearance. In an RNP approach, you have a clearance of two times the RNP value (0.30 x 2 = 0.6 miles) and also a buffer of 0.2 miles either side. In an RNP AR approach, the buffer is taken out of the picture and it only offers two times the RNP number left or right. If the RNP value is 0.10, then the lateral obstacle clearance will be 0.20 miles either side. This allows tighter approaches in areas with high obstacles such as mountains (such as Innsbruck). But because such approaches are a bit demanding you need better equipment. So, airplanes that are not designed for it need extra modifications. These modifications will depend on the manufacturer and it will also depend on the actual procedure. The pilots who fly these approaches need extra training as well, so that they are more accustomed to flying it and to teach them the required contingency procedures. Depending on the operator and the aircraft configuration, an RNP AR approach maybe approved with an RNP value between 0.30 and down to 0.10. For example, Airbus offers A320 and A330 with modifications and approvals for either RNP AR approaches with 0.30 or 0.10. In case of the A350, the aircraft comes with RNP AR 0.10 approval as a basic setting.

For LPV approaches, there is not an RNP value as it is basically an ILS type approach flown to category I minima using the GPS and a ground based GPS signal augmentation system such as WAAS (Wide area augmentation system).


Do the DO-178 standards include guidelines on cyclomatic complexity? Is there any guideline related to the cyclomatic complexity of software that complies with the DO-178C or DO-178B standards?

Short answer is No.

From the note after 5.2.2.g

  The current state of software engineering does not permit a quantitative
  correlation between complexity and the attainment of system safety objectives.
  While no objective guidance can be provided, the software design process should
  avoid introducing complexity because as the complexity of software increases, it
  becomes more difficult to verify the design and to show that the safety-related
  requirements are satisfied.

DO-178C is focused on defining process objectives rather than design guidelines.  Your process (specifically Design Standards) might specify a certain maximum complexity value, but it wouldn't be driven directly from DO-178C


Why don't cockpit windows suffer pressure leaks? The de Havilland Comet had pressure leaks inside the square windows which resulted in 3 incidents of the aircraft breaking apart in mid-air. A cockpit window has a similar square window and should be susceptible to pressure leaks.Source:(prosim-ar.com) Here is a Boeing 737 cockpit outer view. As you can see the windows are square which makes them susceptible to pressure leaks because of their square shape like the de Havilland Comet. So why doesn't the Boeing 737 cockpit windows suffer pressure leaks? And what modern methods do engineers use to reduce pressure leaks and why aren't square windows used on airliners any more?

The Comet did not have pressure leaks.

The original structure of the Comet, which had a pressurised cabin (a new technology in airliners at the time), featured rectangular windows with sharp 90-degree corners. This concentrated the stresses in the aircraft skin at these points resulting in accelerated fatigue, cracking and finally catastrophic failure of the fuselage starting at a window corner. This first happened to BOAC Flight 781

The first prototype de Havilland DH106 Comet at Hatfield. Note the square windows.

(From the Imperial War Museum collection, public domain)

A later Comet 4 which now has rounded windows.

(By Ian Dunster (Own work) [CC BY-SA 2.0 uk (http://creativecommons.org/licenses/by-sa/2.0/uk/deed.en)], via Wikimedia Commons)

The photograph you have posted is not a 737. It's a wooden cockpit mock-up intended for a flight simulator. It will never be pressurised, and the builders are more concerned with the overall shape than detail around the windows.

Take a look at a real 777 cockpit:

(Source: Chris Sloan, AirwaysNews.com)

Notice that the corners at the ends of the widows are rounded, as are all the other corners around the window frame. There is one panel (second from left) that appears not to have this treatment. This panel can be opened (although not when the cabin is pressurised) and is built into its own frame that is fitted into the cockpit window structure. A failure in this frame will not propagate to the rest of the aircraft.


What runway materials reduce skidding in wet conditions? What are the possible materials that can be used on a runway, so as to reduce the plane speed and certainly stop the plane while preventing skidding when the runway is wet?

This question has kept industry awake at night for ages now. A good start is here.

TL; DR: In addition to grooving, some runways (e.g. London City) apply a special surface material that is called a Porous Friction Course.

  An alternative to grooving as a means of facilitating surface water dispersal is the laying of a Porous Friction Course (PFC) which allows water to pass vertically through the surface layer and then move horizontally clear of the runway beneath, but essentially parallel to the surface.

The trouble with all that is that if you test it, build it and then calculate performance taking credit for it, if it doesn’t work for real for whatever unknown unknown, you are likely to find out by having to remove an aircraft from the Thames river which is kind of frowned upon by most. You want to be a bit conservative with runway friction.


What is this WW1 era plane? I'm trying to identify this plane from an old family photograph.  Is it possibly  a DH4?

The tail number doesn't bring any success (well, at least not that I can find).
Any info would be much appreciated.

According to worldmilitair.com (.pdf), serial E8557 was assigned to an Airco DH.9A, a light bomber used by the newly formed Royal Air Force starting in the final months of World War 1.

Source: Wikimedia


How does switching off pumps and putting on MAN switch resolve low pressure? FUEL
  If the center tank is less than 200 kg (440 lb) for the flight: Apply the following procedure, if your airline is affected by FUEL CTR TK PUMPS LO PR cautions on ground or in flight when the center tank is less than 200 kg (440 lb):

  FUEL MODE SEL pb sw ....... MAN

  CTR TK PUMP 1 pb-sw and CTR TK PUMP 2 pb-.....OFF

I came across this while reading Normal Procedures. Could someone kindly explain as to how the centre tanks pump low pressure gets resolved by following the above mentioned procedure?

The procedure switches off the pumps, thereby resolving the low pressure. Low pressure means the pump is running but it is not pulling anything.

Fuel pumps can't draw fuel if the amount is too low. Imagine pumping water out of a swimming pool by attaching the pump to a hose. There will be smalls amounts that the hose can't reach. That amount is called unusable fuel and is part of the plane's operating empty weight (OEW).

See: What are different types of weights of an aircraft?

Some fuel quantity indicators are calibrated to not count this unusable amount of fuel.

If the captain really needs those 200 kg, they it's their decision to add a lot more in the center tank so it would be usable without any problems.

RE: What about the reason for putting the mode selector to MANual?

If the pumps are selected off with the mode selector still in auto, this will cause an auto feed fault warning as the normal operation of the auto mode (which is what causing the issue in the first place) requires the pump switches to be on the on position.

What does the auto mode do? See: Why are the center tank pumps deactivated when slats are extended on the A320?


Wingtip landing light lenses are hazed - what can I do to make them clear again? I have wingtip landing/taxi lights on my plane.  They have acrylic covers to protect the bulbs.  They have started to haze over time, similar to the effect of car headlights.  I am wondering if anyone has experienced this and can provide information on how to correct this issue.  I feel as if this might be affecting the ability of the bulbs to direct light on the runway/taxiway efficiently.  I am starting to eye some of the cleaners that say they can correct this in the automotive aisle, however I am very hesitant.  I don't want to buy new covers, as I am sure there is a better/cheaper option.

The hazing is a collection of very small scratches on the acrylic, and the way to remove them is to actually remove a thin layer of the acrylic - the trick is to make sure you use something that won't damage any of the paint/metal around the cover.

A mild acrylic polish is your best bet, although any mild abrasive will work (including non-"sensitive" toothpaste and a paper towel... although it might take a while!). There are plenty of "Perspex cleaner" style products for a reasonable price to do it.

Use a small amount (multiple small applications are better for controlling the spread) and try to use a thicker "paste" type of abrasive rather than a runnier one, for the same reason. Apply a small amount and some fairly firm pressure with a cloth, as per the instructions of the cleaner, removing any excess regularly.

If you think there are deeper scratches than just a very fine "hazing" on the lens, you can use a fine sandpaper (eg 1600 grit) to sand the acrylic first before using the polish


What is the reason for the fence aft of the static ports on some airplanes and helicopters? Airbus AS350 helicopters and Bell 206 helicopters have a 1/2" long fence behind the static port. CRJ 200/700 have a .060" step on the aft side of the static port. What would be the reason for these?

In yet another example of unique helicopter design complexity, issues can arise when it comes to mounting static ports on the side of the fuselage where they may fall within the rotor wash. Ideally, static ports should be mounted in an area of undisturbed air, but other design considerations sometimes make this impossible. Additionally, helicopters can fly in directions other than forward, including in the direction of the static port itself. To counter pressure differences from sideslip, many helicopters use a second port on the opposite side. To adjust for rotor wash interference and ensure that the instruments read correctly in climbs and descents, small structures may be placed around static ports to speed up air flow with convex surfaces or slow them down with dams (what you refer to as fences or steps).
Although there is no picture provided, I think this may be what you are asking about:

For more information, you may look here:
https://books.google.com/books?id=rxryAgAAQBAJ&pg=PA409&lpg=PA409&dq=helicopter+static+port&source=bl&ots=p-H9klAkUl&sig=ACfU3U2Yda5qCljafTMg97TSlRH_EpC13g&hl=en&sa=X&ved=2ahUKEwj7paLgkLzqAhUIWa0KHZseD2cQ6AEwG3oECAgQAQ#v=onepage&q=helicopter%20static%20port&f=false


When will I hear the sonic boom from a plane flying towards me? When does a person first hear a plane, flying faster than the speed of sound, as it flies directly overhead? Yes, I know there is a sonic boom and I also know the plane will produce sound as it flies away from that person. But what about the sound that the plane produces as it's flying towards the person? Will the sound be different from the sound heard after the plane has passed?

My dad and I have discussed this and looked at a partial explanation on Youtube, but there wasn't anything - as far as we could find - on the sound produced when the plane was coming towards a person.

In case a supersonic aircraft (flying faster than the speed of sound) flies directly over a person, that person (the observer) will not hear anything.

The (supersonic) aircraft produces sound throughout its flight (i.e. while flying towards as well as away from you).

The sonic boom you hear is the sound produced by the aircraft when it was coming towards you.

For how this works, see this question and the Wikipedia page.


Why does the Canard Rotor Wing use a Canard? Why does the canard rotor wing (CRW) use a canard?  The CRW is a helicopter using engine exhaust to drive the rotor.  The exhaust can be used in the normal way to drive the aircraft once high enough speed has been reached.  Why can't the CRW use a tail instead?

The Canard rotor/wing demonstrator, later called the X-50 Dragonfly was a Stop-Rotor Rotary Wing Aircraft made by Boeing and DARPA to demonstrate the principle that a helicopter's rotor could be stopped in flight and act as a fixed wing (like in movie The Sixth Day).

Image from boeing.com

The canard is used in case of Boeing X-50 Dragonfly in order to overcome the transient aerodynamic loads that occurred during conversion from rotary to fixed-wing flight. During transition from rotary mode to fixed wing mode, the rotor is being unloaded and the aircraft has to produce enough lift. The designers used both a canard and horizontal stabilizer to overcome this problem.

From Canard Rotor/Wing Aircraft:

  During development, analysis and testing indicated that the X-wing design encountered high transient aerodynamic loads during the conversion from rotary to fixed-wing flight. The CRW design addresses this issue by generating positive lift on both the canard and horizontal tail during conversion. By deploying the flapped canard and the full-flying horizontal tail, enough lift can be produced to support the weight of the aircraft, and the main rotor can be completely unloaded throughout the conversion maneuver.

The aircraft never made a conversion flight and the program was canceled.


Does anyone know where I can find old TFRs? Looking at the FAA TFR site (http://tfr.faa.gov/tfr2/list.html) I can only view current or future flight restrictions. Does anyone know of a resource where I can find past flight restrictions?

As you know, a TFR is published in FDC NOTAMs, so you can use the FAA's NOTAM search tool's "Archive" mode to search for past TFRs issued via FDC NOTAMs.

Use this link (https://notams.aim.faa.gov/notamSearch/nsapp.html#/). Press the Location button on the left and select "Archive Search." Then select a Date and Location designator to search for the NOTAMs (including TFRs) in effect on that date in that area.

By way of example, I did a search for Riverside Municipal Airport (KRAL) on 2/2/2016, and it returned these old NOTAMS (included TFRs).  See the California Speedway TFR.


How does an aircraft recharge its batteries? I assume aircraft have batteries that power up all electronic devices on board. But how are these batteries recharged? When the aircraft is on the ground, it can be connected and recharged in the airport, but mid flight? Is there a way to charge the batteries in the air ?

Next question, bit trickier: how many devices must be plugged by the passengers (via USB) and how long it takes to drain all energy on board of a 787 ?

The engines turn generators which supply the electrical power to the aircraft, including a battery charger that keeps the battery at 100%.  (Normally, power coming "from the battery" on the diagrams, is actually coming from the battery charger or another component like a Transformer-Rectifier converting AC to DC current.  The battery is very rarely actually being drained in flight.)

If you had an aircraft battery & went to deplete it with USB devices, it would take a lot of them & a really long time.  Aircraft batteries store power way beyond what our normal cell phones & iPads do, and can power all the emergency systems on the jet for an hour or more (radios, INS, flight instruments, minimal lights, GPS, flight management system, etc).  Of course, if you're down to just the battery, in-flight entertainment is NOT one of the systems being powered!


What does "Information " in ATC transmissions mean? When listening to ATC broadcasting (at least for ULLI), phrase "Information <Charlie|India|whiskey|etc.> on board" may be heard. Charlie, India, Whiskey, etc. refer to "C", "I", "W" but what do these codes mean? What kind of information is this?

This is a simple versioning system used in ATIS (Automatic Terminal Information Service).
ATIS messages contain information about the weather at the airport, runways in use etc. This information is transmitted on a dedicated radio frequency and repeated in a loop. The ATIS messages are updated frequently as the weather or operations at the airport change. Every time a new ATIS message is generated, the letter is changed.

Example of an ATIS message:

  This is Schiphol arrival information Kilo, main landing runway
  18Right. Transition level 50. Two zero zero degrees, one one knots.
  Visibility 10 kilometres. Few 1300 feet, scattered 1800 feet, broken
  2000 feet. Temperature 15, dewpoint 13. QNH 995 hectopascal. No
  significant change. Contact Approach and Arrival callsign only. End of
  information Kilo.

Now when for example the temperature or pressure at the airport changes, the ATIS information is updated:

  This is Schiphol arrival information Lima, main landing runway
  18Right. Transition level 50. Two zero zero degrees, one two knots.
  Visibility 10 kilometres. Few 1300 feet, scattered 1800 feet, broken
  2000 feet. Temperature 16, dewpoint 13. QNH 994 hectopascal. No
  significant change. Contact Approach and Arrival callsign only. End of
  information Lima.

The flight crew will listen to the ATIS often before they are in contact with the Air Traffic Control of the airport. They note the information in the ATIS, together with the information version letter.

On first contact with the airport's Air Traffic Control they will state what version of the ATIS they have listened to.

  Schiphol approach, this is KLM1234 with information Lima.

The Air Traffic Controller can then determine whether this is the latest version of the ATIS or if there is newer information available that should be passed on to the flight crew.


Are modern fighter jets bulletproof and armored? Are modern fighter jets bulletproof and armored? One would not be wrong given part of their operations is being on the receiving end of enemy ammunition shooting at them.

With very select exceptions like the Su-34, and the A-10 if you consider it modern, they are not. Even in these cases only the pilot and a few critical parts are armored. Most of the aircraft's skin offers no meaningful resistance even to small arms.

Extensive armor coverage that could withstand modern air-to-air or surface-to-air weapons simply isn't viable for a fighter for weight reasons. Redundancy is used to resist near misses and light hits instead.

The reasons are different depending on the weapon. MANPADS like the Stinger go for the nearest heat source (engine exhaust). Their capabilities against a fighter are limited to fringe cases (flying too low, caught on landing/takeoff, and a very lucky shot), and armoring the engine nozzle isn't viable anyway.

The lightest effective anti-aircraft weapon, the Sidewinder, uses a continuous rod warhead, a ring of steel expanding at almost 1 km/s. Due to high inertia, it will cut through anything except for the engine, major bulkheads and longerons, and hard elements like the A-10's "titanium bathtub". Aramid lining, which is the only thing a modern fighter could afford weight-wise for full-airframe protection, will be cut.

The heaviest is the S-400 SAM, which intercepts the target at 2 km/s with a 180 kg fragmentation warhead. The resulting hypervelocity impact (3+ km/s) has solids behave like liquids, with impact results determined by mass rather than strength. The low odds of surviving a warhead this size aside, when it's mass that determines these odds, it's always better invested in something functional rather than just armor.

Most mid-size AAM and SAM also fly an intercept course at high velocity and use high-brisance explosives for blast fragmentation. Against resulting high impact velocities, mass is better invested in more redundant hardware, not armor. That is very different from low velocities, common on the ground, where thin layers of high-strength materials can prevent projectile penetration.

Larger fighters are indeed more resistant to damage than smaller lighter ones (see the one-wing F-15). Armoring a few critical elements can help improve survivability, but this armor has to be very localized. The Su-27 family has an armor plate between the engines, so that one wouldn't damage the other if it disintegrates after a hit. Close air support (CAS) aircraft that attack tanks and other ground vehicles carry some protection for the pilot - notoriously vulnerable to punctures, unlike most of the plane - against ground fire.

A combination of these factors is why armor has largely become limited to CAS aircraft: high-strength armor is very weight-effective against the low velocities of land warfare weapons, but much less so against hypervelocity impactors and fragments used in air combat.

See a related question for a few more bits of info on aircraft armor and protection:
Why do modern fighter jets use 20mm guns?


How does the fuselage handle the different loads? I have been trying to find out how the loads experienced by a fuselage are transferred between structural members (bulkheads, formers, frame assemblies, skin, longerons, stringers).

I understand that the following loads are typically experienced by the fuselage:

empennage loads due to trim, manoeuvring, turbulence and gusts
pressure loads due to cabin pressurisation
landing gear loads due to landing impact, taxiing and ground manoeuvring
loads due to the weight of passengers and cargo

What I don't understand is how are these loads handled by the fuselage structure. I would really appreciate if someone could help explain the concept to me.

Not sure this is what you're looking for, but if you're looking for a simple big picture analogy, try this:

Think of the fuselage as a very thin walled compressed air cylinder resting on an airbag at about it's mid point, filled with sandbags that are offset toward one end, enough so that the cylinder wants to tip over on that end.  All the way at the other end you add more sandbags (to represent the tail pushing down) until the tube balances out.  Your fuselage will be basically a tubular beam balancing on the air bag with the bottom half in compression and the top half in tension, and the largest compression load will be just on each side of the air bag, but mostly on the "tail" side because the moment arm is longer.

Almost all of the loads of the fuselage pass through the skin, the surface of the cylinder.  The frames and stringers are mostly to keep the "paper thin" tube from buckling or collapsing.

As a compressed air cylinder, pressurization loads are also absorbed by the skin in tension, like a balloon.  Usually at the aft end there is a spherical bulkhead to absorb the pressure while keeping the skins of the bulkhead (mostly) in tension.  Sometimes this bulkhead is flat, usually when there are tail mounted engines that require a support beam running across, which can cause many headaches crack wise (on the RJs, a long sad story).  The front end of the cylinder narrows down to a small flat bulkhead at the front of the cockpit, which being small isn't too much of a problem being flat.

The fuselage usually has a big cutout on bottom to accommodate the wing box
and landing gear, which puts this great big notch in the structure right at the point in the cylinder where the compression buckling loads are highest.  The wing box itself may form part of the fuselage structure there, but farther back the cutout usually continues, to make space for the landing gear.  There is usually a massive beam, called a keel beam, to bridge the cutout aft of the wing box and absorb the compression loads along that section.

As a tube that is trying to sag forward and aft of the wing, there is also a force that wants to squash the tube flat at the peak bending point, resisted by the frames.  There are usually several very heavy frames at this mid section to support the tube where this force is highest, and to tie the wing box into the fuselage structure.

The highest loads on the fuselage are usually compression buckling stress just aft of the rear spar wing attachments when in flight, or the gear attachments when on the ground, and are highest on landing touchdown.  When aircraft have broken fuselages on hard landings (Boeing did it on their company 717 -formerly DC-9 - on hard landing testing and an Embraer 145 broke its back on landing in Brazil about 15 years ago - they did an excellent job keeping it out of the media) they fail in compression buckling right at that stress peak point.

I don't want to oversimplify, and there are lots of other stresses, like from the floor beams and such, but that's kind of the big picture.


If airliners and commercial planes have turbulence, can helicopters have turbulence? I was watching a video of an V 22 (Osprey) taking off and it had that white cloudy stuff that looked like turbulence around the propellers. Can helicopters have turbulence too? I know an osprey is not a full helicopter nor airplane but it has some features of a helicopter.

That 'cloudy stuff' that you say is not 'turbulence', but condensed water vapor, the condensation being due to the sudden cooling caused by the adiabatic expansion of the air due to the aerodynamic disturbance caused by a passing blade (or wing).

That condensation can also be observed as wakes in landing aircraft, and in stationary revving props. It appears when the sudden expansion takes place in humidity conditions close to saturation.


Why are the B-47’s inboard and outboard engines mounted so differently? The B-47 has six underwing GE TG-190 engines, two on each inboard pylon and one on each outboard pylon.

The inboard engines are mounted on a long pylon that extends well below and forward of the wing, such that the inboard-engine tailpipes are (longitudinally) just about even with the wing’s leading edge.

The outboard engines, in contrast, are mounted on much shorter pylons and fit snugly against the underside of the wing, with their tailpipes extending aft of the wing’s trailing edge; they’re placed so far back that their pylons even protrude beyond the trailing edge.

(Image originally by the United States Department of Defense, via Balcer at Wikimedia Commons; modified by me.)

Why the long pylons and forward mounting for the inboard engines, versus the short pylons and rearward mounting for the outboards?

The forward pylon mounting of most jets with wing mounted engines is to use the engines' mass to move the overall center of mass of the wing forward, closer to or ahead of the torsional axis of the wing box.  Boeing discovered this helps with aerolasticity related issues with long, flexible, swept wings.  A bit like mass balancing a control surface, but on the root structure.

The outermost engines, being so close to the tip where the chord is short, have to be farther aft or they would move the wing's center of mass too far forward in that area.