Can a handgun shot take down a commercial airliner? Most airplanes, when taking off or landing, are in range of small arms fire, i.e. your every day handgun.

Is it possible to take down a commercial airliner by shooting at it with a handgun? Would one shot be enough, or would several be needed?

shooting at it with a handgun

That's really going to depend on the handgun.

James Bond carries a Walther PPK - this is a .32 calibre popgun with rather limited use outside of a small office. Direct hits on people are usually survivable unless you take a round in the heart or brain. Chances of even hitting an airliner during takeoff or approach are zero - it simply doesn't have the range, and that tiny subsonic projectile will get deflected by the aircraft's wake.

Skipping the middle range and going straight for the hand-cannons, we might get somewhere with a Desert Eagle or a Smith & Wesson 500. Remember the posters for the first Terminator movie? The one where The Ah-nuld is wearing sunglasses and holding up some light artillery? These are bigger. At close range, both guns can crack an aluminum engine block1 and are one-shot kills on most things smaller than a rhino. Hitting an approaching widebody isn't all that difficult - it's coming toward you, the angle isn't changing, and it's a rather big target.

But aircraft are large, complex systems with a lot of redundancy, so in order to actually take it down we have to hit something critical. Hitting a 767 right in the nose will destroy the weather radar and (maybe, if it has enough energy left) poke a 1/2 inch hole in the forward bulkhead. No one will notice. The flight-deck windows are rather strong, apparently a police sniper tried to take out a hijacker on the flight deck way back in the 1970s and the rifle round (far more powerful) just pock-marked the window. Several hostages were promptly shot as a result.

The best shot (ha!) is the engines. They don't like ingesting bits of metal but just hitting the fan won't do much. You would have to get very lucky to get the round into the engine itself, where it will promptly do a lot of very expensive damage.

But even if one engine literally goes boom it still won't crash the plane. Pilots practice for an engine failure on takeoff all the time, and an engine failure on landing is almost a non-event.

Hydraulic lines for control surfaces are certainly sensitive, but they are well buried in the structure and there are several independent systems. You won't get them all.

So, in conclusion, handguns are sufficiently ineffective at bringing down large aircraft that we really don't need to be concerned about them. There's a reason air forces around the world prefer missiles or 20mm cannon2 for this sort of thing. Geese, however, are quite effective if released in sufficient numbers along the takeoff path. Rather noisy though - you won't hide 500 of them anywhere near an airport without someone noticing.

Internet legends of a .50 cal being able to kill a truck refer to the MUCH larger .50 BMG round, available in explosive and armour-piercing and rightfully categorized as a first-class military weapon.
The projectile's the size of your thumb, and the gun fires 6,000 rounds per minute. It'll do your plane like a chainsaw.

If it pokes a hole, the pressure in the cabin will be lost (which involve possible injury to the ear drums or death of the people inside, as at cruise altitude there is not enough oxygen in the air for the human body, which is why it is pressurized inside and maintained at near sea level pressure).

The aircraft can be damaged to the point of losing a structural panel, but not necessarily crashing. Unless the pilot falls unconscious due to the lack of oxygen. (Oxygen mask only have about 15 minute of oxygen by the way).

If the bullet hits an engine, or poke the wing (i.e. fuel tank), then there is high risk of explosion, with dramatic consequences.


Can one "create" a private interpilot air-to-air frequency to chat? I've been looking for an answer to this for the past couple of hours and can't find anything on the matter. (Yes, I've read this What radio frequencies are used for intercom in formation flying?  , but it didn't answer my question, as I'd like to know about frequencies that are not in use by any authorities or  organisations and are not used for anything official,  such as search and rescue).

A pilot mentioned to me she "created" frequencies to talk in private with her mate (air-to-air interpilot). I found one link that says pilots sometimes use unallocated frequencies to chat , so I assumed she meant that, but I've just read on AOPA that all 760 channels available in the VHF   aviation band are actually allocated (I'm assuming in the USA, as the reference is an FAA circular) even if some are not used at all times.

So, can you "create" a private frequency? If I'm right to assume that that makes no sense, can you join a private frequency (do they exist??) or use an unallocated one to chat privately? In case it's the latter, how does that work if all frequencies are allocated already (or is that just in the USA)?

Thanks a lot for your help

Every frequency is allocated somewhere, in fact the same frequency will be used over and over throughout the world. This works because air band radios are relatively short range, about 200nm at best, so all you really need to do is find a frequency that isn't used anywhere near you and tune to that. This is what formation or group flights do.

That will not be private in the sense that it cannot be overheard, anyone with an air band radio can listen in. It will also not be private in that only you could use it, again anyone could tune into the same frequency for the same purpose. Formations often have secondary frequencies to tune to in these cases, but it's rare they need them.

123.45 is commonly used as an (unallocated) air-to-air frequency, although depending in your location it might not be officially sanctioned

You may use that for chat or formation flying, but it will definitelly not be private


How are pilots alerted to the publication of a NOTAM affecting their flight while in-flight? With the length of flights in this day and age, it is possible for an activity to be published in a NOTAM, that NOTAM to be disseminated via the NOTAM system, and the activity to start taking place after a flight whose PIC should have received that NOTAM took off!  Take for instance a by-NOTAM restricted area activation with the minimum 4 hour advance notice, and a 777 whose crew flightplanned through the restricted area based on it being "cold" at the time they briefed their flight, which was oh, another 4 hours before the NOTAM for activating said restricted area was put out.  Keep in mind that the 777 crew took off 2 hours before the restricted area was activated...

How is the flight crew of our hypothetical 777 supposed to find out that the restricted area they were told was cold during their preflight briefing is now hot?  (Of course, this applies to other kinds of NOTAMs as well, some of which can pop up with even less notice; for instance, a mishap-induced runway closure could occur at any time, and a long-haul flight crew would probably rather get that memo before they seek a landing clearance!)

Also, assume this is in US airspace.

In the case of the 777 (or any flight operating on an IFR flight plan) it is ATC's (and the pilot's) responsibility to vector the flight around restricted airspace. In the event of pop-up NOTAM's, ATC will do their best to inform you, especially on an IFR flight plan. The key is you should be talking to somebody, or at least listening in. I have a feeling though that you want a little more...

Flight Information Services (FIS-B) provides weather, text, graphical weather, NOTAMs, ATIS and other information to aircraft in flight over the UAT link. ADS-B (ADS-B Out mandated for all aircraft in controlled airspace by 2020 in the US) will be able to provide similar data to aircraft in flight all the way down to small GA (if they have ADS-B In capabilities, or ADS-B receivers like Stratus).

So the crew of your hypothetical 777 will be notified multiple ways:

ATC will notify them about the need to vector around restricted airspace.
ATC will notify them about the closure of an entire destination airport as filed
NOTAMs will pop up on the FIS-B display in the cockpit
The ATIS or automated information broadcast will be updated to include NOTAM'd airport or runways
Crew will see the FIS-B display when they check destination airport weather/closures enroute
At the very least, ATC will notify the flight on approach ("Airport is currently closed, please continue to the filed alternate" or something like that)

Now, on the flip side let's say that you are a J-3 Cub with no radio humming around to your local uncontrolled field. The field was just closed due to a water buffalo migration and NOTAM'd out of service until the beasts can be removed from the property. You are on a 5 mile final and all the water buffalo are hiding in the woods next to the runway...

If you land you've violated the NOTAM and airport closure. This may warrant a call to your FSDO but you will probably be able to explain that the NOTAM wasn't active or identified prior to departure and they'll let you off (and the next thing you should do is go buy a handheld radio). At the very least you should make a report in the Aviation Safety Reporting System (ASRS) in case somebody raises concerns.

Either way if possible the FAA recommends a 4 hour minimum notification time for closures that can be planned. Why 4 hours? Most GA aircraft have about a 4 hour endurance and most domestic flights are less than 4 hours long. Obviously not everything can be planned, but ATC will work with aircraft in flight to disseminate the information as required.

There is on one way to get the latest on the spot NOTAMS the instant they are published.  If an aircraft has WiFi and the flight crew periodically obtains NOTAMS throughout the flight is one way.  Critical and pertinent NOTAMS may also be passed along to controllers who will disseminate them to pilots operating in airspace affected by a NOTAM.

Due to the capricious nature of VIP TFRs or other critical NOTAMS, the FAA is generally forgiving if a pilots has conducted a competent preflight and researched applicable NOTAMS to his/her flight prior to departure.  I talked with an F-22 pilot a couple of years back who had some experience chasing errant bugsmashers out of TFRs.  His advice for legal CYA: ALWAYS call flight service an drew request a standard flight briefing, including a NOTAM briefing prior to flight.  You now have a permanent record that there was no knowledge of a NOTAM affecting your planned flight and that can be used as evidence in court.

As for the 777 example you listed above, it is almost certainly going to be operated under IFR rules and will be under the control of ATC who can grant you clearance to enter a restricted area; it's their hinder that's in a sling if it goes SNAFU, and not yours.


Could you land a large airplane on short circular runways? Considering that building runways is expensive and that they take up a lot of space, could you land a large commercial airliner on a circular runway, braking as you go around the curves?

As a matter of physics, there's a certain amount of braking and steering force available to the aircraft after it lands.

Suppose the aircraft touches down moving due north, and continues clockwise around your circular runway. After a quarter-circle it travels due east -- but that means that enough braking force has been applied to reduce the northward component of its velocity to zero, while also giving it some eastward velocity.

If only that eastward force hadn't been applied, the aircraft would be at a halt now -- using less force than what's necessary for it to follow the circle.

So if the plane can keep itself on a circular runway, it will be even easier for it to come to a halt on a straight runway whose length is the radius of the circle.

(Also, the plan ignores that the critical factor in the length of runways is not landing but taking off -- there must be enough runway to brake down safely if the takeoff is aborted just before rotation. And taking off from a curved runway sounds even crazier than landing. If the runway is horizontal, the plane would become airborne in the middle of an uncoordinated turn...)

This will work if the circular runway is rotating.
The runway should be on a pedestal, and spin up to an to an angular velocity such that the tangent matches the aircrafts speed.  then rise up and catch the aircraft, slowly reducing angular velocity and lowering to the tarmac.

Take off would proceed in the reverse manner.  with the aircraft speeding up trying to stay on the rotating runway as it rises and increases angular velocity.

In the event of alien invasion we can use the rotating run ways to launch projectiles such as the airport busses.


Where is the safest seat in the plane? If I'm going on holiday with my family on an airliner such as one of the ones listed on this page, then where is the safest place to sit?

The answers to this question seem to imply that the seats at the rear of the plane would be safer:
Why are flight recorders generally located in the tail?
but of course I can think of quite a few things which might make it different for passengers versus the flight recorder.

I realise that flying is statistically a lot safer than driving, so I don't need to be told not to worry about safety - I'm not worrying, I'm just interested in the difference in safety between different seats in the airliner. Because if I get a choice of seats then (ceteris paribus) I can't see any reason not to choose the safer ones.

I'm guessing it might be affected by things like proximity to the exits, and so might be different in different models of airliner. If that's the case then can you tell me what factors affect the safety of each seat? E.g. is it more important to be close to an exit or to be further back in the plane? What should I look for?

Is it different on long-haul versus short-haul? Or are there other differences in the type of flight (and therefore the more typical type of accident) which change the 'safest place to sit'?

A few articles I've found on the topic:
 - Popular Mechanics, 2007 (statistical review: back is safest)
 - Discovery / Channel 4, 2013 (one-off crash experiment: back is safest)
 - BBC, 2014 (expert opinion: no safest place)
 - CAA, 2006 (statistical review of evacuations: exit rows and adjoining rows are safest)

Before I start, remember that we're talking about statistics of location in the aircraft, within statistics of survival of an accident (95%), which is itself dependent on a statistic of particular type of accident (approx 85% have no fatalities at all), within something which is already a very unlikely event (an aviation accident).

So let us for a moment assume that you're in a semi-survivable accident, ie where there are a mixture of fatalities and survivors - this is already a very unlikely scenario... in the vast majority of accidents either everybody dies (rare), or nobody does (most common).

To give you the straight out answer to your question, in such an accident, the rear of the plane is safest - more passengers have survived plane crashes in those seats than any other. This is due to most accidents being nose-first, so if it's going to be partially survivable your ideal location is near the back: the front of the aircraft essentially acting like a crumple zone. Thus, if an accident is survivable, you are more likely to survive in the back of the aircraft.

Of course, every accident is different - if your aircraft runs off the runway and spins, the back can impact first. Similarly if your aircraft stalls and pitches up. Alternately you could slew sideways into another aircraft in a taxiway etc etc... but in most cases, the impact is more likely to be nose-first, thus the rear is the safest place.

Also statistically, being near to a door but not next to a window gives the "optimum" chance of survival - more people survive in such locations. This is due to ease of evacuation (being near the aisle and the exit), and without being whacked in the head by a crumpling fuselage.

So if you're going on purely "increasing your chances", a non-window seat near the rear door is the best possible place to be in an accident... other than on the ground, having missed your plane.

The thing to remember, though, is that (referring to modern aviation)

Accident rates work out as roughly 1-per-1 million flights
Around 85-87% of aviation accidents involve no fatalities (so fatal accidents are around 15-per-100 million flights)
Even if you're in a crash where someone dies, Aviation accidents with fatalities have a fatality rate of somewhere between 5% and 25% (which is to say, even if we only look at accidents where one or more people do die, 75-95% of people in those accidents survive).

So that gives you a 1/4th to 1/20th of a chance of a 3/20th of a chance of a 1 in a million chance of becoming a statistic in my next post on this subject. So that would take you somewhere between 20 and 100 million flights before you die.

You have a 1-in-100-million chance of being in a fatal accident, and even then you a 75-95% chance of surviving it anyway.

So in short, if it's going to get you it will, if it's not it won't. By the time you've survived enough accidents for it to be anything more then sheer atrocious luck that you die at all, you'd already have developed a healthy fear of flying and have gathered enough air miles to take a taxi....

Though an analysis of a single crash is hardly decisive, its findings did support a study by Popular Mechanics, carried out in 2007. The magazine analysed all crashes since 1971 and found that those in rear seats (behind the wing’s trailing edge) were safest – survival rates were 69 per cent as opposed to 56 per cent over the wing and 49 per cent for those at the front of the plane.


Do you need an instrument rating to fly in IMC in Class G airspace? I’ve been watching IFR checkride prep videos and several of them mention that among the things you can do with your IFR ticket is to:

Fly in Instrument Meteorological Conditions (IMC) in controlled airspace.

If you have your IFR ticket, you can depart Class G in IMC if you have filed a flight plan. So I’m a bit confused about why they put that restriction on the privilege.

But it also got me thinking. You need an IFR ticket to fly in IMC in controlled airspace, but is there a regulation that says you need one to fly in IMC in uncontrolled airspace? Or does NTSB 3935 mean that flying in IMC without your IFR ticket automatically imply careless and reckless flying?

In the US, an appropriate instrument rating is needed to fly when WX conditions are below VFR minimums, and at night for SVFR.  14 CFR 61.3(e)

Additionally, one needs an appropriate instrument rating to fly under an IFR clearance when in controlled airspace, regardless of the WX.

Notes: ATP and things like a category and class in an airship are exceptions. Also an airplane instrument rating works in gliders. Etc.

Addendum: Specific to the OP's citing a NTSB report, the following comment is added: To rely on the mentioned NTSB report, it is important to read the findings, and understand that, a.) the pilot intended to hit VFR conditions before 700AGL and upon entering controlled airspace, b.) the pilot told people he was taking off despite qualified observers noting 200 ft clouds, c.) the airport where approach was based said they were below IFR, which would leave the pilot few options if he needed to land nearby. Given that, it is easy to agree with the NTSB. Just because something is legal, doesn't mean it is best practice, and in this case it is likely the pilot did not do as stated.

One final point, IFR without a clearance in Class G is a decision which should not be taken lightly. Years ago, I routinely did it, and the airspace was uncontrolled at safe IFR enroute altitudes.  Departing a Class G airport, with less than VFR conditions, and expecting VFR before hitting controlled airspace at 700AGL is dubious. The vest majority of my IFR flights in uncontrolled airspace, that were without benefit of clearances, were where there are large areas of uncontrolled airspace, and safe IFR altitudes could be flown enroute. Furthermore, while ATC did not issue clearances for flight in uncontrolled airspace, coordination with ATC where possible, and also with local FSS were done. That is different from popping off an airport that is likely IMC, and stating that one intends to climb into VFR weather before hitting 700AGL and controlled airspace. The frosting on the cake is having the field in the area be below IFR limits for approaches.  Good judgment is hard to legislate.

The videos you were watching did not mean to imply that you can fly in IMC in uncontrolled airspace without an IFR rating.  Rather, it seems that the creators of the videos wanted to avoid suggesting that you can fly in uncontrolled airspace in IMC with an IFR rating.

Some pilots believe that no legal flight in IMC is possible in uncontrolled airspace.  (See for example this ASE answer).  This is simply not so.  If it were, why would some of the approaches into KCVO, which has a Class E floor at 700' AGL, have "minimums" well below that that?  (Click here to download approach plates.)

In fact, it is legal for a pilot with an IFR rating to fly in IMC in uncontrolled  airspace, at least under some circumstances.  However, there is no provision for adequate separation from VFR traffic, which is allowed to fly right up to the edges of the clouds in uncontrolled airspace, at least below 1200' AGL in the daytime.  This strange discordance in the FARs had led the FAA to use FAR 91.13, the "catch-all" regulation against "careless or reckless operation", to sanction some pilots for operating under IFR in uncontrolled airspace in some circumstances, as described in this 1993 NTSB ruling, also referenced in this 2016 Letter of Interpretation from the FAA.

Evidently the creators of the videos you were watching wanted to spare you the possibility of sanction, persecution, or prosecution by the FAA or NTSB, and so they chose language that avoided any suggestion that an IFR rating was a blanket license to operate in IMC under IFR in Class G airspace, and chose not to "muddy the waters" by letting you know there are in fact many situations where the FAA does condone operation in IMC under IFR in Class G airspace.

If the FAA had ever at any point made it clear under exactly what circumstances a pilot may operate in IMC under IFR in uncontrolled airspace without fear of sanction, the creators of the checkride prep videos would undoubtedly have included that information in the videos, but unfortunately this is not the case.

This answer begs another question which has not yet been asked on ASE, though it is addressed in some of the links below: "May an IFR-rated pilot flying an IFR-rated aircraft in Class G airspace violate the VFR cloud clearance requirements without an IFR clearance?"

Related ASE answers --

Can an IFR clearance be issued and flown through IMC in class G airspace?

Can an IFR clearance be issued and flown through IMC in class G airspace?

Can an IFR clearance be issued and flown through IMC in class G airspace?

Do you need an instrument rating to fly in IMC in Class G airspace?


At what point does an aircraft become an airliner? For me, the term 'airliner' conjures up an image of a passenger jet that is capable of hosting a large number of passengers¹ but I'm finding it difficult to source a reputable definition or etymology despite the term has been sourced int the early 20th century.

Since the term has been in use since 1908, jet as in jet airliner may be inferred by the modern user but it obviously wasn't part of the original definition.

The word itself would seem to indicate that airliner refers to an aircraft used by an airline (a company that provides air travel for the public), but would that extend to bush pilots with small prop-driven Cessnas or passenger helicopters?

  airliner - A large passenger aircraft. A large plane for carrying passengers:
  a commercial/commuter airliner   source: Cambridge English Dictionary

  airliner - An airplane operated by an airline   source: Merriam-Webster Dictionary

  Is this an 'airliner'? How many passenger seats need to be added before it becomes one?  (image courtesy of The Bush Pilot Company)

The term seems so vague that even the airliner tag on this site only uses 'large' to define size or capacity.

Some reference work definitions include cargo aircraft but the predominant emphasis seems to be on passenger aircraft with no upper or lower limit criteria.

I suppose that this term could be relative to the observer or situation but if the term 'airliner' was used in FAA regulations then I'm sure there must be an appendix somewhere that provides a definition. My research has come up empty.

Apologies if this is a dumb question and the term is intentionally meant be vague with only 'large aircraft' or 'many passenger' as criteria. Given proper references as support, that could be an answer.

¹ This previous question got me wondering exactly what was the criteria before any aircraft could be called an 'airliner'.

I think there's a misunderstanding here that airliner ("an airplane operated by an airline") is directly derived from airline. While it is a reasonable definition, "airliner" predates airline. (Mirriam Webster: airliner 1908, airline 1910).

An airliner is a portmanteau of air and liner, a liner being any passenger ship plying between ports along regular "lines" (as distinguished from transient ships using those ports). By the early 20th century a liner was generally understood to mean any large, passenger-carrying vessel operating a route between major ports (think Titanic etc.). You'll notice how airliners travel between airports - nautical words and phrases were adopted in the early days of flight.

So an airliner should be

passenger carrying
large
travels between airports, rather than having one fixed point of operation

It's a definition that still fits, and covers any commercial aircraft over a certain size. While there's not a specific point at which an aircraft becomes large enough to be called an airliner, I think it does impart some sense of largeness or capacity that an 'ordinary' plane does not possess.

(You might even say that a better definition of airline is "an air transportation system that operates airliners"!)

For many vague terms without specific, precise legal or regulatory definition, the answer will rarely be binary, True or False. It helps to think in terms of probability. If you ask the question: "Is this an airliner?" to 100 random people, how many would answer yes?


What are these holes in the engine inlet? In the next picture can be seen within the housing, near the inlet:

These are "acoustic liners"; they allow the sound waves to get "trapped" in the holes and make the engine quieter. Sometimes chunks tear loose and they're repaired with what amounts to Bondo.

The holes you are referring to are a part of the engine-inlet anti-icing system. These holes prevent ice accumulation around the engine inlets. Without them, the engines may ingest ice and will also encounter airflow problems. Engines require laminar airflow in order to operate efficiently.


What does this vehicle transport? I've seen on many airports a lot of vehicles transporting some "brown boxes".

Any idea of what is this?
 (I don't ask what's exactly inside this box, just in general)
Source: 123rf.com

What that may be?  Baggage? : in case that boxes contain baggage, why don't they put it as loose baggage or in ULD's. May that be other type of cargo?

Your question is rather broad and somewhat unclear, but what you have is a picture of a baggage tug and cart.

The carts are loaded with passenger baggage or cargo, then strings of them are pulled to the aircraft by the tug before being loaded on to the plane.

Based on the additional pictures, I think what you're after would be called palletized cargo. It's stuff, put in boxes, placed on pallets, wrapped with stretch wrap, with a net thrown over the top and hooked down to keep it from sliding off the pallet.

It is most likely not passenger baggage. That's either loaded one piece at a time by hand, or in a ULD.

If you're asking what's in those particular boxes, you'd have to zoom in quite close to find a label on a box to even have a chance of figuring it out. Just about anything can be and is shipped in a brown cardboard box, from anywhere to anywhere in the world. This question is far too broad and off topic for Aviation.SE, and probably any SE.

shipping batteries could be a possibility.


Where can I find more about a cargo pickup technique that doesn't require landing the plane? A number of years ago, I read about a technique for cargo pickup and recovery at remote sites using a low-and-slow-flying plane.  The plane would enter a slow orbit around a point on the ground and lower a bag to the ground on a long (hundreds of feet?  A thousand feet?) line.  Once the bag was down, the plane would maintain its turn around a point so that the line stayed slack and the bag stayed in position.  Cargo (perhaps mail?) could be retrieved from or placed in the bag, then once the transfer was complete, the plane exits the circle and lifts the bag.  I'm a lot less clear on that part.

However, I don't have any idea where I read about this, and haven't been able to find any more descriptions of this.  Can anyone tell me if this technique is real, what it's called, where/when it's been used?

See the notes on this page under the "Delivery" section. Looks like its formally called " long-line loiter". There is a brief note on it here and a US Patent here and this manual from the 70's that seems to show how to do it and some related research on it.

Here is a nice image of the maneuver

I saw a similar technique to pick up personnel. A balloon was floated up, and a plane with a big V in front collected the line and pulled the person up.
Pictures and more description here CIA's Fulton surface-to-air recovery system (STARS)

This was used in a couple of the early James Bond (007) movies also.

Similar can be done to what banner tow pilots do.  A line is strung between two poles, the plane flies above with a hook on a rope to grab the line and then takes off full power, with lots of slack to give the plane time to climb and gain some speed before the load's drag kicks in.


When do the lift and drag vectors contribute a force component along a glider's path of travel as seen from the ground? It is often said that the lift vector helps to propel a glider forwards.  The lift vector has no component acting parallel to the glider's trajectory through the airmass, but in many cases the lift vector does have a component acting parallel to the glider's trajectory as seen from the ground.  Note that for a given airspeed, the direction of the glider's trajectory through the airmass-- i.e. the direction that is exactly opposite to the "relative wind" that is "felt" by the glider-- is not affected by sustained updrafts, downdrafts, headwind, or tailwinds.  But this is not true of the glider's trajectory as seen from the ground.  A glider in a powerful mountain wave updraft may be rising straight up as seen from the ground, while the trajectory through the airmass will still be the normal glide path that would be obtained at whatever airspeed the glider is flying at.

Under what circumstances does the lift vector of a glider contribute a force component along the glider's path of travel (trajectory) as seen from the ground?

Under what circumstances does the drag vector of a glider contribute a force component along the glider's path of travel (trajectory) as seen from the ground?

Likewise for a powered airplane.

Note -- to avoid any possible ambiguity, please be aware that this question is using the word "lift" exclusively to mean an aerodynamic force generated by the aircraft, not to mean rising air.  These are two completely different things.

Note -- the scope of this question is meant to be confined to linear (but not necessarily horizontal) straight-line flight with constant airspeed and groundspeed, at least over the short term.  We're assuming the glider (or airplane) is flying in an airmass that is locally uniform. In other words, if the glider has entered a thermal updraft or penetrated through an abrupt wind shear or flown from sinking air into a ridge updraft or wave updraft, we're assuming that it has been in the new airmass long enough to come into equilibrium, so that net force is zero for the time being.  This is not a question about "dynamic soaring" as practiced by the albatross over the open ocean, r.c. glider pilots flying in loops on the lee side of the hill, someone trying to put into practice Taras Kiceniuk's ideas about exploiting the boundaries between still air and downdrafts as a source of energy, etc.  Those subjects are worthy of an ASE question, or many questions, but this is not that question.

In thermals.

More precisely: Every time when the updraft strength increases. The glider will continue on its original path due to inertia while the airflow will have a positive angle when referenced to the path of travel. Now the aerodynamic force will point slightly forward and accelerate the glider.

Experienced pilots use this by pulling more than 1 g when flying into a thermal and less when updraft strength decreases. This helps them to gain some more energy from the rising air.

Drag is per definition the part of aerodynamic forces parallel to the flow direction, so for drag the answer is: Always. With changes in vertical airspeed, the drag component parallel to the direction of movement will merely change with the cosine of the angle between the aerodynamic and the kinetic coordinate systems.

If you insist on excluding changes in vertical air speed, your question will have no answer regarding lift.

Assuming steady-state linear flight and the other constraints stated in the question--

The idea that a component of the Lift vector is helping to pull the glider forward along the glider's trajectory as viewed from the ground is only true when the glider's achieved glide ratio relative to the ground is better than the L/D ratio (which is also the still-air glide ratio), or when the glider is climbing in an updraft (unless the glider is drifting backwards over the ground along an achieved climb path that is flatter than the direction of the drag vector/ relative wind). These are the only times the that the Lift vector has a forward component relative to the glider's trajectory as viewed from the ground.

This would only happen in one the following cases -- the glider must be flying in:

1) Air that is neither rising or sinking, but is moving with tailwind component relative to the glider's actual heading (not relative to the "course" or achieved ground track-- the distinction comes into play when there is a strong crosswind component.)

2) Air that has no horizontal motion, but is rising.

3) Air that is rising fast enough to offset the degradation in achieved glide ratio caused by a headwind component.

4) Air that is moving with a tailwind component that is strong enough to offset the degradation in achieved glide ratio caused by a sinking (downdraft) component.

5) Air that is rising and also is moving with a tailwind component

To a first approximation, the idea that a component of the Drag vector is helping to pull the glider along the glider's trajectory as viewed from the ground is only true when the glider is moving backwards over the ground-- i.e. when a component of its "course" or achieved ground track vector points in the same direction that the tail of the glider is pointing, and opposite the direction that the nose is pointing.  This can easily happen in strong wind-- the glider can easily drift backwards over the ground.

Strictly speaking this approximation is only true if the L/D ratio is infinite.  To be more precise, we have to recognize that the true criteria is that the Drag vector will help pull the glider along the glider's trajectory as viewed from the ground whenever a component of the actual three-dimensional flight path as viewed from the ground-- not just the "course" vector or achieved ground track-- points in the same direction that the Drag vector is pointing.  Since the Drag vector is parallel to the flight path through the airmass or "relative wind", it is inclined relative to the horizon.  Therefore for non-infinite glide ratios this criteria can be achieved when the aircraft is rising straight up vertically relative to the ground with zero horizontal groundspeed in a strong updraft, or even when the aircraft is creeping forward very slowly over the ground as it rises rapidly upwards.  Also, there can be cases where the criteria is not met even if the aircraft is drifting backwards very slowly over the ground, if it is also sinking very rapidly due to a strong downdraft.  But for reasonably high glide ratios, it's a fairly good approximation to say that the Drag vector is helping pull the aircraft along its trajectory as viewed from the ground only when the aircraft is actually drifting backwards over the ground.

What is really going on here is that we have a closed vector triangle of Lift, Drag, and Weight, with Lift smaller than Weight and inclined slightly forward relative to weight.  Drag is perpendicular to Lift and acting at right angles to Lift, as illustrated in this related answer.  There is no question that net force is zero, and net acceleration is zero, from any inertial reference frame, including the airmass reference frame and the ground reference frame.  But the velocity vector is different in different reference frames-- for example consider the case where the glider is slowly rising straight up as seen from the ground reference frame-- so the issue of which forces are contributing a component along the glider's direction of travel depends on which reference frame we choose.

Note that it is often said that gliders are powered by gravity.  This is always true as seen from the airmass reference frame-- of the three forces Lift, Drag, and Weight, Weight is the only one with any component parallel to the glider's path of travel through the airmass.  But as seen from the ground-- i.e. in relation to the path of travel relative to the ground-- gravity is exerting a force component parallel to the trajectory only in cases where the glider is losing altitude.  If the glider is climbing, then the Weight vector is exerting a force component against the direction of glider's trajectory as seen from the ground, not in the same direction as the glider's trajectory as seen from the ground.  If the glider is flying horizontally, then from the viewpoint of the ground-based reference frame, the Weight vector is exerting no force component along the direction of the glider's trajectory, and is doing no work on the glider.

This means that whenever a glider's velocity vector as seen from the ground is constant and is purely horizontal, the forces exerted along the direction of travel by the Lift and Drag vectors must be exactly equal and opposite.  Whenever a glider is travelling horizontally and the glider's velocity vector as seen from the ground has a component toward the nose of the glider rather than toward the tail of the glider, the Lift vector is helping to pull the glider along its path of travel, and the Drag vector is resisting that pull in a way that exactly cancels it out.  Conversely, if the glider is travelling horizontally and backwards as seen from the ground, then Drag vector is helping to pull the glider along its path of travel, and the Lift vector is resisting that pull in a way that exactly cancels it out.

The Lift and Drag vectors are fixed in their geometric relationship to the relative wind-- Lift is perpendicular to the relative wind, and Drag is parallel to the relative wind.  Furthermore, for any given L/D ratio and Weight value, the Lift and Drag vectors are fixed in magnitude.  The fact that, from the ground reference frame, the force component exerted in the glider's direction of travel by the Lift and Drag vectors varies according to the wind direction and updraft or downdraft velocity, is really just an artifact of the fact that direction of the relative wind is not fixed in relationship to the direction, and magnitude, of the glider's velocity vector as seen from the ground.

The work done by a force is defined as distance traveled times the force component exerted in the direction of travel, and it takes energy to do work on a glider or any other body.  It doesn't really make sense to ask what energy source is responsible for any one particular aerodynamic force component, such as the component of the Lift or Drag vector that is acting parallel to the flight path as viewed from the ground.  Ultimately, as seen from the ground, any NET energy transferred from the atmosphere to the glider, via the combined action of the Lift and Drag vectors, can only come from updrafts.  Only in the presence of an updraft can a glider gain altitude while flying at a constant velocity, regardless of the horizontal wind speed and direction.  Of course, as the glider extracts Kinetic Energy from the updraft and converts it into its own gravitational Potential Energy, the updraft must be slowed by an infinitesimal degree.  The only instance where the glider is neither increasing nor decreasing the Kinetic Energy of the surrounding local airmass is when the glider is travelling purely horizontally, rather than sinking or climbing.

And the ultimate energy source that powers updrafts-- whether they are thermal updrafts or "ridge lift" updrafts or mountain wave-- is the sun.  This power may be conferred either via direct solar heating of the ground directly below, or by complex meteorological processes involving the entire global weather system.

The question also asked about powered aircraft.

Assume for simplicity that Thrust acts exactly opposite in direction to Drag.

In the case where Thrust is less than Drag, the aircraft is exactly like a glider, except we substitute "(Drag minus Thrust)" wherever "Thrust" appears in the explanation.

In the case where the Thrust and Drag are equal, the explanation regarding the Drag vector is exactly the same as the "first approximation" explanation (assuming infinite L/D ratio) above.  And in this case, the Lift vector is adding helping to pull the aircraft forward along its trajectory relative to the ground anytime the aircraft is in rising air, without exception, and without regard to headwind or tailwind.

In the case where Thrust is greater than Drag, then the idea that a component of the Lift vector is helping to pull the aircraft forward along the aircraft's trajectory as viewed from the ground is only true when the aircraft's achieved climb ratio relative to the ground is better (i.e. a lower number than) than the L/(T-D) ratio.  This will require an updraft, or a headwind, or both, and a weak updraft may not be enough to offset a strong headwind, and a weak headwind may not be enough to offset a strong downdraft.

In the case where Thrust is greater than Drag, then the idea that the Drag vector will help to pull the aircraft along the aircraft's trajectory as viewed from the ground can only be true when the aircraft's trajectory as viewed from the ground has a component pointing in the same direction as the drag vector.  For ratios of L/(T-D) that are not too high (i.e. the still-air climb angle is not extremely good), to a first approximation this means that the aircraft is drifting backwards over the ground, but this approximation will not apply well to extremely high still-air climb angles.  An aircraft that could climb straight up at a high rate of climb in still air could, in that same configuration, only move (as seen from the ground) in the direction of the Drag vector in the presence of an incredibly strong downdraft.


Are there any low time lucky CPL holders who operate as PIC on medium or heavy jets? CPL holder can be PIC on any plane if it's not a commercial air transport. So if the flight is positioning, acceptance, testing, delivery or any other without cargo and passengers any CPL holder provided he/she has TR and special training for the left seat can act as PIC? Do airlines allow this maybe as bonus for young first officers?
Another option is big business jets like 737 747 BBJ, A319...

There have been periods of time in the past (and I think at present as well) where a lack of experienced pilots meant airlines had to resort to hiring CPL's as SIC's on turboprop and some regional jets. I know that American Eagle did that for a while -- they used to call them the 500 hour wonders. Some were hired with even less time if they showed exceptional skills.

However, to log PIC time in an aircraft that requires an ATP ticket, you have to have the ticket. Sole manipulator of the controls is not enough for a CPL to log the time as PIC on those aircraft.


Has the Boeing 737 MAX MCAS system successfully prevented a stall or approach to stall condition outside testing? The Boeing 737 MAX MCAS system has been in the press a lot as it has been implicated as a possible cause of 2 fatal crashes. The MCAS system was put in to prevent a wing stall caused by excessive Angle of Attack. My question is, outside of testing, has the system ever operated as intended and prevented a stall or approach to a stall?

We don't know - until we hear about such a case!

A positive answer would require

a trace of MCAS action
sufficient evidence that the pilot could not have handled the (near) stall without MCAS.

This information could come from

a pilot who

notices the trim interference, and
perceives the situation as dangerous, and
reports the event

or

a flight data recorder that is analyzed, and

corrective MCAS action is found, and
it is evident from the data the pilot(s) did not / could not control the situation.

[ Am I missing any other way how MCAS could leave a trace? ]

A well-trained vigilant pilot can handle (near) stall situations in most cases.

MCAS is designed not only to get out of a stall, but to prevent the airplane from stalling in the first place. That means that
 the plane should never get into a critical stall situation, if MCAS does its preventive work.

All that just to say, it would be possible to answer "yes" as soon as there is knowledge of one averted incident, but it is impossible to conclude "no" from the absence of information.

I am very well aware that I wrote this theoretical answer while sitting in an arm chair, and I will happily remove it as soon as anybody with hands-on knowledge can provide more satisfying information.

MCAS is designed only for extreme abnormal flight attitudes, normal activation would be rare. Activation of this sort is also likely to show on data logs and it seems likely that it may even set some an alert flag for the operation even if the pilot fails to report such an abnormal flight condition. Engines have been remotely monitored while in flight for many years(basic temp, pressure, and RPM), with some lag as it must traverse several layers of electronics and sat links.


Which direction of a forward slip is safer during a turn to final? Suppose I'm making a turn from a left base to final and, realizing I need to kill quite a bit of excess altitude, decide to use a forward slip with full rudder deflection (along with full flaps).

Which direction of rudder deflection would be preferable?

I suppose that yawing left would result in less bank and therefore a lower load factor, which may be safe at low speed and low altitude. But yawing right, will result in a steeper bank which (under the same IAS) will result in an even higher rate of descent thereby getting me to the ideal glide slope earlier and leaving more time to fully stabilize before the threshold.

I use slipping turns all the time.

If you want to do a slipping turn you yaw the nose OUT of the turn, being careful to keep G loads low. It's NOT a dangerous maneuver if you keep speed up and nose down while doing it although you should use a bit more aggressive nose down than in a straight side slip to account for the slight extra G from the turn.  If you keep the wings largely unloaded and let the airplane fall semi-ballistically, you aren't going to stall.

To do it safely however, you do have to know how to fly the plane.

"Supposing I'm making a turn from left base to final, and realizing I need to kill quite a bit of excess altitude" 4th Edition

Pushing the rudder into the turn is not an option due to stall/spin risk.

An aileron slip while turning is possible.  To execute the maneuver, one would carefully increase roll to the left (left pattern) without increasing load on elevator, resulting in an increased rate of descent. The inside aileron goes up more, the outside down.  Your approach speed is kept at a safe margin over stall.

What could go wrong??  If you practiced a simulated approach well above AGL, you would know!

The slip maneuver is safer for aircraft with low wing loading and ample tail volume as the tail will naturally lower the nose with increased VV and the wings will help reduce VV when you roll out.  With practice, you do these all day long in a glider.

If you are flying a "brick", VV can get out of hand and high loaded wings will be much more prone to stalling if care is not taken to monitor VV, limit slip angle, and unload or even pitch down elevator while rolling out.

In all cases, the "round out" should start at a higher altitude due to the higher angle of descent (power on low angle landing can be "greased in" with hardly any round out at all).

Try it first at altitude.  Practice makes perfect,


Is is possible to land a midsize commercial plane on an Ford Class aircraft carrier? I'm story-boarding a novel. I'm toying with the idea of contriving a situation in which a midsize commercial aircraft is forced to make an emergency landing on an aircraft carrier. I read here that the largest plane to land on an aircraft carrier is a C130 Hercules, which it did as a series of tests. Its maximum weight when performing these tests was 121,000 lbs. At this weight, it required only 460 feet to land. And it did so without use of a tailhook, or any other arresting mechanism. The brand new Gerald R. Ford class carrier is 1,106 feet long. The landing strip on the carrier is somewhat shorter. I couldn't find numbers on this, but from photos, I would estimate that the landing strip is a little more than 800 feet long. Which leads me to my first question:

In an emergency situation, could an aircraft use the full length of the aircraft carrier's deck to land?

Assuming the answer to the above is yes, this means that our commercial aircraft has somewhat more than twice the distance to make its landing than the C130 did. Let us assume that the commercial aircraft is a B737 Classic, weighing aobut 140,000 pounds, just a little more than the C130 did during the tests. This leads me to my main question:

Could a 737 Classic safely make an unarrested emergency landing on a Gerald R. Ford Class aircraft carrier?

By safely, I mean without any passenger death. I expect that any such landing attempt would result in significant damage to the aircraft and the carrier. On one hand, the numbers seem to suggest that if a C130 can do it, so could a B737. On the other hand, I would expect that the C130, as a military aircraft, is more robust than a commercial aircraft, and perhaps able to better meet the conditions required to land on a carrier.

Also, I realize that in an emergency situation it may be safer for the aircraft to make a water landing near the carrier, and then for the passengers to evacuate onto inflatable rafts, finally to be brought aboard shortly thereafter. I will have to invent a situation which requires the plane to land on the carrier.

A Classic 737's maximum landing weight is about 114,000# for a -300, less for a -500, more for a -400.  But that is max. Down to "almost empty" gas tanks, knock another 8 or 10,000# off of that. Less than full pax/cargo load, several more thousand pounds off.
With a stiff breeze, say 20 knots, plus the carrier at max speed, you have maybe 50 knots of headwind. With that sort of headwind, can you stop a mid- to light-weight 737 in 1000' using full flaps, max reverse, and max brakes?
I don't have the charts with me, but I suspect you totally could, and in fact it wouldn't even be all that close. No margin for error if equipment fails, but if everything works right, those jets can stop quite well. And 50 knots on the beak is a LOT of headwind... "groundspeed" over the deck would be like 60 or 70 knots, and you can lose that in a lot less than 1000'.
I've flown C-130's and 737's, and the rate of descent on an assault landing in the Herk is nothing the 737 couldn't handle.
So... carrier landing... good to go?
Not so sure.  Compared to carrier aircraft, the 737 wingspan in WIDE -- even for the "little" -500.  That limits where you can plan your rollout so that you don't have an unacceptably high risk of clipping the island. Without playing with scale drawings of a 737 and the Ford's deck, I couldn't even guess if or how the geometry would or wouldn't work out.
Alternate plan that is probably no more risky for the passengers and might be a LOT safer for the carrier: ditch your (nearly out of fuel) jet close to the carrier, and have the carrier's helos and the small boats of the battle group pull the survivors off the wings or out of the water. With calm water, you get a "Sully" scenario (except with wings filled mostly with air, rather than jet fuel... Better buoyancy). In rough water, the risks go up. Way up.
But in that sort of weather, the deck landing gets lots more risky as well.
I suspect the deck landing would neither be allowed nor requested, but not for reasons of stopping performance.

The simple answer is no. Although the C-130 was successful in the carrier landing, the C-130 is an extreme STOL aircraft (Short Takeoff and Landing) design. Meaning it is designed to land in an extremely short distance. We currently do not have any commercial airliners that match this performance parameters. The question of using an arresting array of cables as an emergency procedure would not be a great idea as the commercial aircraft would probable be too heavy and not properly reinforced in critical areas for a successful deceleration.


How do paper airplanes create lift if their wings are flat? Paper airplanes' wings are completely flat, unlike the droplet shaped curvature of a full sized wing. How does a flat wing generate any lift if both sides have the same air pressure?

Paper airplanes create lift just like any other airplane. Don't let the wing cross section confuse you; what the air "sees" is a different shape.

When a flat plate flows through air at a positive angle of attack, the stagnation point of the flow (where air splits into an upper and a lower flow path) sits slightly below the forward edge of the wing. Now the streamline just above the one which hits the stagnation point has to negotiate the sharp corner of the leading edge, which will cause it to separate from the surface. This creates a small separation bubble which to the streamlines further up looks like a round leading edge, and since the rest of the wing has no curvature, the flow will attach again soon. The bigger the angle of attack gets, the farther back the stagnation point will be and the bigger the separation bubble becomes. At some point, which is promoted by leading edge sweep, the separated flow will not reattach, but produce a vortex, like the vortex which provides lift on delta wings.

The picture above is from a simulation; see the full video here. Note the red color on the forward part of the upper side: This indicates low local pressure, which is needed to make the outer air negotiate the leading edge plus the separation bubble (see here for an explanation on the molecular level). In the separation bubble, low pressure is not accompanied by high flow speed since the separation turbulence has eaten up most of the kinetic energy.

The actual nose radius of the paper wing does not matter, and it might even be zero (like when you fit razor blades to the leading edge): The air will create its own airfoil shape by padding with separated air what cannot be reached with attached flow.

Paper airplanes have a low enough wing loading that their angle of attack is low and the separation does not become too big. All it needs is a center of gravity at the quarter chord of the wing; that is why you need to fold the piece of paper first.

It is interesting to watch a slightly tail-heavy, delta-shaped paper airplane fly: It will gradually reduce speed and will start to rock its wings when it approaches stall. Once one wing is going down, the sudden increase in its local AoA will increase lift so much that the rolling motion reverses, without noticeable yawing. The delta-shaped flat plate has effective roll damping right into the stall. The lack of yaw is mostly due to the high yaw inertia, but indicates also that drag buildup is moderate at best.

Of course, after a few periods of rocking the nose will drop and it will pick up speed like any well-behaved airplane.

It is true that a flat wing can produce lift, but a paper airplane's wings are not flat on top. They have a folded piece on top that bows slightly up. Air has to flow a longer distance over that slight curve as compared to the bottom of the wing, just like an airplane. The faster air causes a pressure drop relative to the bottom of the wing, thus lift.


How much runway does an amphibious Cessna Caravan require for VFR part 135 operations? I am trying to determine the length required for us to build a simple paved runway for a Cessna Grand Caravan EX Amphibian to depart for water landings and and return (VFR/VMC only, Part 135).

Previous questions about for "runway length required" only refer to Part 135 and Part 91 for Large Transport Category.

The minimum required runway length for a Cessna Caravan is *762m/2500ft. Considering that the Caravan has a stall speed of 60 knots, and for a safety margin, you would approach at 70 knots, this makes sense. Assuming that it slows at a rate of around 1 knot/ 10m, You can do some basic calculations: 70/1, which is 70, resulting in a 700m long runway, plus 62m for a safety margin.

EDIT: But now we're talking about amphibious, so water could help the plane slow down, but at the same time a longer takeoff roll would be required. And it turned out that the website I used wasn't reliable. Sorry about that. Anyways, it is said that an amphibious Cessna Caravan needs a takeoff roll of 2,341 feet/714m. However, the actual requirements force a 3660ft/1116m runway. I didn't get the landing distance required, but I think it's safe to say that the landing would be shorter than the takeoff roll.

*the actual runway length required of a normal caravan is 2,055 feet/ 626m, and you can expect to rotate at 1,160 ft/ 354m.

*Some (incorrect) information was from: http://www.sgl.com/208.html
And more correct info from: https://cessna.txtav.com/en/turboprop/caravan

Please tell me if I should delete the first paragraph.


What happens to the spinning wheels of the landing gear after takeoff, before retraction? What happens to the spinning landing gear right after V2 on takeoff? Are they automatically or manually braked to a halt before gear retraction? Or are they allowed to spin to a halt in the wheel wells?

I am asking this question in the context of common passenger planes, such as the Boeing 737-800 or the Airbus A320.

In most common passenger aircraft brakes are automatically applied when the gear is retracted. The wheels stop spinning before they enter the wheel well. This is particularly important because a spinning damaged tire may cause damage to hydraulic and fuel lines which are usually routed near the wheel bay.

The nose wheels have no brakes, so they are spinning while they are being retracted. Most aircraft have rubber brake pads (snubbers) fitted in the nose gear bay that stop the spinning of the gear once the wheels are retracted.

Actually, the main reason brakes are applied to stop spinning wheels is the centripetal and centrifugal forces. These forces are a reaction to the spinning of wheels. As the gear is retracted, the forces that would be generated would cause uncontrollable vibrations throughout the aircraft. The result would be catastrophic. Want an example of how this works, find a fidget spinner. While it is spinning, rotate the spinner on an axis. You will feel the spinner "wobble". Now imagine the landing gear and the speed they are rotating at.


Has it happened before the EASA and the FAA disagreed on the airworthiness of a plane? With the Continued Airworthiness Notification and with the updated statement https://pbs.twimg.com/media/D1fZ4GTW0Ac4Qsr.jpg in contrast with EASA   2019-0051-E it is crystal clear the EASA and the FAA disagrees on the airworthiness of the 737 MAX at this time. Had this happened before?

It seems FAA and EASA did disagree since 2016 about 737max certification, and not just after the crashes. EASA certification was conditioned by training and more information to the pilots, which hasn’t been done.

Please refer to the following website:

https://www.cnbc.com/2019/03/29/reuters-america-insight-regulators-knew-before-crashes-that-737-max-trim-control-was-confusing-in-some-conditions--document.html

You read the following:

  The European Aviation and Space Agency (EASA) certified the plane as safe in part because it said additional procedures and training would “clearly explain” to pilots the “unusual” situations in which they would need to manipulate a rarely used manual wheel to control, or “trim,” the plane’s angle.

You may also read about EASA:

  The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said:

  It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s centre console rather than an electric thumb switch on the control yoke.

So when agencies give their approval it doesn’t mean the approval conditions are the same, it was not the case since the origine  for the 737max, and not just after the crashes.

Yes with respect to the CONCORD supersonic which had different limitations due to the produced supersonic subsonic transitional external sound.

As you say recently with respect to the 737 max, since it has been forbidden flight in Europe irrespective of FAA decision which came well later on.


What is this line found above the door on many aircraft? Many aircraft have this line above the door as seen in the photos below. What is it, and what is its purpose?
Source

Source

These are rain gutters.

They are designed to catch rain that runs off the upper surface of the aircraft fuselage and channel it away from the open aircraft door so that the water does not enter the cabin.

This is a close up of the rain gutter over the main cabin door of a Beechcraft King Air B200, showing the channel that would catch and redirect water flow to the side of the door opening:

Source: own work

A jet-bridge may partially deflect rainwater in some cases, but the gutter helps mitigate water exposure for the aircraft interior and passengers. Even gutters cannot completely eliminate such exposure since their size is necessarily limited by aerodynamic constraints. Many operations do not have the luxury of jet-bridges, making the rain gutter especially helpful in reducing the amount of water that might otherwise enter the cabin and pose a hazard to the aircraft or onboard systems.

"Rain Gutters" is the technical name, at least for Boeing aircraft, such as the B777 depicted in the photos in the question.

Here is a Boeing reference to such gutters on the 787:

  Rain gutter: When customers found that water was not being properly deflected
  over a passenger entry door, they requested a change. Boeing relocated the
  gutter to function more efficiently. The solution means happy customers, who had
  fewer water and maintenance issues, and happier and drier passengers. This
  was a significant enough change to process to require certification work with the FAA.

It's used so when they are attaching the air bridge, it helps the top of it to align.


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

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

  4-8-6. CIRCLING APPROACH

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

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

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

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

If approaching an uncontrolled field at altitude a pilot can over fly the airport in order to determine wind direction and traffic in the pattern. The pilot can then begin circling while descending to pattern altitude and enter the pattern merging with other traffic. General aviation airports are not usually located where there are obstacles that have to be avoided during the approach unless there are mountains nearby that prevent a straight in final approach.


Does weight and balance of an airplane matter more on takeoff? On a few international flights with non-full cabin, the crew has asked no one to move to open seats until the plane has finished take-off. I assumed this was just to force people to sit down during take-off and not cause problems, but when I asked I was told it was because of weight. The claim is, the airplane is balanced by where people are sitting. The next claim is by the end of the flight, there is less fuel, so if people have switched seats there is less of a problem. This weight balance apparently uses all information, people, baggage, and fuel.

I believed this, until recently a plane had lost at least 30 peoples baggage, some people had more than 1 bag. The bags were left at the previous airport, and on arrival no one knew that the bags were not on the plane.

If the lack of knowledge for bags not being on the plane (which seems logical, why would they depart without the bags), how did this change in weight not show in the take-off, or was this a dangerous occurrence?

By regulation, a "weight & balance" must be computed before departure to ensure that the weight is within the weight limits for a number of different conditions and that the weight is distributed such that the c.g. (center of gravity) of the airplane is also within the longitudinal c.g. limits for those conditions. That c.g. is typically computed assuming the the passengers are in the seats they have been assigned. If the passengers are seated elsewhere, the computed c.g. is not accurate. In other words, the pilots wouldn't know where the c.g. is or whether it is even within limits.

The takeoff trim setting is affected by the c.g. If the pilots set the trim according to the computed value, and the c.g. is considerably aft of that, on liftoff the nose is going to want to pitch up more than the pilots expect. If the c.g. is so far aft that it is well beyond the aft limits, serious control problems can occur. If the c.g. is far forward of what the pilots are expecting, they're going to have to pull harder on the elevator control than expected to get the aircraft to rotate, and that is going to extend the takeoff roll.

Different makes of aircraft vary widely in c.g. limitations and how tolerant they are limit wise of passenger placement in aircraft that are not full. For example, when I was flying Metroliners, if you had few passengers, you had to ask them to sit toward the rear. I once got a strange look from Rosey Grier (Google him if you don't know who he is) when I had to ask him to sit in the back of the aircraft.

Once you're in the air, the movement of passengers is of lesser concern balance wise. You're at speed, the elevator is fully effective, and the pilots or the autopilot keep the airplane in trim. In other words, the trim is what it is rather than a computed value that might not reflect reality.

Also, on large aircraft, once you're in the air in level flight and if you're so inclined, you can work out your weight by using your fuel flow, pressure altitude, and airspeed. Then with the weight and your trim setting you can come up with your c.g. I would guess that most of the glass cockpit aircraft probably do that for you automatically if you request it. My experience stopped with the 747-200, which is now a near relic.

The c.g. limits are defined conservatively, and usually in terms of the percent of the mean aerodynamic chord. It's not a case of that if the aft c.g. limit is 33% mac (a common value for 747s) and you're at 33.1% you're going to crash. It's just a case that you're beginning to move into an area that gives you a lesser degree of controllability than is deemed necessary for some situations. Continued movement to an extreme value would eventually make the aircraft uncontrollable.

Insofar as the bags-left-behind part of the question, I don't think their absence would cause a problem. If the computed c.g. including the bags was right right up against the aft c.g. limit, their absence could conceivably put the c.g. beyond the aft limit. The pilots might notice it, but handling it would not be difficult. However, aircraft are typically not loaded such that their c.g. is close to a limit. A preferred aiming point for 747-100s, -200s, and -400s is 26.6% mac.

Just for the heck of it, I did a weight & balance for a 747-400, loading it so that the takeoff c.g. was 26.6% mac. I then took 5000 lbs (100 bags x 50 lbs each) out of the most forward baggage area. This changed the the c.g. to 28.0% mac, still within limits. On the 747, a given weight of baggage misplacement has less potential to affect the c.g. than the same weight of passenger misplacement because the passenger seating extends over 20 feet forward of the most forward baggage compartment and over 17 feet aft of the most aft baggage compartment.

The airplane must have enough power and "lift" so that it can clear any obstacles at the end of the runway.   The more weight at takeoff, the more lift is required.   The "lift" of an aircraft is dependent on temperature, barometric pressure, and humidity.  These things affect the density of the air.  And, the lower air density produces less lift, so the total weight of the aircraft must be reduced to insure a safe takeoff.


What happens when lightning strikes an airplane? What happens to an airplane when hit by lightning?

Since the aircraft is not grounded, the physics are the same as with a car that gets struck by lightning. The aircraft's skin will be charged, but very little current will be induced in internal parts in the process. The outer skin will act as a Faraday cage and protect the inner parts from the electric field.

This is greatly helped by a metal structure. Composite aircraft parts need a copper mesh over all external surfaces to give them sufficient electrical conductivity. The terminals of that mesh must be connected to the metal parts of the structure to make sure that all parts will have the same electrical potential in case of lightning strike.

However, the ionized air caused by lightning will leave burn marks on the surface, and an inspection of the affected part and the electrical equipment is advisable.


Why are prop blades not shaped like household fan blades? My two projects right now are learning to fly and 3D modeling a replacement blade for antique fan, and a question occurred to me:

Why are airplane propeller blades not shaped like household fan blades?

They're both designed to push the same fluid with some degree of efficiency, but household fan blades look a lot more like ship screws/boat propeller blades. Is it a weight issue? A visibility issue? A drag issue, and if so, how?

EDIT - This is not a duplicate of this question because the proposed answers a question about angle of attack and pitch, which are aviation jargon words that may mean nothing to novices. Pitch and angle of attack also have little to do with the front-view shape of a household fan or prop blade, which is likely the most obvious difference between the two types of blades in the eyes of someone without an aeronautical engineering degree.

1) Airspeed, 2) Forward motion, 3) Size constraints. Just to begin with.

Household fan blades are extremely slow, so they need more chord to push a meaningful amount of air. Aircraft propellers approach the speed of sound at their tips, and low drag is critical. All things equal, more span and less chord is more efficient. Reducing the airspeed for props has diminishing returns, because aircraft themselves move forward through the air, so a very large propeller adds drag to the aircraft however slow it is.

In household fans, size constraints are more critical than efficiency. A high-span, low-chord (narrow) blade would be more efficient in a household fan as well - indeed, you can find household fans with such blades. But they have to either spin faster for the same amount of airflow, which adds noise, or be larger in diameter. Ceiling fans, which can be larger, have longer narrower blades than desk or floor-standing fans.

Ship propellers move in an extremely dense and viscous medium, which changes things even more. The drag of moving through water is extremely high and proportional to V³ power-wise. The thrust force they produce is at most proportional to V², and can be lost to cavitation at high velocity. So their velocity has to be kept as low as possible. The drag cost of adding more chord is also relatively small in water.

On large ships, propellers are already as large as they can be made without sticking out of the water at low draft (on merchant vessels) or reducing the number of shafts that can fit (on combatants). This allows them to spin slower and lose less power to drag.

[If you can not explain why wind turbines should be made with aerofoils, please have the sense or decency to not edit this answer.]

A fan blade pushes air.

An aerofoil is a special device.  It passes through the air, leaving a partial vacuum behind it.  The trailing part is designed so that this vacuum, instead of pulling the thing backwards, pulls it perpendicularly.  It is an ingenious device.  It does not push air.  If they wanted it to push air, they would have made it like a fan blade.  It is not like a fan blade.  It does not push air.  It is a cleverly designed device.

Actually, what happens is, not that the vacuum pulls, but that the air pressure pushes; the difference is that things [otherwise] almost always have equal air pressure on all sides.

Again… .  A wing is not lifted by laminar flow.  A wing is not lifted by vacuum.  A wing is pushed up, from below, by air pressure.  This works because of the ingenious design of a wing, that makes it so that, as it passes through the air, a (partial) vacuum is formed… and this partial vacuum functions to lift the wing, rather than dragging it backwards, because of the clever and special design of an aerofoil.

The reason that a propellor-driven aeroplane can not do more than about 400km/hr is that the air pressure can not push it any faster.

The reason that a wing at a steep angle of attack works less well is not about laminar flow, nor turbulence; it is about the fact that (a) the horizontal area being pushed upwards is smaller and (b) the surface being pushed is on an angle, which compromises the pushing upward concept.  Again, no amount of managing laminar flow and turbulence will change the above two facts.  (Note also that a wing at a steep angle of attack has (more) drag backwards (so to speak).)

Note that there is a systematic problem with propellors, being that they are passing through the air such that the clever vacuum side is moving towards {air at normal pressure}.  (I don’t know if that is a freebie or not, but it does not make much difference.)

If you understand aerofoils now, and no one ever explained this properly before, please leave a tick or a positive comment or something.  It would be a pleasant change from people looking at me funny or getting angry, and I would really appreciate it.


How would an Inverted Delta Wing planform behave, Straight part forward? Are there wings shaped this way? There is info about straight edge wings, swept back wings, swept forward wings, delta wings.
Is there also info about a Delta wing with a straight leading edge and a tapered trailing edge, the point rearwards, like a manta ray?

To answer your second question first: Yes, these are mainly used for ground-effect craft. The idea is to have positive angle of attack, a tapered wing and a trailing edge with constant, low distance from the ground. This question covers their sweep angle.

The RFB X-114 ground-effect craft. Forward is right.

The wing would behave just like an ordinary, forward swept, low aspect ratio wing. It would have a small lift curve slope and a lousy stall behavior, because the low tip chord will make the tips stall first, causing the whole wing to roll. Unfortunately, the low leading edge sweep will prevent vortex lift, which is a typical feature of regular delta wings, to develop, so the stall angle will be only little higher than that of a straight wing with the same aspect ratio.

Manta rays are more diamond-shaped, so their "wing" has positive leading edge sweep and almost zero quarter-chord sweep. Their stall behavior would also be nasty if they were rigid, but the wings are actually fins and used for propulsion, not lift, so stall does not mean that the fish plunges to the ground. The picture above shows an Atlantic manta and was shamelessly copied from Wikipedia.

The reverse delta wing configuration, which Dr. A. Lippisch proved to be very stable in ground effect, creates a very steady position of the Cm (pitching moment) when the wing is close to a surface.


Do aerodynamic forces depend on the materials of the wing surfaces? I think that statement is true, but in the book "Fundamental of aerodynamic", page 35: the aerodynamic forces R don't depend on the skin material, 5 variables are mentioned but none of them is relate to skin material of wings. So why is that???

Surface roughness has a minor impact, except in a range of Reynolds numbers mostly encountered in General Aviation. Only when you use an airfoil that has been designed for laminar flow but experiences an early transition will the performance be impacted significantly.

The book you quote assumes a certain minimum standard of build quality: Flush riveting, smooth surface and a solid, stiff structure. Once you move to more advanced topics, more variables will need to be added, such as elastic deformation in flight or substandard surface quality. At the introduction level some simplifications are justified. The lessons derived from this reasoning are valid and valuable. When you progress, they will be expanded with more complex concepts. Trust me.

Generally, skin materials will impact the local deformation of the surface of the airfoil (think parachute).  They also can impact the local boundary layer, with things like surface texture (think Reynolds number changes). Generally speaking these effects are tertiary, and not a prime mover in R.

If you are looking for something more comprehensive, then I will have to break open the book.

Addendum #1
Understood the question is redefined. In this case, I agree with @mins and his commented reference to the Jones and Williams paper. You can find it readily on the net.  It addresses the surface texture and corresponding Reynolds number impact.  There are some recent works which take that further and model it nicely, but that is beyond the scope of the question.

In general, there are two regions of Reynolds, laminar and turbulent, and how they impact drag is different.  Perhaps the question should be reformulated.


Why is this equation for TAS only valid for subsonic flow? I came across the equation for calculating True airspeed (TAS) on Wikipedia:
$$ \text{TAS} = a_0 \sqrt{\frac{5 T}{T_0} \left[ \left( \frac{q_c}{P} + 1 \right)^{2 / 7} - 1 \right]} $$
But this equation is apparently valid for only subsonic flows.
Why is that? Does this have something to do with pressure?

Because you didn't read the text in the Wikipedia page above that equation ;-)
That equation works for subsonic flow only because you have replaced the Mach number of the original TAS formula:
$$ \text{TAS} = a_0 M \sqrt{\frac{T}{T_0}} $$
with the formula of Mach number for subsonic flow:
$$ M = \sqrt{\frac{2}{\gamma - 1} \left[ \left( \frac{q_c}{p} + 1 \right)^\frac{\gamma - 1}{\gamma} - 1 \right]} $$
which exploit the simple form of Bernoulli's formula, working only for incompressible flows, i.e., subsonic flows. Check the Mach number and the Bernoulli formula pages for further details.

Probably because Supersonic flow is totally different than subsonic flow. Subsonic, the movement of one molecule, (just from it's kinetic motion due to temperature), is faster than the flow velocity, so the effects are felt by other molecules downstream before the flow reaches them. This is in fact the speed of oscillations in the fluid, i.e., the speed of sound. The molecules in front of the flow therefore, move away from the ones pushing them before the flow reaches them. The flow is incompressible.
In supersonic flow, otoh, the flow is faster than the speed of the molecules, so the molecules don't feel the effects of the flow until the flow itself reaches it. The flow is, therefore, compressible, since the molecules "bunch-up" in front of the flow. This bunching up is in fact the shock wave created by the Supersonic flow.


Can a passenger jet be towed to safety if it breaks down in midair? A car can be towed by a tow truck to the repair shop when it breaks down on the highway. Is it possible for a passenger plane to be towed by an emergency tow plane along the air corridor to the next nearest airport assuming it cannot reach there on its own power (example if it runs out of fuel or both pilots are incapacitated)?

Update:
This question actually consist of two parts. Whether it is feasible and whether this is practical. I have seen a small tugboat towing a barge; a man pulling a firetruck. Before writing off this idea completely, can someone address what's stopping a fighter plane from towing a passenger jet if a proper point of harness is installed?

Towing is quite feasible, and even landing while in tow is possible - I've done it myself in gliders. The hard part is catching up with the stricken plane.

To stay with Keegan's Boeing 777: It most likely will travel at Mach 0.8 and somewhere between 30.000 and 39.000 ft when disaster strikes. Let's further assume that power fails instantly, so the aircraft, traveling at 240 m/s, will develop a sink rate of 15 m/s just to continue flying. When it sinks, the increasing density allows it to slow down, so at sea level it will fly at 130 m/s, sinking with 8.125 m/s. For simplicity, assume a linearly decreasing sink rate, and the remaining time aloft will be less than 1000 sec or around a quarter of an hour.

Now the towplane has 15 min to scramble the crew, run up the engines, get take-off clearance and to actually fly an intercept to the stricken airliner. How realistic is that? How many airports are needed to have 777-sized towplanes ready, and how many otherwise unproductive 777s would this scheme need?

A car can stop and wait a couple of hours for the tow to reach it. An aircraft can not.

With enough engines and all the right hardware...sure, it's possible.  See this report on tow launching an F-106: http://www.nasa.gov/centers/dryden/history/pastprojects/Eclipse/index.html

The problem is that it's just not practical.  Nevermind the actual execution of a pickup maneuver.   As mentioned in previous answers the costs of a standby fleet required to execute reliable pickups are very high.  There would be zero utilization of such a fleet.

Your edit asks about a fighter jet serving as the tow plane.  Yeah, sure, it's technically possible to build a trap of some kind to be a tow rig on an airliner.  If afterburner is required to keep both planes aloft tow time would be extremely limited though.


How do pilots stay disciplined and use checklists, even after they have become routine? I originally asked the same question on Space Exploration but will repeat it also here:

Before every takeoff and every landing, there are several routine checks which need to be done in order to provide the highest maximum safety to passengers.

But after certain number of repetitions, these tasks can feel as tedious especially if you are flying a plane which goes in "turns"

(Real example of what I mean is commercial line between Prague (PRG) and Frankfurt (FRA) where plane lands, people go out, new people go in and plane goes back. And so on)

How do pilots and other personnel make sure that they actually do go through the check every single time without "It was OK last time, so it will be ok this time. Skip it and say it is ok" ?

You stay vigilant by having seen things go wrong.

Like Jamiec mentioned the importance of a proper preflight is drilled into you by your primary instructor from day one in light aircraft, and that mentality carries through all the way up to heavy transport-category aircraft: You want to find any problems you can while you're on the ground, because if you take a problem into the air with you it's a decision you can quickly come to regret.

Personally I've found all sorts of "interesting" things on preflight inspections of light aircraft, including (but not limited to):

Mud wasps building a nest in the pitot tube
Mud wasps building a nest in the fuel drain
(still don't know what was up with that)
Missing screws from a fairing
Flat-spot on a tire
A tennis ball stuffed in between two cylinders
(a common flight instructor trick to see if students are doing a thorough preflight)
Position light lenses installed backwards
(another common CFI trick, but this one was actually a maintenance shop screw-up)
Various light bulbs burned out
Water in the fuel after a heavy storm.

I've also heard some scary stories:

Migrating colony of bees hanging out on the tail of the aircraft
(described as "The tail was literally dripping bees")
Flight controls rigged backwards
Big gouge in the wingtip
(acquired by the previous pilot scraping the tip against another plane's tail taxiing in on the ramp)
The ever-classic "ran out of fuel because you didn't visually check the tanks"

On commercial flights, key checklist items are forced by using a procedure call a "cross check". The one pilot must "challenge" another for those specific check list items. For non-cross checked items, pilots have discretion to implement checklists as they see fit.

Each pilot has their own practice. Some are sloppy and do nothing. Others fastidiously follow every item. Most commercial pilots tend to blow off most checklist items, especially anything outside the cockpit. For example, I fly American a lot and when the flight crew arrives I always watch to see if they do a walkaround and I have NEVER seen them do it. I complained about this to one of the top flight instructors at American and he said, "Well, they are supposed to do the walkaround." Yeah, they are SUPPOSED to, but do not do it.

It comes down to the personal discipline and attitudes of the pilot involved and how pressured they are to get the plane up in the air and on schedule.

On non-commercial flights and cargo flights often crews will become very lax. For example, in 2014 a Gulfstream crashed at Hanscom airfield killing everyone on board because they tried to takeoff with a gust lock engaged. This is impossible if you do even a minimal checklist because a control check is like the #1 or #2 thing on any checklist. A review of the tape showed that the crew in that case did ABSOLUTELY NOTHING verbal for cross checking. Furthermore, a review of the flight recorder for the aircraft showed that the crew had NEVER done a control check in 98% of the flights in that aircraft. Such laxity is not unusual in non-commercial flights.


What is the advantage of the blisk design in the new Trent XWB engine? What advantage does the "blisk" or "bladed disk" type rotor have over the usual rotor system?

Since the blades and rotor disc are a single piece, there are no additional fittings (bolts, screws etc) to attach the blades reducing complexity and weight.  The blade root joints are also a source of failure through manufacturing defects and fatigue cracks.

The disadvantage is that any damage to a blade requires the entire assembly to be replaced rather than a single blade.

The above answers give very good technical reasons for blisks.  Less parts means less rotating weight which has a high impact on overall weight.

However, the technical benefits are not the only considerations.  As engines age eventually they are worth more in parts than as an engine and the parts are salvaged and sold.  The compressor blades can bring in a tidy sum.  However some will have erosion and other nicks that will not meet the manual limits for returning to service.  So not 100% of the parts removed from a compressor can be salvaged and sold.  There are several thousand blades in a modern high pressure compressor.  This salvage market is competition to the aero engine manufacturers.  The manufacturers would rather you buy a new part from them than a used part from someone else.

Aircraft engines follow the same economic model as razor blades.  The initial purchase may be sold at a loss but the replacement parts will be profit drivers.

A salvaged Blisk will have airfoils with erosion and or nicks and dents on the airfoils (this can happen after only a few hours of operation)  So the blisk assembly will not be able to be inspected serviceable.  The engine operator will need to send the part to the Engine Manufacturer for a proprietary repair.  This gives the engine OEM a better control on the used material on the market.

In conclusion, less weight is always good but the blisk also provides an opportunity for the engine OEMs to ensure that they sell parts and make a return on their initial investment.


How do I find the floor of controlled airspace on sectional charts? So, I understand finding floors and such when seeing something a fraction like 45/10 or 39/SFC, but I've been reviewing and gotten a couple of questions that look like this:

What is he floor of the controlled airspace overlying the Coeur D'Alene Airport?

The answer is "At the surface" but I can't figure out where it says that?
The second question said this:

What is the floor of the controlled airspace overlying the Sandpoint Airport?

The answer is "700 feet MSL". Cant find that either.

It does not say it anywhere because the answer is not variable like class B airspace but lets dig in.

The question here is really trying to get you to identify the airspace that surround Coeur D'Alene Airport and we can see the dashed magenta line encircling it. If you take a look at the legend you will see that denotes

  Class E (sfc) Airspace.

The same goes for Sandpoint Airport which is surrounded by a faded magenta line. The legend again denotes

  Class E Airspace with a floor 700ft. above the surface

In the example figure 22:  Coeur D'Alene Airport is the thick Magenta lined airspace.
(thick Magenta according to legend = airspace with floor 700ft above surface.)
Brooks airport ( dashed magenta)  is "overlying" Coeur D'Alene Airport.  According to the legend dashed magenta = "Class E surface airspace"
what confuses people here is that you also have the VOA (hashed blue line circle/compass rose) overlying the class e airspaces.
the correct answer is "at the surface"  because dashed magenta airspace overlying Coer D'Alene is Class E surface airspace. Surface to 700 AGL


Why do we refer to "power" for turboprop engines and "thrust" for turbojet/fan engines? In jet engines, I read that thrust is related to the fuel flow rate, whereas in turboprop engines power produced is related to the fuel flow rate. What is the reason and brief math behind this?

Ultimately what you want from all three types of engines is quantification of thrust available to push an airplane through the sky. The turbofan/jet engines are self contained and produce thrust directly but a turboprop engine requires the addition of a propeller, which may have differing characteristics based on the installation.

Since thrust is not known for a turboprop until its installation has been determined, manufacturers instead quantify the power available to drive a prop. This allows engines to be compared so that an airframe manufacturer can make the proper selection.

Fuel flow is then related to either thrust or power as a measure of efficiency, depending on type of engine. There is generally no published mathematical relationship between power and thrust for a given engine. It is not needed for turbofans/jets and not possible to determine at time of manufacture for turboprops.

I thought even the engine design would be a bit different depending on your application.  For a pure turbojet, you want maximum thrust out the rear, so maximizing torque turning a shaft isn't the goal.    On the other side of the spectrum, going past the turbo fan to the turbo prop or even max torque application like a turbine generator, you want to minimize thrust and convert all expansion gas energy to torque.
That is why most turboprops use centrifugal compressors at at least one stage, and pure turbo jet engines use axial compressors.   The former converts most of the expanding gas energy to circular motion (we will call it torque) and the latter accelerates that expanding gas for maximum air/gas velocity out the back for thrust.
A turbocharger in an auto engine or the turbo charger in a piston aircraft engine use centrifugal compressors similarly because they want to generate circular motion (torque) on an air compressor to increase the amount of oxygen in the combustion chamber.
I'm not sure if this is correct, because I just thought of it, but I did some research to confirm that pure jets want thrust, and axial flow, and high gas velocity at exhaust, and turboprops want torque, to turn a shaft into a psru then a propeller, so these engines have different compressors that convert the energy into different types of motion.


E class airspace(transition area) Extension area(E airspace) shown as shaded magenta start from 700AGL but how to measure 700AGL over the water like in this picture?
Cuz there’s no land…

While the documentation uses AGL when talking about class E airspace the chart legend uses the slightly different phraseology of

  above surface level

This avoids the ambiguity of, ground or ocean.

Interestingly the glossary uses both ground level and sea level but explicitly defines neither.

Looks to me like the chart is showing controlled airspace as 100/60, 100/50, 100/40 -
which means the highest you can go is 4000, 5000, 6000 feet MSL and stay clear of the Class C, not AGL. So the height would be read off the airplanes altimeter.
Go to www.skyvector.com, enter HAF at the top left, then select San Francisco.  To the left of HAF is the explanation showing T/30, and 70/30 - in MSL.


What happens if a parachute does not open while skydiving? While skydiving we have to jump from the airplane at a certain altitude above Earth. So if for some reason the parachute doesn't open, then will that person (or any object similar to a human body) fall to Earth or will it be incinerated/vanish in the sky due to air-friction in space before coming down to the Earth?

The majority of the heat developed during reentry of a space-vehicle comes from air compression not friction. Remember, to go into space, you don't have to go very high, but to orbit you will have to go very very fast, and it's not the fall that produces the heat, it's the slowing down part.

If you had a human fall without a chute, the terminal velocity (where air resistance cancels gravity and you continue downward at a constant speed) would be around 100-200 mph, not nearly enough to cause any kind of heat (or cars would burn up by going normal cruising speeds). To get higher terminal velocities, you'd have to go a lot higher, where the air is a lot less dense, higher than any skydiver have, and as far as I can tell Felix Baumgartner did not experience any incineration during his jump.

The unfortunate skydiver will almost certainly die, but there'd definitely be a body, and it'll probably be in one piece.

Well, if you skydive in pairs, which as I understand is recommended, and if you catch the error before your partner opens his/her chute, you could signal to them that your equipment is not in working order, and then you could essentially bear hug them while they open their chute.

This is obviously not ideal and could even dislocate the shoulders of the skydiver whose chute did not open.