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I recently saw a movie in which what appeared to be a Lockheed C-130 Hercules was suddenly without occupants. They did not move or exit the craft, they simply vanished. I assume that the craft remained pressurised, and no modifications to controls were made. I assume that any auto-pilot-like features remained either on or off as they were at the time, unless they have some kind of system to automatically engage in certain situations. At the time of the incident, the aircraft appeared to be cruising.

I am wondering how long it would take for a cruising aircraft take to crash if everyone spontaneously vanished?

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    $\begingroup$ The death of Payne Stewart is a real life example similar to your scenario. en.wikipedia.org/wiki/Payne_Stewart#Death In that case, the aircraft continued flying until it ran out of fuel. $\endgroup$ – James Aug 19 '15 at 16:18
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    $\begingroup$ Other cases: the Cornfield Bomber (plane landed in a field with minimal damage after the occupant literally left the aircraft) and this crash of a TBM 900 last year (flew until it ran out of fuel.) $\endgroup$ – reirab Aug 19 '15 at 18:24
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    $\begingroup$ Alternate question: "How long would a cruising aircraft take to crash if everyone awake spontaneously vanished?" $\endgroup$ – Michael Aug 19 '15 at 19:34
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    $\begingroup$ @Michael Awake? As in a scenario in which one person who was asleep wakes up on an otherwise empty plane? Sounds like the plot for a mediocre movie starring Liam Neeson. $\endgroup$ – Lewis Goddard Aug 19 '15 at 20:05
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    $\begingroup$ @LewisGoddard Yes. This was actually a reference to the 1995 TV Mini-Series "The Langoliers" starring Dean Stockwell (edit: and Bronson Pinchot). (except there were more like 10 or so people that were sleeping) $\endgroup$ – Michael Aug 19 '15 at 20:09
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There is a far more important question (and the real deciding factor)

How much fuel is on board?

If left unattended a plane will fly until it runs out of gas quite literally, the more gas in the tanks the longer you have. This has happened on occasion before. The Helios Airways Flight 522 is about as close as you can get to this hypothetical situation in real life. The plane may travel farther (since its lighter) but it will go until its out of gas. The same thing applies to smaller GA planes as well.

This is all of course assuming that the plane is not intercepted before running out of gas. Helios 522 was intercepted by F-16's before it crashed and they visually established that the crew was incapacitated (although a flight attendant tried to save the plane). I don't know what the procedure is for interceptions like this but if the plane were headed for a highly populated area they may not let it run out of fuel.

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If the autopilot was on, it's a question of how much fuel is on board as related in another answer (unless high ground was in the way).

If the autopilot is off, even if in perfect trim, the aeroplane will usually gradually drop a wing (this process will start within 15-30 seconds) and develop an ever steepening spiral dive and crash at high speed into the ground.

Sometimes something different has happened, perhaps if a dead pilot has his hands on the controls, but in most cases there is insufficient stability for it to remain wings level for long without pilot input.

I'm thinking of an example in the States where an airliner's wing sliced through the cabin of a light aircraft and cut the three occupants into two. That aircraft managed to land itself intact in a school playing field I think it was, with blood covering the fuselage and wings, but no fuselage top and no visible occupants.

Also, in a small plane the trim might be affected by people disappearing (due to a change in CG) but I think the usual result would be to drop a wing as related.

I am not aware of any systems that would turn an autopilot on automatically if it was off. Finally, many light aircraft only have wing leveller autopilots, so in this case, even if it was out of trim on the elevators, it would take longer (perhaps much longer) to crash, it just might fly long enough to run out of fuel, assuming no mountains were in the way.

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    $\begingroup$ Another example of a light airplane (albeit a military one) in which the occupant actually did vanish (he ejected after the aircraft entered a spin) is the so-called cornfield bomber. After the pilot ejected, the aircraft recovered from the spin on its own and landed wings-level with minimal damage in a field. It was repaired and returned to service. And that was a fighter, which typically has much less aerodynamic stability than non-aerobatic civilian aircraft. $\endgroup$ – reirab Aug 19 '15 at 18:17
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    $\begingroup$ @reirab: I don't know about the details, but I would bet a considerable amount that on the day of the "Cornfield Bomber" event the weather was calm and no gusts were shaking the airplane. $\endgroup$ – Peter Kämpf Aug 19 '15 at 18:37
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    $\begingroup$ Many thanks for the grammer correction edits $\endgroup$ – Philip Johnson Aug 19 '15 at 20:25
  • $\begingroup$ I would just like to add that in the case of an aircraft entering a spiral dive there is no guarantee it will reach the ground in one piece. It may break up in the air, due to either excessive G forces, or a structural failure caused by exceeding VNE (never exceed speed). This depends on the aircraft type but would not be helped by going downhill with cruise power set. $\endgroup$ – Philip Johnson Aug 19 '15 at 21:23
  • $\begingroup$ Your decapitation accident was the 1986 Cerritos incident but the Piper did not land, it fell. The NTSB report says "‘The Piper PA-28 was destroyed by the collision and ground impact ... The pilot and two passengers in the Piper were found in the remains of the airplane’s cabin; they were strapped in the left front seat, the right front seat, and the right rear seat. All three occupants had been decapitated". See www3.gendisasters.com/files/files/newphotos2/… (wreck, not occupants) $\endgroup$ – RedGrittyBrick Aug 20 '15 at 13:49
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I remember reading about another one--GA aircraft, a drug runner, the pilot jumped once he was intercepted. The plane continued on until it ran out of fuel. Someone involved (it's been too long since I read about it to remember who) was trying to get permission to shoot it down as it was heading for a city. It was not shot down, it flew over the city and went down at sea when the fuel ran out.

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