In this comment it's mentioned that pilots in instrument conditions may wind up in a situation where they're inverted due to spatial disorientation, and be completely unaware of it until they fly out of the cloud.

Is that really something that can happen so easily?

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    $\begingroup$ A untrained pilot in unexpected IMC won't survive for 3 minutes. $\endgroup$ – ratchet freak Jan 30 '15 at 19:27
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    $\begingroup$ One of the flight instructors I took lessons from told me this actually happened to him (he took off into clouds and his artificial horizon died as soon as he was in the soup)...he popped out a few hundred feet later "at a crazy angle"... $\endgroup$ – Tim Jan 30 '15 at 21:01
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    $\begingroup$ huh, why does the AI always fail the second you go into the soup? $\endgroup$ – rbp Jan 31 '15 at 0:02
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    $\begingroup$ Isn't this exactly what happened to JFK Jr.? As shown in this week's Air Crash Investigation/Mayday. $\endgroup$ – Danny Beckett Jan 31 '15 at 0:45
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    $\begingroup$ @DannyBeckett: According to wikipedia, yes it was spatial disorientation. $\endgroup$ – mins Jan 31 '15 at 0:50

The short answer is "Yes", if the pilot is not properly trained, doesn't trust their instruments, or has an instrumentation failure.

Without a visual reference (such as the real horizon or artificial horizon) it's very easy for a pilot to place their aircraft in an "unusual attitude" -- something other than the expected straight-and-level flight or commanded turn.
As humans we naturally rely on our inner ear and sense of balance to tell us what's "up" or "down", but in a moving aircraft you can move through a whole range of attitudes in "1G maneuvers" (where your body thinks down is where your seat is).
Little micro-movements we unconsciously make on the controls (or larger movements we may make consciously in response to turbulence or other perceved disturbances in the aircraft's attitude) based on what our ears and butts are telling us will eventually get us into trouble.

It's so easy that often instructors will have students create their own unusual attitudes in training by having the student close their eyes and simply try to fly straight and level - within a few minutes at most the student will invariably no longer be in level flight.

There's an exercise you can do on the ground to illustrate how imperfect your sense of balance and direction is, but it requires a large, flat, open area. We'll use an American Football field for this example:

  • Stand on one of the goal lines
  • Have a friend stand at the 50 yard line (about 45 meters away)
  • Close your eyes or put on a blindfold, count to ten, and walk directly toward your friend
  • Stop when you reach your friend (or your friend yells "STOP" because you're about to walk into a goal post)

Most people will not make it to their friend on the 50 yard line, because without a visual reference it's very hard to walk a straight line: You will veer off one way or the other, and some folks will even manage to walk in a complete circle (which is what eventually tends to happen over a large enough distance).
The same principles apply to flight, except there's a third dimension (vertical) to contend with.

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    $\begingroup$ And note that walking with closed eyes is still a lot easier, because the only motion is due to your legs and your brain can correct by correlating position and forces exerted by your legs. In an aircraft your platform is moving with you, so that this only adds to the confusion instead. $\endgroup$ – Jan Hudec Jan 31 '15 at 23:16

Yes, definitely. This is from an NTSB accident report:

During the right turn, the pilot stated that he looked to his right to cross check the GPS and set up the autopilot for a coupled approach. He felt the airplane start to accelerate rapidly, and he looked back to the Primary Flight Display (PFD) which was “showing all brown with no sky and 6-7 chevrons, indicating a severe unusual attitude.” He tried to correct the unusual attitude, but said that he had severe vertigo, and was unable to regain control of the airplane. He elected to deploy the ballistic recovery parachute, and the aircraft impacted terrain in a nose low attitude in a creek bed.

The roll wasn't fully inverted, but it was extreme (120 degrees to the left):

The downloaded data showed that, just prior to the accident, the airplane went through a series pitch and roll oscillations, with maximum pitch values of approximately 26 degrees nose up, and 75 degrees nose down. The airplane reached maximum roll values of approximately 83 degrees right wing down, and 120 degrees left wing down.

So yes, it's absolutely possible for the aircraft to end up in an extreme attitude very quickly and easily: in this case the pilot just looked away to set the autopilot. The reasons why this can happen so easily have been discussed and commented in detail in this question.

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    $\begingroup$ and pulling out of a 75-degree dive is how you break the wings off $\endgroup$ – rbp Jan 31 '15 at 0:03

Yes. Remember that once you get into a weird position, like upside down, you may be turning at the same time, creating a false gravity, so it does not feel like you are upside down. Usually you just feel "cockeyed", and if you try to adjust based on that "feeling", you will make the WRONG input which will make the situation worse. If you strap yourself inside a ball in an amusement park and get turned upside down, even then it can be difficult to tell exactly what is going on.

Many pilots have difficulty trusting their instruments and have to be trained to do so very strictly. When I was put "under the hood" for the first time, my instructor told me "you will go into a spin". I said "no I will not". He said, "Yes you will, everybody does the first time." As a matter of fact I kept the plane straight and level and did not go into a spin, but I was an exceptional case, 95% of students will start spinning the aircraft uncontrollably if they cannot see the horizon. JFK, Jr. was killed that way.

Let me just say, that first time under the hood I was sweating bullets and focusing my mind like I was trying to play speed chess against Bobby Fischer. Flying by instrument is INSANELY hard, if you have not trained on it for hours and hours. You have 5 different needles that you have to keep lined up at the same time. Even experienced pilots find manual instrument flying grueling and mentally taxing (thank god for autopilot).

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    $\begingroup$ FWIW: I've been told, and tend to agree, that PPLs who play Flight Simulators more are more comfortable looking at and interpreting the instruments, because we get no sensory cues from our desk-chairs. :) $\endgroup$ – abelenky Jan 30 '15 at 20:46
  • $\begingroup$ "spin" or "spiral"? I suspect your instructor said or meant to say graveyard spiral $\endgroup$ – Steve V. Jan 31 '15 at 5:37

Yes, very easily... Although 'sideways, nose down and at 200 knots into the ground before becoming a massive fireball' is more common

If it's only a short period they may just find themselves coming out of the other side of the cloud at a completely different attitude than they went in (and, likely, altitude too), but with anything more than just a quick cloud, it can be far more serious.

Pilots without an instrument license have something like a 3 minute life expectancy once they get into instrument conditions.

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    $\begingroup$ Before I flew into a cloud I thought I could trust my senses just a tiny bit. When I came out the other side, my attitude was completely different in more than one way. $\endgroup$ – Steve V. Jan 31 '15 at 5:39

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