|Quoting PlanesNTrains (Reply 72):|
So does that mean that they weren't looking at their instruments but rather just going by what they felt?
It's not a matter of not looking at the instruments. The human body unfortunately has a piss-poor "attitude indicator" if you will. You may be looking at the instruments yet the other sensory inputs are sending your brain contradicting information, at a much louder tone than what is coming through the eyes. The instruments are the logical input which should be correct, yet your brain will find it very difficult to ignore the other inputs which may be telling it a completely different (and wrong) story.
To sense motion and balance, your brain receives inputs from 3 different sources:
-From your eyesight
-From sensations in your muscles and joints (for example, feeling pressure points like your weight on your rear end, accelerationsdeceleration pressing against certain parts of your body held by seat belts, etc)
-From the vestibular system in your ears, which essentially detect the direction of "gravity", linear movement, and rotational movement.
Your brain constantly processes the 3 inputs combined and based on that is normally able to conclude what your position in space is, and your motion, and all is well.
For pilots, during situations when there is poor visibility and the only visual reference is reduced to the instruments in front of him, the visual input is now severely degraded. The cross-checking your brain does of the above 3 inputs now becomes compromised because the eyesight is no longer a strong input. The muscle/joints and vestibular system take over in importance to your brain.
The problem with that is that the muscle/joints and vestibular systems by themselves are not suitable for controlling the motion of an aircraft in flight. The force of gravity your body "feels" is no longer necessarily "down", it's governed by the flight path of the aircraft (for example, an aircraft pulling out of a vertical dive with a 1G pull will feel like straight and level to your body). Linear acceleration will be interpreted as a pitch up motion and a climb, deceleration as the opposite, even if the plane is only accelerating/decelerating in level flight.
Similarly, the vestibular system sends erroneous messages as it unfortunately is not a biological attitude indicator. It merely senses changes in motion and transmits that signal to the brain, and your brain figures out what your attitude should be based on that change in motion it just felt. The problem is, once you lose visual references your brain eventually will lose the "level horizon" reference point and the vestibular system will send erroneous information regarding attitude.
The above can all result in vertigo, or spacial disorientation. The also render the pilot susceptible to illusions such as the one Pihero well described. Again those 2 sources can be very overpowering to the brain and believing what the eyes see through the instruments becomes a difficult task. With a good visual input (such as discernible horizon), this is not a problem because the eyesight overrules any erroneous inputs from the other 2.
Having some outside visibility can have even a further negative effect in disorienting a pilot, especially in areas where the ground is sparsely lit and there is no discernible horizon. Having some outside reference, not matter how vague or useless, tempts the eyes to try to conclude or verify what is happening based on that limited view. The problem is, the lights of that town on your "left" could actually be a town "below"; the "star" you see ahead is just the light of a farm in the middle of empty fields. Trying to fixate on what little detail can be seen to determine orientation of the aircraft can be disastrous.
Pilotaydin and Pihero both gave excellent explanations of the plausible scenario based on what is known. A sudden change in motion with poor visual references can be very disorienting and challenging, particularly if you have a fatigued crew.