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I'm not a pilot.

In the following excellent explanatory video I understand everything except for a single point.

The presenter says that in cloud, if the instruments tell the pilot to level the wings when the plane is tilting to the left, the pilot's reaction will be to bank even more steeply to the left, because they will believe they are in a severe right-hand bank.

Spin, Spin, Spin! 2:00 (Video)

Given that, according to the presenter, the inner ear is telling the pilot that they are currently horizontal, why would the pilot assume or feel anything about which way they should correct? Isn't it 50/50 whether they choose to roll in the right sense or the wrong sense?

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    $\begingroup$ Spatial disorientation is hard to describe even when you've experienced it first hand. I'm looking forward to someone doing so here, because I'm struggling to. $\endgroup$
    – Jamiec
    Mar 17 at 13:27
  • $\begingroup$ @Jamiec How did I do? $\endgroup$
    – StephenS
    Mar 18 at 2:26
  • $\begingroup$ @StephenS amazing. I learned a lot about the cause. $\endgroup$
    – Jamiec
    Mar 18 at 8:39
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    $\begingroup$ Do you ever notice how weird it is that you can have a cup of coffee sitting flat and level on your meal tray in an aircraft when the plane is banked 30-deg into a turn? If you couldn't look out the window, would you believe the plane was halfway sideways? $\endgroup$
    – J...
    Mar 18 at 10:31
  • $\begingroup$ @J... -- yes, because I weigh more! $\endgroup$ Mar 19 at 13:53
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Your vestibular system can only recognize acceleration. Your brain combines that with visual inputs to produce spatial orientation. (If the two disagree, you may get motion sickness.)

When you are in a sustained left bank, your vestibular system stabilizes and feels no acceleration. Without a usable visual input, your brain will convince itself that you are instead straight and level. This is spatial disorientation.

If you then look at the instruments and try to correct yourself to actual straight and level flight, the vestibular system feels that change in acceleration as starting a climbing right bank, which now feels wrong. It orders you to bank back to the left even harder to make itself feel “normal” again.

It’s not a 50/50 chance; once disorientation sets in, your body will consistently tell you to do exactly the opposite of what you should do. That’s why it’s so deadly.

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    $\begingroup$ This is probably correct, but explained in a weird way. There is certainly acceleration present in any turn, coordinated or not. Without acceleration we fly on a line. $\endgroup$
    – Vladimir F
    Mar 19 at 7:51
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    $\begingroup$ @VladimirF I think he's talking about angular acceleration for the most part. That makes up accelerating in three of the six degrees of freedom we usually consider for motion: pitch, roll, and yaw. We can also sense linear acceleration in the other three degrees of freedom: thrust (forwards), slip (sideways), and heave (upwards), but roll is the most important in this context. $\endgroup$
    – Chromatix
    Mar 19 at 10:14
  • $\begingroup$ You have a good hunch but a bit of the detail is lacking. The vestibular system has three linear accelerometers and three angular velocity sensors. So it can recognize both linear acceleration and angular rotation. In fact, the angular velocity sensors are oriented in plane of the ocular muscles, so that the ocular muscles can stabilize the image on our retina with a minimum of neural network latency - IIRC there's a single synapse or two between the semicircular canals and the ocular muscles. $\endgroup$ Mar 19 at 20:02
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To enlarge slightly upon StephenS's answer, if the banked turn is properly coordinated, then the pilot feels only the G-force of the turn and no shear force in his or her seat, and concludes that (s)he is not turning left but climbing.

The hapless pilot then notices that (because the plane is in a banked turn) it is showing a descent on the VSI gauge. Pilot then tries to climb by pulling back on the yoke, which for a steep turn tightens the turn and increases the G-force, making the pilot feel the plane is climbing more steeply.

By that point the altimeter has begun to unwind while at the same time the pilot feels as if the plane were still climbing. If (s)he happens to look at the attitude gyro, (s)he will then see the plane is in a very steep left bank and in a nose-down attitude even though the G-force makes it feel like the plane is climbing.

Cognitive dissonance sets in and the pilot panics and reefs back still harder on the yoke. Now the plane is in a high-speed spiral dive and if the cloud bank extends all the way to the ground, the plane flies into the ground in a hard bank, nose-down, with the VSI pegged at -3000 ft/min or thereabouts.

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    $\begingroup$ @supercat a properly trained pilot could do that with several of the instruments, but probably not the mag compass, it has several turning errors that make it pretty difficult to interpret in any regime other than straight and level flight. $\endgroup$
    – nexus_2006
    Mar 18 at 1:36
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    $\begingroup$ @supercat no a simple compass doesn't show that. For example, if you don't change your heading at all, but change your speed, the compass will show a turn, then swing left and right before it settles back to the original heading. If you're facing (it's been a while so I might have it precisely backwards) south and start a turn, the compass will show a turn the other way. These errors aren't hard to memorize, but they're very hard to remember and account for in the heat of the moment. The tool you're looking for is the DG/HSI - a gyro-stabilized compass card. $\endgroup$
    – nexus_2006
    Mar 18 at 18:06
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    $\begingroup$ @supercat yaw speed indicators exist and are commonly available in aircraft as "turn coordinators." $\endgroup$
    – Erin Anne
    Mar 18 at 22:36
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    $\begingroup$ @supercat As an IR student...a LOT more than you might think. The inner ear is extremely powerful in its insistence that the integrated worldview generated from it is correct at all times. The only thing more powerful by nature is the horizon lock generated from the Mk1 eyeball, which acts to nearly immediately reset the integrators used to process the inner ear acceleration inputs (this is why if said VFR pilot pops out sometimes they regain control). A significant amount of IR training (and IR currency) is acclimating to using the instruments vs. natural senses at all times. $\endgroup$ Mar 19 at 19:14
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    $\begingroup$ @supercat The only condition where those would be comparable is in absolutely glassy smooth air with no preceding false horizon type inputs (i.e. uniform lighting top to bottom out the window, etc.). In that case sure, SD is unlikely to occur if straight and level flight is maintained. In reality cloudy air is rarely so nice -- the acceleration inputs from turbulence etc. cause rapid integrator drift and wham you have a nice case of SD. $\endgroup$ Mar 19 at 20:35
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To elaborate even a bit further on the existing answers, there are a couple of items that should be kept in mind.

First, the entire human spatial orientation system works as a sort of sensor fusion between fast (but prone to drift) sensor inputs and slow (but accurate) sensor inputs. Fast / drift-prone inputs come from acceleration sensors -- the inner ear (rotation), the "seat of the pants" feeling (G-force, linear acceleration). Slow but accurate inputs come from the eyes (horizon lock) and proprioception (when standing upright, pressure sensed in feet + autonomous balance inputs required to stay upright). To make matters worse, our inner ear is designed only to function in two dimensions, and lacks certain features of avian inner ears that would help reduce the drift in three dimensions.

If this sounds similar to the systems spacecraft use, it's because it is. They have gyros and star trackers, and like our own senses the gyros drift while the star trackers are too slow to maintain orientation under motion.

Fundamentally, the problem here is that the brain is performing a double integration automatically, every waking second of every day, using the visual system to keep drift in check. It is very, VERY hard to discard the bad state vectors generated from the system without being able to see a horizon. Without visual references, as the integrated values from the inner ears drift more and more away from reality, expected position (i.e. predicted path of travel and even current location) will start to drift as well.

An untrained brain (or even one sufficiently far out of IR currency) will use the synthetic state vectors (predicted path of travel, integrated attitude) generated by this autonomous system, even in many cases overriding the gyro in front of it ("it's broken", etc.). If you try to override, that same internal autonomous system will continue to drift and will scream even louder and louder at you that you're making the problem worse with your inputs.

To make matters even worse, many other higher level functions within the human body rely on those internal state vectors being accurate. When you're running for example and start turning, you'll notice your eyes start to lead out the turn (looking in the direction of the turn) and there are subtle posture shifts that occur in anticipation of the turn. If those vectors are wrong, you can actually end up looking slightly away from the object you want to see, or unconsciously start putting in a bit of compensatory yoke pressure.

This integrated positioning system is so deep within the brain and so fundamental to daily life (without it, jumping, running, or even walking would be out of the question) that when an IR pilot gets "the leans" it can take every ounce of effort and focus to just override that system. A pilot not familiar with that experience is likely to panic and unconsciously ignore the instruments, even if they were able to interpret what the instruments were saying under normal conditions.

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TL;DR: Beware the double whammy: spins caused by spatial disorientation can cause inappropriate vestibular nystagmus that then causes loss of visual acuity making you unable to read small features on instruments potentially depriving you of critical instrument feedback needed to reacquire spatial awareness.

In addition to the other answers, the yet other confounding (and deadly) factor is vestibular nystagmus. Nystagmus is the sawtooth (pendular) motion of the eyes, and is normally a desirable oculomotor function, used to both reacquire the image on the retina (the fast slope of the sawtooth), and to stabilize the image on the retina (the slow slope of the sawtooth). Unfortunately, the vestibular inputs experienced in flight, and especially during aerobatics, spins, or other high-g maneuvers, are not mostly inappropriate for stabilizing the image on the retina. Such vestibular input may cause inappropriate and undesirable nystagmus. Suppression of this nystagmus can be learned under feedback (e.g. biofeedback from an eye tracker), and is also naturally present to an extent. Some substances, like alcohol, decrease the effectiveness of the suppression, and lower the threshold for vestibular nystagmus to occur. Individual threshold and sensitivity to various substances in that respect seems to be highly variable and is influenced by genetic and environmental factors.

Inappropriate nystagmus is bad news, because it decreases the visual acuity - i.e. decreases the maximum spatial frequencies we can visually resolve, while, paradoxically, increasing the apparent contrast of low spatial frequency features. Under nystagmus, we lose the ability to see small details, while the scene overall seems to have higher contrast. This causes a dangerous sensation of "seeing better" while the opposite is happening.

If the instruments have sufficiently low contrast and/or small features, e.g. the thin vectors that the glass cockpits and HUDs love for some inexplicable reason, the retina won't be able to resolve them. Alas, our visual system is used to filling in the blanks with expected information, and will gladly substitute whatever we'd deem "expected" based on prior experience and our idea of spatial orientation - based in part on the inappropriate vestibular inputs. This, unfortunately, is a good recipe for controlled flight into terrain, and is a real problem for e.g. military fighter pilots.

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Spatial disorientation, spins, and illusions are complex subjects each. In this case, the initial explanation in the video is straightforward. If one gets into a particular flight attitude, especially slowly, and doesn't feel one's self getting there (slowly creeping into a bank), the body doesn't know it's in that attitude (eg, bank). The body thinks nothing has occurred to move it from level flight. The sensory perception in the inner ear hasn't indicated a change from level to the bank, so when one sees the instrument indicating a bank, the natural inclination is to believe the one instrument that's always been true...the brain and the body.

Of course, this isn't true. The instrument is correct. But one perceives the instrument to be incorrect; after all, how did one get into that bank, without sensing it. If one suddenly rights the airplane according to the instrument, or in other words returns the airplane to level flight, now the body senses a movement. This movement is contrary to what the body believes; the body believes it was level, but the movement now is sensed as a bank.

When the body senses a bank, but the instrument is showing level, this is confirmation bias: the body believed it was correct before (even though it wasn't), and thus that the instrument was incorrect. Now, having moved the airplane back to level flight, the body is receiving a signal of movement, and believes itself now banked...the instrument says it's not in a bank. This confirms the previous believe, or is confirmation bias, implying that the body was correct all along, and the instrument wrong. This furthers distrust of the instrument.

What has really occurred is that the pilot has returned the airplane to level flight, but perceives himself or herself to have entered a bank. Now a sensory conflict exists. Which is correct? Instrument training will tell the pilot to believe his instruments. A lifetime of standing upright will tell the pilot to believe his inner ear. If the pilot does not follow the instrument indications, he can roll back into the bank where he was, and even steepen it; each upset or correction can make the sensation worse.

In the process, the upset is further worsened if there are changes in pitch, or in speed. Changes in speed will usually be perceived as changes in pitch. As bank angle increases, these changes are further complicated by increasing G load, or unload, which may be perceived as pitching moments, further complicating the matter, and a pilot in a bank who begins to apply pitch can further complicate the matter, steepening the bank, unloading the wing, increasing or decreasing senses of bank, speed, and pitch.

The immediate solution is simple in description, harder when experiencing strong conflicting sensations (disorientation). Fly the instruments. Easier said than done, especially initially when one wants to believe one's own senses. I have seen very experienced pilots, on several occasions, become disoriented even in visual conditions, or visual conditions with illusions. In one case, in a 747, I took the controls briefly when the pilot became disoriented with a sloping horizon illusion. We were at night, departing Bahrain, and a combination of dark, fires on oil rigs in the gulf, and stars in the sky combined to disorient the pilot. It only took a moment for him to regain his bearings, but it does happen. I saw the same thing once flying into Liege in the daytime in reduced visibility, but still visual conditions, when turning final. I have experienced the same thing myself, both as a student and as a seasoned professional, and it's very real. The solution: fly the instruments, but again, easier said in some cases, than done, and it doesn't change the strong, sometimes seemingly overpowering sensation that what the body feels is correct, and what the instruments are telling you, is not.

This is the reason we don't just look at one instrument, but crosscheck them, and interpret them: to ensure that what we're seeing is correct, and that we're seeing the whole picture.

In a spin, the effects are magnified significantly, with numerous factors coming into play. To complicate that, spins are not stead-state events, staying the same; they change dynamically due to a number of factors, and physical and psychological factors combine to cause very powerful illusions that can lead one to make incorrect choices and further aggravate the situation, rather than effect a recovery.

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    $\begingroup$ This finally clicked. I'm sure reading all the answers contributed but, reading this, the penny dropped. As land-dwellers, we don't bank!, or at least not for long. We tend to remain laterally upright and gravity always comes from the same direction. When flying, "gravity" comes partly from the Earth and partly from inward acceleration. If I have understood then, provided you can ignore your inner ear long enough to fly straight and level for a few seconds - by the instruments, everything will fit back together again. Am I right? So in low visibility use instruments constantly. $\endgroup$ Mar 21 at 0:29
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    $\begingroup$ I don't know that a few seconds is the fix, but understanding the instruments and using them to maintain orientation is the correct choice. It's equally important to cross check them, so that one does not follow an incorrect instrument (a bad attitude gyro, for example). If you've ever attempted to stand after laying down or sleeping, and found yourself struggling for balance, you get the concept. A stable reference is needed. Most of us would grab something and hang on, or sit back down, or let someone steady us. In flight, we do that by focusing on instrument indications. $\endgroup$
    – Will
    Mar 21 at 0:34

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