Flyin' the ointment.

An aviation medicine 'Dear Doctor'.
Dr. Dougal Watson.




From the March 1996 issue of the AOPA (Australia) magazine.


This month Flyin' the ointment considers a member's question concerning the relatively common in-flight illusion - The Leans .

THE LEANS.

"Last week during an instrument training flight, under the hood, I experienced an amazing sensation of flying left-wing-low when all the instruments, and my instructor, assured me I was straight-and-level. My instructor explained that this was an illusion called 'the leans' and that it occurred because of confusion in the balance organs inside my ears. I don't really understand how this could happen. Could you please explain the leans?"

It really is an amazing feeling isn't it? I well remember my first 'leans'. Despite a thorough understanding of the physiology I was totally unprepared for that powerful false feeling of roll when I knew that my wings were level.

Your instructor was correct in that the leans do result from limitations in our natural balance mechanisms. More detailed descriptions of these limitations can be found in several articles I wrote for the, then, CAA (details below).

Our eyes are primarily responsible for our orientation during flight. We have balance organs in our ears called semicircular canals and otolith organs but they are not terribly effective at sensing orientation during flight.

A scenario in which 'the leans' might occur is:
  1. Flight in IMC or under the hood;
  2. A very slow, gentle roll to the right;
  3. Right-wing-low attitude noticed on instruments;
  4. Rapid correction roll to the left.
  5. 'Leans' to the left when straight-and-level.

During flight in IMC (or under the hood) we lose the balance and orientation cues provided by our eyes - The horizon being the most important.

A very slow roll to the right will not be detected by our non-visual balance organs. These organs are designed to detect such rotation but only down to a certain level, or threshold. Such a slow roll would be called below-threshold or sub-threshold because our balance organs wouldn't detect it.

Despite flying, say, 5 degrees right wing low our body would still think we were straight-and-level.

When we notice the roll attitude on our instruments and correct, rolling to the left, we will probably do so at a rate that our balance organs can detect ( above- or supra- threshold). The aircraft will now be flying straight-and-level and our body will think it has rolled to the left. This is because the roll to the right (subthreshold) was not detected while the correcting roll to the left (suprathreshold) was.

Without an horizon to check against we will be sitting there certain that we are 5 degrees left wing low when all our instruments, and our instructor (Not under the hood) will know that we're straight and level.

This illusion is named 'the leans' both because it feels as if you're leaning to one side, or the other, when you're not and because some pilots attempt to correct the sensation by leaning their body in the opposite direction to the illusory aircraft roll - leaning to the right in your case to make everything feel straight-and-level.

The danger in an illusion such as this is that the false sensation can be so overpowering that we modify our flight so that everything 'feels' normal. In this case that would involve rolling the aircraft to the right again so our body feels straight-and-level. If this process were to continue it's not difficult to imagine getting into quite extreme attitudes by following our misleading body cues rather than our instruments.

Everyone who flies in IMC will suffer from the leans at sometime. It's not possible to avoid the illusion altogether, all we can do is avoid it causing us problems.

To avoid problems from the leans it is important to get onto the instruments, minimise head movements and, if possible, fly straight-and-level for a minute or so. This will allow your body's balance mechanisms to reset and stabilise and will strengthen your faith in your instruments.

The leans is only one example of the many illusions that can occur during flight. All of these illusions occur because our body's balance mechanisms were not designed for flight. An understanding of these illusions and how to avoid them causing problems is important for all pilots.


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