Under the Illusion

Defined as an erroneous mental representation, the inability to reliably process the sensory inputs is a critical issue for instrument pilots.


Our body is traditionally said to have five senses—all of which, it can be argued, are involved in piloting an airplane. The problem occurs when the inputs from two or more contradict each other thereby causing an erroneous mental picture.

During instrument training most of us have experienced “the leans” where our eyes convey one message (attitude indicator showing wings level) while the inner ear tells us we are in a left turn—to name one example. All the while, the sense of touch (literally the “seat of your pants”) may say you’re tipping to the right. A more scientific word is kinesthetic—the sense that detects body position.

This can be an intensely disconcerting and confusing feeling, not to mention potentially dangerous. The good news here is that these feelings will gradually be overcome the more you are exposed to the instrument flight environment. The bad news is that this process seems to be highly individual with some pilots never being fully and truly at ease and confident on the gauges.

An old timer I knew was of the firm opinion that your abilities as an instrument pilot to overcome various sensory illusions are inversely related to your sense of balance—the worse balance you have the better you do on instruments. This applies to me but for the theory in general I cannot vouch.

Minimizing The Risk

Instrument flight is largely a mental exercise founded in knowledge. Leaving the familiar visual surroundings is a leap of faith that requires implicit trust in your equipment. It is therefore vitally important that the qualified aviator has an intimate knowledge of how the flight instruments and navigation systems work as well as standby and backup equipment and procedures.

Many practical things can be done to make for an uneventful IFR experience. First, don’t neglect your own personal physical condition. Even the affects of a simple cold, often feels multiplied when flying and may increase your susceptibility to illusions. Second, try to set up a comfortable work environment in your cockpit. This will help avoid sudden head or body movements such as picking up a chart from the floor or turning to reach a Jepp binder on the right seat. And third, make sure your body is properly anchored to the airframe. There is no worse feeling than flailing on top of your seat, instead of in it, when encountering turbulence. Make sure your lap belt is snugged-down tight and keep the shoulder harness on even in cruising flight to provide a more physical connection to the airplane. Any of these three aspects can induce vertigo.

Somewhat within our control is of course also the type of equipment we fly. The philosophically inclined may wonder why we so easily suffer vertigo when in IMC, but rarely (except maybe for the first few exhilarating experiences of flight) when in the clear?

Part of the answer simply lies in the size of the horizon, be it displayed outside by nature or inside the cocoon of the cockpit in the form of an attitude indicator. It is said that normally 90 percent of the input to our senses is visual information—much of it no doubt is peripheral vision.

In flight, our life long ingrained familiar sense of gravity can be compromised. We need to rely exclusively on our eyes for orientation while suppressing other senses. Anyone flying a modern large screen Primary Flight Display (PFD) with the prominent representation of aircraft attitude is at a clear advantage over those with the traditional small diameter Attitude Indicator (AI).

The Gravity Of The Situation

Every pilot should be familiar with somatogravic experiences caused by gravity as well as various visual illusions that occur in, for instance, low visibility conditions. While extensive flying experience helps acclimate us, there is no way that the comparatively short time we enjoy piloting our aircraft (and the relatively brief periods of IMC) can ever overcome those mental patterns to which we have been conditioned during our lifetime.

Consequently the best way of dealing with much of what we are discussing, is to avoid it.

An important part of learning to fly aerobatics, for example, is forcing yourself to look in the right place for the proper cue during maneuvers. You look up and behind you to catch the horizon coming over the top during a loop, or out at the wingtip to establish a vertical line. Instrument pilots too must often consciously force themselves to look at the right flight instrument for the maneuver being performed and at those that provide supporting information with purposeful glances.

Where Not To Look

While flying with many pilots, ranging from students to qualified airline crew members, I have often observed the natural tendency to want to search for the ground. Glimpsing a bit of farmland or some stray lights below on final approach. It may be psychologically and subconsciously rewarding, seemingly increasing the chances of a successful conclusion to your flight. However, unless you are in a helicopter, it does no such thing; on the contrary it may distract your concentration and increases the risk of illusions.

So when should you look for the runway environment in low IMC? The answer is simple—at the approach minima and no higher. One may feel that visually acquiring the lights as early as possible would be safer, but that is not so. Poor visibility will often produce a tendency to “duck under” the glideslope, obviously a dangerous place to be. Some types of restrictions to visibility, such as radiation fog, tend to be layered with different visibility at different altitudes. Descending through this may cause you to go high. Add to the equation, rain on the windshield, and it is clear that the visual cues presented to the eyes may be undependable.

Even after having acquired the required references in the runway environment at the minimums, do not abandon your instruments altogether. The PAPI or VASI is often bright enough to provide vertical guidance even in dense fog, however there is always a tendency to fly towards what we’re looking at. This can cause the pilot to drift off the centerline since the light projectors are mounted some distance off to the side of the runway. On an ILS you can verify your position with a quick glance at the Nav head for both the Localizer and Glide Slope until over the threshold—50 feet is the lowest altitude at which the ILS glide slope is available.

Dealing with and overcoming illusions in instrument flight is the successful result of knowledge, training and discipline.

Bo Henriksson is a captain with a regional carrier and has more than 15,000 flight hours.

This article originally appeared in the September 2014 issue ofIFR Refreshermagazine.

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