Which Illusions Can Pilots Encounter While Flying?

Humans are, by design, land creatures. This means that when we try to do something unnatural to us, like flying, some of our organs can give incorrect information, resulting in illusions.

As pilots, understanding possible illusions, at least at a basic level, is quite important. This is because failure to appreciate them could lead to an undesired aircraft state.

It is a balancing act

If you ask anyone what the ears do, they will tell you that they are there for hearing. This is true. However, the ears also have another crucial function in that they help to maintain balance by sensing direction and acceleration.

Inside the inner ear are what is called a vestibule and semicircular canals. Together they form the vestibular apparatus.

The vestibule consists of the utricle and saccule, which detect linear accelerations. In the utricle and saccule are structures known as the macula, which comprise small stone-like particles called otoliths that are embedded in a gel-like fluid. This fluid also contains sensory hair cells.

When acceleration is detected in either the vertical or the horizontal plane, the otoliths pull the fluid, which in turn causes the hair cells to move. This movement of hair cells causes excitation in the sensory nerve fibers, which transfer the information to the brain that there is an acceleration. The job of the vestibule is to ensure that the human being knows what is right or wrong in terms of movement. For instance, if you tilt your head up, your sensations will tell you that it is not the normal position of the head and vice versa.

When flying, this can lead to an illusion called Somatogravic Illusion. This happens when engine power or thrust is increased by the pilot during take-off, go around, or in a normal climb. When thrust is added, there is an acceleration that is detected by the otoliths causing the hair cells to move. When this happens, the brain thinks that the head has tilted back, giving a sensation of climb. A similar thing happens in a deceleration. The hair cells move in the opposite direction giving the pilot a sensation of descent.

This is particularly dangerous in an aircraft with powerful engines when power is added, for instance, in a go-around. In a go-around, the engines are pushed to their maximum, giving a false sensation of a climb even though the aircraft maintains altitude.

A few years back, there was a crash involving a Boeing 767 operated by Atlas Air. The aircraft was on the approach to George Bush Intercontinental Airport in Houston. While the aircraft was being configured for landing, the Pilot Flying (PF), in this case, the first officer, accidentally pushed the throttles to maximum or go-around thrust. This arrested the descent, and the aircraft went to a near-level attitude. The first officer sensed the sudden acceleration of the aircraft as a climb and got disoriented, putting the aircraft on a steep descent path and causing the aircraft to crash.

The other part of the vestibular apparatus is the semicircular canal. The semicircular canals detect right or left movement or angular accelerations. In the semicircular canals are the ampulla, which house something called the cupula. The ampulla is also filled up with a fluid known as endolymph and consists of sensory hair structures like that of the macula. When the head moves either to the left or to the right, the cupula bends to the direction causing the hair cells to move, which triggers the brain to detect that there is a head movement.

When flying an aircraft, the semicircular canals can give an illusion called Somatogyral Illusion. Here is how it works.

If a pilot were to put the aircraft in a prolonged turn or a bank, the hair cells initially move to the direction of the turn giving a correct sensation of the turn. However, the lengthy turn may, at some point, cause the hair cells to return to their normal erect position giving the pilot a sensation that the aircraft is flying straight and level. When the pilot takes the aircraft out of the roll and puts it in straight and level flight, the hair cells move in the opposite direction giving him/her a sensation of an opposite bank or a turn.

If the pilot reacts to this sensory illusion by giving an opposite bank to get back straight and level the flight, the aircraft will go into a spiral dive in the previous turn direction. This is called a graveyard spiral, and a failure to correct it will most definitely lead to a crash.

John F. Kennedy Jr.’s plane crash in 1999 is a famous example of somatogyral illusion.

In Instrument Meteorological Conditions (IMC), that is, with no visual cues to refer to, it is almost impossible for a normal human being to prevent these illusions. Hence, to fly in IMC, pilots are required to be instrument rated; they need to be trained and checked out for their instrument flying skills.

The instruments of aircraft have no sensations, and they are always right (if there are no mechanical failures). Over the years, many accidents have occurred because of non-instrument-rated pilots flying into IMC and getting disoriented.

What are some other illusions?

Autokinesis

During night flights, if you were to look at a still light source with little to no other visual references, the light may appear to move. This is known as autokinesis. So, a pilot may assume a lone star in the skies is a moving object or an airplane. To prevent this, it is essential to avoid looking at single light sources for prolonged periods during night flights.

Judgment of approach angle

When approaching for landing with no instruments or other guidance, pilots rely on visual judgment of the glide slope. However, visual judgments can cause illusions.

Upsloping runways may give the pilots an illusion that the approach angle is high, and he/she may put the nose down to get the correct angle, leading to a low approach and risking a runway undershoot. A downsloping runway, on the other hand, makes the pilot believe that he or she is lower than the optimum glide. The normal reaction of the pilot, in this case, will be to climb, which puts the aircraft in a higher-than-normal approach.

The width of the runway can also make the pilots misjudge their glide on the approach. If a pilot is used to landing on wider runways, he/she may feel high on approach when approaching a narrower runway. And if he/she is used to landing on narrower runways, he/she may feel low on a wider runway.

Illusions due to atmospheric conditions

During night approaches in heavy rain, water droplets on the windscreen can cause the runway lights to bloom, giving the impression that the runway is closer than it is. This makes the pilot feel that the aircraft is approaching the runway faster than expected, and due to this, the pilot may tend to make a shallower approach.

Fog, mist, and haze can also lead to sensory illusions as they can obscure objects, terrain, lights, etc. This may give the pilot the impression that things like the runway are further away than they are.

What are the safeguards against illusions?

• Having an understanding of possible illusions.
• Use all resources available to prevent an illusion from happening. Even in visual conditions using your instruments rather than visual cues can be a great help.
• Avoiding IMC conditions if you are not instrument rated.