Vestibular Illusions

#FlySafe GA Safety Enhancement Topic

Published in

Cleared for Takeoff

8 min readOct 25, 2024

Which way is really up? To some, this may sound like a silly or odd question, but for pilots, this can be a real concern. Statistics show that between 5 to 10% of all general aviation accidents are attributed to spatial disorientation, and 90% of those are fatal. Spatial orientation is our natural ability to maintain our body’s orientation and/or posture in relation to the surrounding environment (physical space) at rest and during motion. This #FlySafe topic examines the vestibular illusions that you may encounter and what you can do to avoid these dangerous situations.

Fly Safe illustration. — Don’t get disoriented by vestibular illusions.

Vestibu-what?

The vestibular system includes the sensory organs contained within the inner ear that detect the relative motion of the head in space within its axes of movement. It consists of two major components: the semicircular canals that detect changes in rotational acceleration, and the otolith organs that detect linear (straight) acceleration. Your vestibular system’s primary function is to detect rotational and translational movements of the head and generate a corresponding response signal. The semicircular canals are three half-circular, interconnected tubes located inside each ear that are the equivalent of three gyroscopes located in three planes perpendicular (at right angles) to each other. Each plane corresponds to the rolling, pitching, or yawing motions of an aircraft.

The vestibular system’s semicircular canal cross-section.

Inner ear with semicircular canals shown, likening them to the roll, pitch and yaw axis of an aircraft. — The vestibular system’s semicircular canal cross-section.

But this system was designed to function on the ground in a 1G environment (normal gravity). Accidents can occur due to a combination of vestibular illusions and poor visibility. When the body is subjected to certain forces that cause a vestibular illusion, vision is often the only sense that can contradict these false perceptions. However, in darkness or other poor visibility conditions, it is much easier to be deceived by an illusion, often making it difficult for a pilot to tell which way is up. When this occurs it’s important to ignore the conflicting signals your body is giving you and instead rely on your experience and trust your instruments.

Don’t Trust Your Gut

Without visual references (e.g., VFR at night/low visibility and IFR flying), pilots can become disoriented, especially in situations like low visibility or turbulent weather, where sensory inputs can be conflicting or misleading. When visual cues are absent, your body will turn to your vestibular system for information. The vestibular system is complex and can be easily deceived in certain flight conditions. When motion makes this system unreliable, pilots experience vestibular illusions. These dangerous illusions are the most likely culprits of spatial disorientation.

There are six types of vestibular illusions you may encounter while flying IFR. The most common illusion, “the leans,” occurs after a sudden return to level flight after a gradual and prolonged turn. If the rotational acceleration of the turn is 2 degrees per second or lower, your vestibular system will not detect this movement. When you level out after a turn like this, you may experience the illusion that your aircraft is banking in the opposite direction. If you rely on what your body is telling you, you might lean in the direction of the original turn to regain what you think is the correct vertical posture.

If a pilot is in a turn long enough for the fluid in the ear canal to move at the same speed as the canal, the Coriolis illusion can occur — the most dangerous vestibular illusion. A sudden head movement, such as looking down at something you dropped during a prolonged turn can give you the false sensation of rotation or acceleration on an entirely different axis. When disoriented by this illusion, you might maneuver the aircraft into a dangerous attitude while trying to correct your aircraft’s perceived attitude. This is why it’s so important to practice moving your head as little as possible during instrument cross-checks or scans. Make sure you keep your head as still as possible when reaching for charts and other objects on the flight deck.

Illustration of the Coriolis illusion showing the inner ear and a pilot spinning. — The Coriolis illusion.

A prolonged coordinated constant-rate turn could cause the sensation of flying straight and level. This is when you are in danger of experiencing the graveyard spiral. Aircraft tend to lose altitude in turns unless you compensate for the loss in lift. When making a constant-rate turn, you may notice a loss of altitude, even though you aren’t experiencing the sensation of turning. This creates the illusion of being in a level descent. Your gut might tell you to pull back on the controls in an attempt to climb or stop the descent. If you listen to your gut instead of trusting your instruments, the spiral will tighten and increase the loss of altitude. This could lead to a loss of aircraft control.

Illustration of an airplane in a graveyard spiral. — The graveyard spiral.

The somatogravic illusion occurs during rapid acceleration and creates the same feeling as tilting your head backward. Pilots experiencing this feeling can mistake it for a climb, especially while flying IFR. This disorientation could make you want to push the aircraft into a nose-low or dive attitude. A rapid deceleration could make you feel the opposite sensation and urge you to pull up, putting you in danger of a nose-up or stall attitude.

When you make a sudden return to straight and level flight after a climb, it can feel like you are tumbling backward. This is known as inversion illusion. The disorientation you feel from this might lead you to push your aircraft abruptly into a nose-low attitude, which can intensify the illusion.

Like the inversion illusion, the elevator illusion is also caused by an abrupt change. A sudden upward vertical acceleration, as can occur in an updraft, can stimulate your otolith organs and create the illusion of being in a climb. This could make you want to push the aircraft into a nose-low attitude. An abrupt downward vertical acceleration, usually in a downdraft, has the opposite effect making you want to pull the aircraft into a nose-up attitude.

Illustration of the elevator illusion. — The elevator illusion.

Impeding Illusions

Your first line of defense against spatial disorientation is practice, practice, and more practice. Undergo regular training on spatial disorientation recognition and recovery techniques so you will be aware and prepared for potentially disorienting situations. Consider experiencing the illusions firsthand, either with a flight instructor or in a simulator. You could also immerse yourself in the vestibular illusions that you might encounter at a spatial disorientation laboratory. Many universities and the military use labs to simulate various flight conditions and scenarios to train pilots to recognize and cope with spatial disorientation. Experience the disorientation in a controlled environment, and practice overcoming what your body is telling you so you can commit to trusting your instruments. Learn about training offered by the FAA here.

Set yourself up for success — to help prevent spatial disorientation, pilots should:

🛩️ Obtain training and maintain proficiency with flying instruments before flying with less than 3 miles visibility.
🛩️ Use and rely on your flight instruments, especially at night, in reduced visibility, and in featureless and sloping terrain. Be sure to test your flight instruments before each flight as well as during your preflight and taxi.
🛩️ Maintain night currency if you intend to fly at night. Include cross-country and local operations at different airports.
🛩️ Do not attempt VFR flight when there is the possibility of getting trapped in deteriorating weather.
🛩️ If you are flying with another pilot and start to experience spatial disorientation, transfer control. Pilots rarely experience visual illusions simultaneously.
🛩️ Plan your transition to instrument flying before you enter IMC. Start your instrument scan while you are still in visual conditions.
🛩️ Avoid movements in the cockpit that are prone to cause spatial disorientation when flying by reference to instruments. Sudden head movements, or the classic “reaching down to pick up a dropped pencil” may bring on sudden disorientation.

In addition to these tried-and-true methods of combating spatial disorientation, it is also important to:

🛩️ Study and become familiar with unique geographical conditions in areas where you plan to operate.
🛩️ Check weather forecasts before departure, enroute, and at your destination. Be alert for weather deterioration.
🛩️ Consider practicing maneuvers that illicit illusions with your CFI to maintain proficiency.
🛩️ Contact your FSDO for opportunities to use a full motion simulator and experience the illusions you might encounter.
🛩️ Set personal minimums for VFR and IFR flight designed to minimize your exposure to conditions that increase your risks.