Definition
A false sense of an aircraft's position or motion caused by the body's nerves and muscles sending misleading signals to the brain about acceleration, gravity, or orientation. It occurs when the pilot relies on physical sensations of pressure and movement instead of the flight instruments, particularly during flight in clouds or at night when outside visual references are absent.
Plain English
An illusion where your body 'feels' the airplane is doing one thing when it is actually doing something else, because the seat-of-the-pants sensations of motion and gravity can fool you when you cannot see outside.
Context Anchor
Encountered in instrument flying, night flying, flight in clouds, and any situation where the pilot cannot rely on a clear outside horizon.
Derivation
From Latin 'soma' (body) and 'sensorius' (relating to sensation). The word literally means 'body-sensing.' It describes illusions that come from what the body feels, as opposed to visual or inner-ear illusions.
Why Pilots Care
Trusting these feelings instead of instruments can produce incorrect control inputs and loss of control.
Grounding Statement
When an aircraft accelerates down the runway, the push you feel against your back can momentarily feel exactly like the nose pitching up, even though the aircraft is still level.
Intuition Check
Do not read “illusion” here as only a visual trick. A somatosensory illusion is a false body-feel message; when outside cues are poor, trust the flight instruments over the sensation.
Example Sentence 1
During the night takeoff into low clouds, the pilot fought a strong somatosensory illusion that the aircraft was pitching up and resisted the urge to push the nose down.
Example Sentence 2
After a long turn the returning wings-level sensation was only a somatosensory illusion that the pilot ignored by watching the attitude indicator.