Definition
A change in speed along a straight line, without a change in direction. In flight, this includes speeding up or slowing down along the aircraft's path of travel, such as during takeoff roll or rapid power changes in level flight.
Plain English
Going faster or slower in a straight line. The aircraft is not turning, climbing, or descending — just changing speed along the path it is already on.
Context Anchor
Seen in instrument flying discussions about the inner ear, balance, and why a pilot’s body can give false motion cues when outside visual references are limited.
Derivation
Linear' comes from the Latin linea, meaning 'line.' 'Acceleration' comes from the Latin accelerare, meaning 'to hasten' or 'speed up.' Together: a speed change along a line. This helps separate it from angular acceleration, which involves rotation rather than straight-line motion.
Why Pilots Care
Linear acceleration can produce a powerful illusion called the somatogravic illusion. During a rapid takeoff or sudden forward acceleration, the inner ear interprets the sensation as a nose-up pitch. A pilot reacting to that false sensation may push the nose down — straight into the ground. Recognising linear acceleration as a known illusion trigger is essential for instrument flying at night or in low visibility.
Analogy
When a car speeds up quickly, you feel pressed back into the seat. Your body feels that straight-line acceleration, but it does not automatically know whether you are speeding up or tilting.
Grounding Statement
When a car presses you back into your seat as it speeds up on a straight road, that is linear acceleration. No turning, no tilting — just a change in speed along the line you are already travelling.
Intuition Check
Linear does not mean smooth or gradual here; it means along a straight line. Acceleration does not only mean speeding up; in this context it can include slowing down along that line.
Example Sentence 1
During the takeoff roll, the linear acceleration down the runway can create a false sensation that the nose is pitching up.
Example Sentence 2
During a missed approach the linear acceleration can create a false sense of climbing when the aircraft is actually level.