Definition
A change in velocity in which the speed of an object decreases over time, or the velocity changes in a direction opposite to the current motion. In aviation, it commonly refers to deceleration during flight, such as when an aircraft slows down on landing rollout or reduces airspeed in level flight.
Plain English
Slowing down. The aircraft is still moving, but its speed is decreasing rather than increasing.
Context Anchor
Seen in basic aerodynamics and Newton’s laws discussions when explaining how forces change an airplane’s motion.
Derivation
From Latin 'negare' (to deny or reverse) and 'accelerare' (to hasten). 'Negative' here doesn't mean bad — it means reversed in direction. So negative acceleration is acceleration acting opposite to motion, which slows the aircraft down.
Why Pilots Care
Recognizing negative acceleration allows correct anticipation of control responses and energy management during deceleration phases such as landing rollout.
Analogy
It is like a car slowing when you press the brake. The car is still moving forward, but its speed is decreasing, so its acceleration is negative relative to its forward motion.
Intuition Check
Negative acceleration does not mean the airplane is accelerating in a bad or unsafe way. It means the change in motion is opposite the chosen positive direction; in common flight examples, that means slowing down.
Example Sentence 1
After touchdown, the pilot applied the brakes to produce the negative acceleration needed to stop the aircraft within the available runway.
Example Sentence 2
Pulling the propeller control to high pitch in a descent can produce negative acceleration if drag exceeds thrust.