Definition
The forward thrust still produced by an engine when the throttle is at idle. Even with the power lever fully retarded, a piston engine at idle RPM or a turbine engine at flight idle continues to generate some thrust, which keeps the airplane moving forward rather than allowing it to slow purely under aerodynamic drag.
Plain English
The small amount of push the engine still produces when the throttle is all the way back. Idle is not zero thrust — the engine is still pulling or pushing the airplane forward a little.
Context Anchor
Encountered during the landing flare, especially when discussing why an airplane may float above the runway after power is reduced.
Derivation
From Latin residuum, meaning 'that which is left over.' Residual thrust is the thrust that is left over after you have reduced power as far as the throttle allows.
Why Pilots Care
This remaining thrust can cause the airplane to float longer than expected and increase landing distance.
Grounding Statement
Picture the throttle at idle in the flare, but the airplane still getting a slight forward push instead of slowing immediately.
Intuition Check
Residual thrust does not mean the pilot is still intentionally adding power. It means some forward push remains even after power has been reduced.
Example Sentence 1
She held the airplane off the runway during the flare, but residual thrust kept it floating until the airspeed bled off.
Example Sentence 2
After landing, reverse thrust was used to overcome residual thrust and slow the jet on the runway.