Definition
The high-velocity stream of air pushed rearward by a spinning propeller. This airflow extends behind the propeller in a roughly cylindrical, slightly spiraling pattern and can move loose objects, debris, dust, and people with significant force.
Plain English
The strong blast of air a turning propeller throws backward. It is fast enough to blow dirt, papers, chocks, and even people around behind the airplane.
Context Anchor
In hand propping, propeller wash is encountered immediately when the engine starts and the propeller begins moving air around and behind the airplane.
Derivation
Wash' here comes from the older sense of a moving stream of air or water left behind by something pushing through it -- the same way a boat leaves a wake. The propeller 'washes' air rearward as it spins.
Why Pilots Care
The propeller wash can push an unsecured aircraft forward during engine start, creating a safety hazard if the pilot is not positioned correctly.
Analogy
It is like standing in front of a very powerful fan, except the air can carry debris and the propeller itself is dangerous.
Grounding Statement
When the engine starts, the spinning propeller throws air backward; that moving air is propeller wash.
Intuition Check
“Wash” does not mean water or cleaning here. It means a flow of air pushed and stirred by the propeller.
Example Sentence 1
Before increasing power for the run-up, the pilot checked behind the airplane to make sure the propeller wash would not blow debris toward the hangar.
Example Sentence 2
The propeller wash from the idling engine moved the lightly loaded airplane a few feet forward on the ramp.