Definition
An older style of engine-driven air pump used to power vacuum-operated flight instruments, in which steel vanes slide in and out of slots in a rotating steel rotor to draw and push air. Because the steel vanes rub against the steel pump housing, these pumps require oil for lubrication and produce an oil-laden exhaust, which is why they are sometimes called wet-type pumps.
Plain English
An older type of pump on the engine that creates suction for some flight instruments. It uses small steel blades spinning inside a steel housing, and it needs oil to keep the metal parts from wearing each other out.
Context Anchor
Seen in discussions of vacuum pump systems that power gyroscopic instruments such as attitude and heading instruments.
Derivation
Named for what it is made of and what it does. The vanes (flat sliding blades) are made of steel, and the pump moves air rather than liquid. The steel construction is the key detail because it explains why these pumps need oil, unlike the newer dry-type pumps that use self-lubricating carbon vanes.
Why Pilots Care
Loss of vacuum from pump failure removes power from the attitude and heading indicators, forcing the pilot onto partial-panel techniques or backup instruments.
Analogy
Think of a small paddle wheel inside a case. As the paddles turn, they carry air from one side to the other and create suction or pressure.
Intuition Check
Do not read “air pump” as a pump that fills a tire. Here it means a pump that supplies suction or pressure for aircraft instruments.
Example Sentence 1
The older trainer was equipped with steel-vane air pumps, so a small amount of oil discharge at the exhaust port was normal and expected.
Example Sentence 2
During the preflight inspection the pilot checks the vacuum gauge to confirm the steel-vane air pump is producing adequate suction for the gyros.