Definition
The readings shown on the manifold pressure gauge, expressed in inches of mercury (in. Hg), which display the absolute pressure of the fuel/air mixture inside the engine's intake manifold. In aircraft with a constant-speed propeller, these readings indicate the power being produced by the engine and are controlled primarily by the throttle.
Plain English
The numbers on the manifold pressure gauge that tell the pilot how much power the engine is making. The pilot adjusts these numbers by moving the throttle.
Context Anchor
Seen during power changes, straight-and-level instrument flight, and the pilot’s regular scan of the engine instruments.
Derivation
Manifold comes from Old English manigfeald, meaning 'many folds' or 'many parts joined together' — the intake manifold is the branching pipework that delivers the fuel/air mixture to each cylinder. The 'pressure' is what's measured inside that pipework, and the 'indications' are simply what the gauge shows.
Why Pilots Care
Incorrect manifold pressure leads to altitude deviations, over-stressing the engine, or insufficient performance in instrument conditions.
Grounding Statement
When the throttle is moved, the manifold pressure indication is one of the first signs that engine power has changed.
Intuition Check
Manifold pressure is not oil pressure or fuel pressure. It is also not engine power by itself; it is an intake-air pressure reading used to help set engine power.
Example Sentence 1
After leveling off at cruise altitude, the pilot reduced the throttle and watched the manifold pressure indications settle at 23 inches.
Example Sentence 2
During the climb the manifold pressure indications dropped as the throttle was left unchanged, requiring adjustment.