Definition
A cockpit instrument that displays the absolute pressure of the fuel/air mixture inside the engine's intake manifold, expressed in inches of mercury (in. Hg). It is used on reciprocating engines equipped with a constant-speed propeller to set and monitor engine power.
Plain English
A gauge that shows how hard the engine is being worked by measuring the pressure of the air entering the cylinders. The higher the reading, the more power the engine is producing.
Context Anchor
Seen on the instrument panel during engine run-up, takeoff, climb, cruise, descent, and instrument flying power changes.
Derivation
Manifold comes from the Old English manigfeald, meaning 'many folds' or 'many parts joined together.' In an engine, the intake manifold is the branched pipework that splits the incoming air among the cylinders. The instrument reports the pressure inside that pipework.
Why Pilots Care
It lets the pilot set and monitor engine power accurately and avoid overboost damage in turbocharged engines.
Intuition Check
Do not read manifold pressure indicator as a fuel-pressure gauge or as an engine-speed gauge. It shows pressure in the engine’s intake passage, not fuel pressure and not revolutions per minute.
Example Sentence 1
After takeoff, the pilot reduced power by setting 25 inches on the manifold pressure indicator and 2,500 RPM on the tachometer.
Example Sentence 2
On the takeoff roll the manifold pressure indicator read 29 inches, confirming full available power.