Definition
A unit used to measure manifold pressure in a piston aircraft engine, expressed in inches of mercury (in. Hg). It indicates the absolute pressure of the air-fuel mixture inside the engine's intake manifold, as shown on the manifold pressure gauge in aircraft equipped with a constant-speed propeller.
Plain English
It is the reading on the manifold pressure gauge, measured in inches of mercury, that shows how hard the engine is being worked by the throttle.
Context Anchor
Seen during power setting changes in airplanes with constant-speed propellers, such as takeoff, climb, cruise, descent, and engine management procedures.
Derivation
The unit comes from how pressure has historically been measured: by how high a column of mercury (chemical symbol Hg) rises in a tube under that pressure. One inch of mercury equals the pressure needed to push the column up by one inch. Aviation kept this unit because early instruments used mercury, and the numbers (typically 15 to 30 in. Hg in flight) are easy to read at a glance.
Why Pilots Care
It shows the power output of the engine and must be coordinated with RPM to prevent overboost or inefficient operation.
Intuition Check
Do not read “inches” as a physical length on the airplane. Here, “inches” is a pressure unit, and “manifold pressure” means air pressure inside the engine intake system.
Example Sentence 1
After leveling off in cruise, the pilot reduced the throttle to 22 inches manifold pressure and set the propeller to 2,400 RPM.
Example Sentence 2
Before takeoff the checklist called for 30 inches manifold pressure at 2,400 RPM.