Plain English Meaning

Hg is the symbol for mercury. Pilots see it written as 'in. Hg' (inches of mercury), which is just a way of measuring pressure — both the pressure of the air outside and the pressure inside the engine's intake.

Full Definition

Hg is the chemical symbol for the element mercury. In aviation, it appears as part of the pressure unit 'inches of mercury' (in. Hg), which expresses pressure as the height (in inches) of a column of mercury that the pressure can support. Standard sea-level atmospheric pressure is 29.92 in. Hg. The same unit is used to indicate manifold pressure in piston engines, including turbocharged engines where manifold pressure may exceed 30 in. Hg during ground boosting.

Context Anchor

Seen in turbocharged engine discussions, manifold pressure readings, settings, and pressure .