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.
Plain English
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.
Context Anchor
Seen in turbocharged engine discussions, manifold pressure readings, altimeter settings, and weather pressure reports.
Derivation
Hg comes from the Latin 'hydrargyrum,' meaning 'liquid silver' — the old name for mercury. Mercury was historically used in pressure-measuring instruments because it is dense and liquid at room temperature, so a column of it responds visibly to changes in pressure. The symbol stuck even after most modern instruments stopped using actual mercury.
Why Pilots Care
Manifold pressure directly indicates available engine power; turbocharger systems are designed and operated around specific Hg targets.
Analogy
Think of it like using a ruler to measure how strongly air is pushing. The “inches” are not measuring airplane length; they are measuring the height of mercury that the pressure could support.
Grounding Statement
In this context, Hg is simply telling you what pressure scale the gauge or report is using.
Intuition Check
Hg does not mean a special turbocharger setting or a type of altitude. It means mercury, used as part of a pressure unit.
Example Sentence 1
Before takeoff, the pilot set the altimeter to the local pressure of 30.05 in. Hg.
Example Sentence 2
The altitude turbocharger maintained 30 Hg all the way to the service ceiling.