Altimeter
The altimeter is an instrument that measures the height of an aircraft above a given pressure level. A stack of sealed aneroid wafers comprises the main component of the altimeter – an aneroid wafer is a sealed wafer that is evacuated to a constant internal pressure. These wafers are free to expand and contract with changes to the static pressure. A large static pressure presses down on the wafers and causes them to collapse. A lower static pressure allows the wafers to expand.
A mechanical linkage connects the wafer movement to the needles on the indicator face, which translates compression of the wafers into a decrease in altitude and translates an expansion of the wafers into an increase in altitude.
Types of Altitude
- Indicated Altitude: Read directly from the altimeter.
- True Altitude: The vertical distance of the aircraft above sea level – the actual altitude.
- Absolute Altitude: The vertical distance of an aircraft above the terrain, or above ground level (AGL).
- Pressure Altitude: The altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92 “Hg.
- Density Altitude: Pressure altitude corrected for variations from standard temperature.