This is a pressure sensitive device generally with an aneroid capsule construction. As the aircraft climbs the pressure decreases causing the sealed capsule to expand and the mechanical pointer to indicate an increased height. A barometric scale in inches of Hg is on the case. A calibration screw is used to compensate for local variations in the atmospheric pressure. By definition:
- True altitude is the exact distance above sea level (ASL).
- Absolute altitude is the distance above the ground level (AGL).
- Indicated altitude is the altitude above mean sea level displayed on an altimeter when it is set to the current barometric setting.
- Pressure altitude is the indicated altitude when the altimeter is set to the standard pressure of 29.92 inches of Hg.
- Density altitude is the pressure altitude corrected for non- standard temperature variations.
The indicated altitude is the same as the true altitude only when the sea level pressure and temperature are standard. A 1000 foot change in altitude is approximately equal to a 1 inch change in pressure. For example if the actual pressure is 30.12 inches Hg and the standard pressure is 29.92 inches Hg, then if the altimeter is set to 29.92 it will indicate a height of 200 feet below the actual altitude.
Temperature also effects the indication. For every 11 degrees Celsius change in temperature there is a 4% error in the indicated altitude. If the air is colder than standard, the aircraft will be at a lower altitude than indicated. In terms of safety during flight the common saying is:
“Cold and Low, look out below. Warm and High, clear blue sky.”
Density altitude is often important to glider pilots who are aerotowed with aircraft with normally
aspirated engines. Hot, humid weather adversely affects the performance of this type of engine and can make for longer takeoff runs and slower rates of climb. This can have serious implications in clearing obstacles.