The Altimeter contains aneroid wafers that expand and contract as atmospheric pressure changes, rotating pointers on a dial through a shaft and gear linkage. Altimeters are subject to limitations of nonstandard pressure and temperature. For IFR, the maximum allowable altimeter error is 75 feet off field elevation.

Indicated Altitude is the altitude read on the altimeter face that indicates the approximate height above mean sea level.

True Altitude is the actual height above sea level. Airport, terrain, and obstacle elevations found on aeronautical charts are true altitudes. True altitude may vary from indicated altitude if the altimeter does not have the correct setting. The altimeter setting is the value to which the scale of the pressure altimeter is set so the altimeter indicates true altitude at field elevation. If the altitude indication is off by more than 75 feet from the surveyed elevation, the altimeter must be sent to a certified repair station for recalibration. Make sure to check the pressure altitude against field elevation before takeoff. If the altimeter is set too high it will read too high and you will be lower than you intend to be. If the altimeter is set too low it will read too low and you will be higher than you intend to be. 

During flight, always update your altimeter setting to a station within 100 nautical miles of your airplane. Turning the knob to increase the numbers in the pressure window (or Kolsmann window) will move the altitude hands higher and increase the altimeter reading. This can be remembered as "Wind on inches, wind on altitude.” Conversely, turning the knob to decrease the numbers in the Kolsmann window will move the altitude hands lower and decrease the altimeter reading.

When the aircraft is flying in air that is warmer than standard, the air is less dense and the pressure levels are farther apart—true altitude is therefore higher than indicated altitude. If the air is colder than standard, it is denser and the pressure levels are closer together—true altitude will be lower than indicated altitude.

When an aircraft moves to an area of lower pressure and the pilot fails to readjust the altimeter setting (calibrating it to local pressure), the true altitude will become lower as the pressure decreases. The altimeter will still indicate the previous altitude, but the aircraft will be lower because it is flying a constant pressure surface that is sloping down towards the low pressure area.

WHEN FLYING FROM HOT TO COLD OR FROM A HIGH TO A LOW, LOOK OUT BELOW. WHEN FLYING FROM LOW TO HIGH (TEMPERATURE OR PRESSURE) LOOK TO THE SKY.

Pressure Altitude is the altitude read on the altimeter face when the Kollsman Window is set to 29.92 Hg. This adjusts the altimeter to indicate the height above the standard datum plane (a theoretical level where the weight of the atmosphere is 29.92 in. of Hg.)

Absolute Altitude is height above the ground and may be measured directly on a radar altimeter.

Density Altitude is pressure altitude corrected for nonstandard temperature.

If the static system becomes blocked, the altimeter needles will remain fixed at the altitude where the blockage occurred due to the constant static pressure trapped in the system.