However, this does not reflect conditions found at this height under standard conditions. and the altimeter is adjusted to this value, the altimeter indicates exact height above sea level. For example, if the local altimeter setting is 30.12 in. True altitude means the vertical height above mean sea level and is displayed on the altimeter when the altimeter is correctly adjusted to the local setting. The difference between true altitude and pressure altitude must be clearly understood.
Pressure altitude, as opposed to true altitude, is an important value for calculating performance as it more accurately represents the air content at a particular level. Pressure altitude is the indicated height value on the altimeter when the altimeter setting is adjusted to 29.92. Pressure altitude is displayed as the height above the “standard” datum plane, which, in this case, is a theoretical plane where air pressure is equal to 29.92 in. It can also be interpreted as pressure altitude corrected for nonstandard temperature differences. Density altitudes the altitude above mean sea level at which a given atmospheric density occurs in the “standard atmosphere”, which is defined as the atmosphere at 15 o C / 59 o F and barometric pressure of 29.92 in. As the density of the air increases, engine power output, rotor efficiency, and aerodynamic lift all increase. The density of the air directly affects the performance of the helicopter.
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