Your ability to predict the performance of a helicopter is extremely important. It allows you to determine how much weight the helicopter can carry before takeoff, if your helicopter can safely hover at a specific altitude and temperature, how far it will take to climb above obstacles, and what your maximum climb rate will be.

A helicopter’s performance is dependent on the power output of the engine and the lift production of the rotors, whether it is the main rotor(s) or tail rotor. Any factor that affects engine and rotor efficiency affects performance. The three major factors that affect performance are density altitude, weight, and wind.

The density of the air directly affects the performance of the helicopter. As the density of the air increases, engine power output, rotor efficiency, and aerodynamic lift all increase. Density altitude is the altitude above mean sea level at which a given atmospheric density occurs in the standard atmosphere. It can also be interpreted as pressure altitude corrected for nonstandard temperature differences.

Pressure altitude is displayed as the height above a standard datum plane, which, in this case, is a theoretical plane where air pressure is equal to 29.92 in. Hg.

Pressure altitude is the indicated height value on the altimeter when the altimeter setting is adjusted to 29.92 in. Hg. 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. The difference between true altitude and pressure altitude must be clearly understood. 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.

For example, if the local altimeter setting is 30.12 in. Hg., and the altimeter is adjusted to this value, the altimeter indicates exact height above sea level.

However, this does not reflect conditions found at this height under standard conditions. Since the altimeter setting is more than 29.92 in. Hg., the air in this example has a higher pressure, and is more compressed, indicative of the air found at a lower altitude.

Therefore, the pressure altitude is lower than the actual height above mean sea level.

To calculate pressure altitude without the use of an altimeter, remember that the pressure decreases approximately 1 inch of mercury for every 1,000-foot increase in altitude. For example, if the current local altimeter setting at a 4,000-foot elevation is 30.42, the pressure altitude would be 3,500 feet. (30.42 – 29.92 = .50 in. Hg. × 1,000 feet = 500 feet. Subtracting 500 feet from 4,000 equals 3,500 feet).

The four factors that most affect density altitude are: atmospheric pressure, altitude, temperature, and the moisture content of the air.

Due to changing weather conditions, atmospheric pressure at a given location changes from day to day. If the pressure is lower, the air is less dense. This means a higher density altitude and less helicopter performance.

Density Altitude— Pressure altitude corrected for nonstandard temperature variations. Performance charts for many older aircraft are based on this value.

Standard Atmosphere— At sea level, the standard atmosphere consists of a barometric pressure of 29.92 inches of mercury (in. Hg.) or 1013.2 millibars, and a temperature of 15°C (59°F). Pressure and temperature normally decrease as altitude increases. The standard lapse rate in the lower atmosphere for each 1,000 feet of altitude is approximately 1 in. Hg. and 2°C (3.5°F). For example, the standard pressure and temperature at 3,000 feet mean sea level (MSL) is 26.92 in. Hg. (29.92 – 3) and 9°C (15°C – 6°C).

Pressure Altitude— The height above the standard pressure level of 29.92 in. Hg. It is obtained by setting 29.92 in the barometric pressure window and reading the altimeter.

True Altitude— The actual height of an object above mean sea level.

As altitude increases, the air becomes thinner or less dense. This is because the atmospheric pressure acting on a given volume of air is less, allowing the air molecules to move further apart. Dense air contains more air molecules spaced closely together, while thin air contains less air molecules because they are spaced further apart. As altitude increases, density altitude increases.