Understanding Altitude: Difference between revisions

'''Pressure''' altitude is the altitude shown on an aircraft [[Instruments/altimeter|altimeter]] when it is set to the ''International Standard Atmosphere'' ('''ISA''') standard pressure" (29.92 inches of Mercury). When flying in Class-A airspace (above 18,000 MSL) pilots are required to set their altimeters to ISA, and fly at a "flight level" assigned by Air Traffic Controllers. Using this standard setting helps pilots to maintain more accurate vertical separation between aircraft at higher altitudes. Pressure altitude is also used to calculate density altitude.

Density altitude is pressure altitude adjusted for temperature, and it is the altitude that determines how the aircraft performs. Cold temperatures make the air more dense than it is in the ISA, so (a) there is more oxygen available for a piston engine (and it develops more power), but (b) the plane flies more slowly through the dense air. Hot temperatures make the air thinner than it

* At a high density altitude, a piston-engine plane will need a longer runway to take off and will climb slowly, but once it is cruising, its true airspeed will be relatively high (assuming that the engine can develop sufficient power). The ideal density altitude for cruising in a normally-aspirated piston aircraft is 7000-8000 ft DA: that's where the plane finds the perfect compromise between thin air and oxygen for the engine. On a hot summer day, that density altitude could occur as low as 4000-5000 ft MSL; on a cold winter day, it could occur at 10000 ft MSL or higher. Turbocharged propeller aircraft, which compress the air to provide more oxygen for the engine, often fly between 14000-18000 ft DA to take advantage of the thinner air.