Understanding Altitude: Difference between revisions

The indicated altitude is the altitude shown on the altimeter when it is using the correct altimeter setting. It will be identical to absolute altitude when the plane is sitting on the ground, but, unless the temperature lapse rate is exactly the same as ISA, it will become increasingly inaccurate with altitude: on a cold winter day, the altimeter can overread by 1,000 ft or more at normal cruising altitudes, so that pilots are flying much lower than they think; on a hot summer day, the altimeter can underread by 1,000 ft or more at normal cruising altitudes, so that pilots are flying much higher than they think. Since all altimeters experience the same error, and the error diminishes near the ground, the difference between indicated altitude and absolute altitude does not normally cause a problem for crusing and approaching: as long as everyone is flying at, say, 8379 ft MSL, it does not much matter if they think that they're at 9000 ft. The one exception is flying around mountains in the winter, where the altimeter error can cause collision with terrain.