The altimeter is essentially a barometer, a device that measures the difference between two pressure levels.

It’s better known as “that thing in a plane that tells you how high up you are”. Exactly what that means varies, but in aviation there are basically two ways of expressing it:

Altitude, or QNH, is your height in feet above mean sea level. Sea level is very often lower than the ground below you, but QNH is useful because it gives a common level to work with between airfields, and constantly updating your setting isn’t really feasible.

Height, or QFE, is your height above the ground level at a certain location, generally an airfield. Hence a good way to remember the two: QNH is Nautical, QFE is airField. The uses of calibrating your altimeter to the ground level are obvious (especially when landing), but of course the ground doesn’t stay at that level and away from the airfield the reading isn’t always useful. Continue reading →