Page 36 - Beginner's Guide to Aviation

QFE
Stands for Question Field Elevation. It is pressure observed at the airfield datum point. When set on the altimeter subscale, the altimeter will indicate 0ft.
QNH
Stands for Question Nil Height. It is pressure observed from the Mean Sea Level (MSL). When set on the altimeter
subscale, the altimeter will indicate an airfield’s elevation.
QNE
Stand for Question Nil Elevation. It is pressure observed at the standard altimeter setting. A standard pressure of 1013.2mb is set altimeter subscale. 1013.2mb is the International Standard Atmosphere for Mean Sea Level pressure!
Flights report their elevation in Feat (ft) while using QNH and Flight Levels (FL) while using QNE. (Elevation is the measurement of height of an aircraft from sea level or airfield.) This correlates to the fact that aircrafts use different pressure altimeter settings in different phases of flight.
As you can see from the table, when an aircraft is climbing,
it uses QNH and QFE because it is still closer to the ground. Hence, it is able to report its elevation in ft. As compared to when an aircraft is cruising, it uses QNH and QNE and reports in FL as the aircraft is now really high above ground. For this reason, reporting in FL would cause less confusion.
Phases of flight
On ground and Taxiing Climbing
Cruising
Descending
Landing
Altimeter setting
QNH / QFE QNH / QNE QNH / QNE QNH / QNE QNH / QFE