How is QNE used in aviation?

Answer 1

QNE is a special use Qcode designator which means "What height will my altimeter read on touchdown if I set my pressure subscale to Standard Pressure".

The 'translation' needs detailed explation for those who are neither skilled pilots nor experienced Aerodrome Controllers. Some will never experience use of QNE in a lifetime.

The total scope of a traditional altimeter subscale is 50 to 60 millibars, 25 to 30 mbs each side of standard pressure (1013.2mbs or 29.92"). Most of the time this scope works fine. In the approach and landing phase of a flight it is critically important for the pilot to know exactly how high above the surface the wheels are at the moment of flare out. Various ways of knowing this piece of information all involve the altimeter but in different ways.

The altimeter can be set so as to read zero [feet] at touchdown. (QFE). The pilot simply sinks the aircraft to [almost] zero feet and flares out. Alternatively, the altimeter could be set to indicate zero [feet] at Mean Sea Level. The pilot would use the published touchdown elevation as reference. (QNH) fly the aircraft down to [just above] that elevation and then flare out.

Both methods require that athmosperic pressure at that time is within the limits which allow the altimeter subscale to be thus set. In certain conditions of exceptionally high air pressure at a field near sea level or a runway with an exceptionally high elevation above sea level it is not feasible to use either QFE or QNH. Under those conditions the pilot will be advised to leave his altimer subcale at the standard pressure setting and will be told what altitude his altimeter should read at touchdown. (QNE)

In all three cases the pilot can gauge his flaps, throttle, airspeed and visual references to allow the aircraft to sink under total control so that it touches down gently at exactly the right place and time. It is important to note that while QNH and QFE are given as barometric pressure, QNE is given as an altitude and this is what sets it apart.