More dust from the sands of Africa. I would draw your attention to the obvious use in practical terms of QNE in order to determine flight levels and its redundancy as a datum for ground operations.
Note: the letters in the Q-code nomenclature have no literal significance, these are remnants of an extensive notation system from the days of wireless-telegraphy. There were some 200 three letter Q-codes each representing a sentence, a phrase or a question, for instance QRM "I am being interfered with"!. Some 30 Q-codes are still used by amateur radio / morse code enthusiasts and the four below, plus QDM (the magnetic bearing to a station), still survive in aviation. For a full listing of Q-codes try www.cbug.org.uk/allqcodes.htm
. The following four codes relate to altimeter settings.
QFE: the barometric pressure at the station location or aerodrome elevation datum point. If QFE is set on the altimeter pressure-setting scale while parked at an airfield, the instrument should read close to zero altitude – if the local pressure is close to the ISA standard for that elevation. However the use of QFE is deprecated and anyway, if the airfield elevation is higher than perhaps 3000 feet, older/cheaper altimeters may not be provided with sufficient sub-scale range to set QFE.
QFF: the mean sea level [msl] pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature and relative humidity at the location are the long term monthly mean, the temperature lapse rate is ISA and the relative humidity lapse rate is zero. This is the Australian Bureau of Meteorology method; QFF calculations differ among meteorological organisations. QFF is the location value plotted on surface synoptic charts and is closer to reality than QNH, though it is only indirectly used in aviation.
QNH: the msl pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature at the location is the ISA temperature for that elevation, the temperature lapse rate is ISA and the air is dry throughout the the column.
The Australian aviation regulations state that when an 'accurate' QNH is set on the pressure-setting scale at an airfield, the altimeter indication should read within 100 feet of the published airfield elevation, or 110 feet if elevation exceeds 3300 feet; otherwise the altimeter should be considered unserviceable. However due to the inherent inaccuracy possible in QNH, this may not be so. The difference between QFF and QNH when calculated on a hot day at a high airfield in Australia can be as much as 4 hPa, equivalent to about 120 feet. The advantage to aviation in using the less realistic QNH is that all aircraft altimeters in the area will be out by about the same amount, and thus maintain height interval separation.
The Local QNH at an airfield is normally derived from an actual pressure reading, but the Area QNH used outside the airfield zone is a forecast value, valid for three hours, and may vary by up to 5 hPa from any Local QNH in the same area. Either Local QNH or Area QNH may be set on the altimeter pressure-setting scale of all aircraft cruising in the Altimeter Setting Region; which [in Australia] extends from the surface to the Transition Altitude of 10 000 feet. The cruising levels within the Altimeter Setting Region are prefixed by 'A' e.g. A065 = 6500 feet amsl.
When there is no official Local QNH available at an airfield, and the site elevation is known, the Local QNH can be derived by setting the sub-scale (when the aircraft is on the ground of course) so that the altimeter indicates the known airfield elevation. The use of Local QNH is important when conducting operations at an airfield as the circuit and approach pattern is based on determining height above ground level [agl].
Note that it is not mandatory for VFR aircraft to use the area QNH whilst enroute. You may substitute the current local QNH of any aerodrome within 100 nm of the aircraft or the local QNH at the departure airfield. See 'Acquiring weather and QNH information in-flight'.
The purpose of the transition layer is to maintain a separation zone between the aircraft using QNH and those using the standard pressure setting. If Area QNH was 1030 hPa there would be about 500 feet difference displayed between setting that value and setting standard pressure. The transition layer extends from the Transition Altitude to the Transition Level which, in Australia, is usually at FL110 but it may extend to FL125 – depending on mean sea level pressure.
QNE: common usage accepts QNE as the ISA Standard Pressure setting of 1013.2 hPa. However another definition of QNE is the 'altitude displayed on the altimeter at touchdown with 1013 set on the altimeter sub-scale'. Also referred to as the 'landing altimeter setting'.
Within the latter meaning the term is only likely to be used when an extremely low QNH is outside an aircraft's altimeter sub-scale range, and the pilot requests aerodrome QNE from air traffic services. In Australia, such extreme atmospheric conditions are only likely to occur near the core of a tropical depression/cyclone and as QNE is not listed in the ICAO “Procedures for Air Navigation Services" air traffic services would not provide QNE on request.
However QNE can be calculated by deducting the QNH from 1013, multiplying the result by 27 [the appropriate pressure lapse rate per hPa] and adding the airfield elevation.
For example: QNH 960 hPa, airfield elevation 500 feet, pressure setting 1013.
QNE = 1013 –960 = 53 × 27 = 1431 + 500 =1931 feet [the reading at touchdown].