3.2 Altitude and Q-code definitions

Altitude

Altimeter indicated altitude: the approximate height of the aircraft above mean sea-level [amsl], calculated in accordance with the ISA.

Calibrated altitude: the indicated altitude, corrected for internal instrument error and static vent position error.

Density altitude: a calculation used to determine possible aircraft performance — see section 'High density altitude' below. This is the pressure altitude adjusted for variation from standard temperature, or the height in ISA having a density corresponding to the location density, then called density height.

Declared density altitude: seasonal charts showing regional values to be added to airfield elevation to give declared density altitude were published in section 20.7 of the Civil Aviation Orders. For example the summer chart shows regional values of 2000 feet on the eastern coast and 3600 feet in south-west Queensland. These regional values are to be used only if there are no other means of calculating current density altitude.

Pivotal altitude: is not associated with altimeter setting; it is a term used by the proponents of 'ground reference' manoeuvres such as 'eights on pylons'. It is a particular height above ground at which, from the pilot's viewpoint, the extended lateral axis line of an aircraft doing a 360° level turn (in nil wind conditions) would appear to be fixed to one ground point, and the aircraft's wingtip thus pivoting on that point. The pivotal altitude in nil wind conditions is easily calculated by squaring the TAS in knots and dividing by 11.3. So an aircraft circling at 80 knots would have a pivotal altitude around 550 feet, no matter what the bank angle.

When an aircraft is turning at a height greater than the pivotal altitude, the wingtip appears to move backwards over the landscape. When an aircraft is turning at a height less than pivotal altitude (i.e. usually close to the ground) the wingtip appears to move forward over the landscape. For more information see 'pivotal altitude and reversal height'.

Pressure altitude: the altimeter reading when the pressure-setting scale is set to 1013.2 hPa. It is the ISA Standard Pressure setting, sometimes termed pressure height. Standard pressure is also the standard factory setting for altitude encoding devices. All aircraft cruising in the Standard Pressure Region — above a transition layer that (in Australia) commences at 10 000 feet — use the standard pressure setting, and the subsequent altimeter reading is normally referred to as flight level [FL]. However an aircraft maintaining a constant altitude using 1013.2 hPa, or any other fixed setting for that matter, is following an isobaric surface whose height amsl will vary according to atmospheric conditions. An aircraft maintaining FL145 (i.e. 14 500 feet), and flying towards a lower pressure area, will actually be descending at a rate approximating 40 feet per one hPa decrease in surface level pressure.

True altitude: the calibrated altitude corrected for atmospheric temperature conditions. But as the correction will assume standard pressure and temperature lapse rates between the surface and the aircraft level, it will not be an accurate reflection of the aircraft's height above mean sea-level. If you maintain a particular altitude, you will be following an isobaric surface and not maintaining a constant height. The only way to measure height accurately is by triangulation — and that can only be done by a GPS receiver in the aircraft. However, there are still problems in determining the vertical datum. See geoid-ellipsoid separation.
Q-codes

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. This is derived 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 method used by the Australian Bureau of Meteorology; 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 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) 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. Cruising within the Transition Layer is not permitted. 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 Area QNH. More detail is available in 'Aeronautical Information Publication (AIP) Australia' section ENR 1.7; downloadable from Airservices Australia.

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'. It is 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).

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