Transition layer, altitude and level AIP ENR 1.7
The system of altimetry used in Australia makes use of a transition layer between the transition altitude (which is always 10,000 ft) and the transition level which is typically FL110 (but can be up to F125 depending on the QNH) in order to separate aircraft that are using QNH from those using 1013.2 hPa as a pressure datum.
For all operations at or below the transition altitude, the altimeter reference will be:
- the current local QNH of a station along the route within 100 nm of the aircraft; or
- the forecast area QNH if the current local QNH is not known.
For cruising in the standard pressure region, the altimeter reference must be 1013.2 hPa.
The positions to change between QNH and 1013.2 hPa shall always be in the Standard Pressure Region on climb after passing 10,000 ft and prior to levelling off, or on descent to a level in the Altimeter Setting Region prior to entering the Transition Layer and is shown in the diagram below.
QNH is available from a reporting station, the ATIS, TAF, ARFOR, AERIS or from ATS.
Cruising within the transition layer is not permitted.
Flights cruising at or below the transition altitude must change the area QNH altimeter setting when advised of a change by ATS. Pilots of aircraft not using radio may obtain local QNH by setting the altimeter to aerodrome elevation before take-off.
Setting the altimeter to the aerodrome elevation does not constitute an accuracy check of an altimeter if there is no reference made to an ‘accurate QNH’ (that is, QNH provided by ATIS, ATC or an AWS).
Not available when area QNH is below 963 hPa
Not available when area QNH is below 980 hPa
Not available when area QNH is below 997 hPa
Not available when area QNH is below 1013 hPa
Local QNH of a station along the route within 100 nm of the aircraft.
Area QNH is a forecast value which is valid for a period of three hours and normally applies throughout an area QNH zone (AQZ).
Area QNH zones will be subdivided, if necessary, to meet the following standards:
- Area QNH forecasts are to be within ± 5 hPa of the actual QNH at any low-level point (below 1000 ft AMSL) within or on, the boundary of the appropriate area during the period of validity of the forecasts.
- Area QNH must not differ from an adjoining area QNH by more than 5 hPa.
Local QNH, whether provided by ATS, AWS or aerodrome forecast (TAF) or by using the altimeter subscale to indicate airfield elevation AMSL, is used as shown on page 3.4.
Heights measured from a QNH or area QNH datum must be expressed in full, for example: 3000 ft as ‘three thousand’ and 1800 ft as ‘one thousand eight hundred’ adding, if necessary, ‘on (QNH)’.
Expressions of height measured from the 1013.2 hPa datum must always include the words ‘flight level’.
Pre-flight altimeter check AIP ENR 1.7
Whenever an accurate QNH is available and the aircraft is at a known elevation, pilots must conduct an accuracy check of the aircraft’s altimeter prior to take-off. In order of priority, the pilot should use the following elevations for the check:
- threshold or
- airfield reference point elevation.
If the first check indicates that an altimeter is unserviceable, the pilot is permitted to make a further check at another location on the airfield; for example, the first on the parking position and the second at the runway threshold (to determine altimeter serviceability).
VFR altimeters AIP ENR 1.7
With an accurate QNH set, a VFR altimeter(s) should read site elevation to within 100 ft (110 ft at test sites above 3300 ft) to be accepted as serviceable by the pilot. If an aircraft fitted with two VFR altimeters continues to fly with one altimeter reading 100 ft (110 ft) or more in error, the faulty altimeter must be placarded unserviceable and the error noted in the maintenance release.
VFR altimeters are not permitted for aeroplane operations above FL200. VFR flights operating above FL200 must be equipped with an altimeter calibrated to IFR standards.
Accurate QNH and site elevation
A QNH can be considered accurate if it is provided by ATIS, a tower or an automatic remote-reporting aerodrome sensor. Area or forecast QNH must not be used for the test.
Site elevation must be derived from aerodrome survey data published by Airservices Australia (see AIP DAP) or supplied by the aerodrome owner/operator.