Transition layer, altitude and level
In Australia, there is a system of altimetry in place that separates aircraft using QNH from those using 1013.2 hPa as a pressure reference. The transition altitude, which is always set at 10,000 ft, and the transition level, typically FL110 but occasionally reaching FL125 depending on the QNH, serve as the boundary between these two groups of aircraft.
For all operations below the transition altitude, the altimeter reference will be either the current local QNH of a station within 100 nm of the aircraft or the forecast area QNH if the current local QNH is not known. This information can be obtained from various sources including reporting stations, ATIS, TAF, ARFOR, AERIS, or ATS. Flights cruising at or below the transition altitude must change their altimeter setting when advised of a change by ATS. Pilots of aircraft not using radio can set their altimeter to the aerodrome elevation before takeoff to obtain the local QNH.
However, when cruising in the standard pressure region, the altimeter must be set to 1013.2 hPa. It is important to switch between QNH and 1013.2 hPa while in the Standard Pressure Region, either during the climb after passing 10,000 ft or during descent to an altitude within the Altimeter Setting Region before entering the Transition Layer. Cruising within the transition layer is not permitted.
Positions to change between QNH and 1013.2 hPa
The transition layer
|FL125||Not available when area QNH is below 963 hPa|
|FL120||Not available when area QNH is below 980 hPa|
|FL115||Not available when area QNH is below 997 hPa|
|FL110||Not available when area QNH is below 1013 hPa|
What is Area QNH?
Area QNH is a forecast value that is applicable for a three-hour period and is typically used for the entire Area QNH zone (AQZ). If necessary, the Area QNH zone may be divided to meet the following standards:
- Area QNH forecasts should be within ± 5 hPa of the actual QNH at any low-level point (below 1000 ft above mean sea level) within or on the boundary of the relevant area during the forecast’s period of validity.
- The Area QNH should not differ from an adjacent Area QNH by more than 5 hPa.
What is Local QNH?
Local QNH is used as follows, whether provided by air traffic services, automated weather stations, or an aerodrome forecast (TAF), or by setting the altimeter subscale to indicate the airfield elevation above mean sea level.
Heights measured from a QNH or Area QNH datum should be fully expressed, for example: 3000 ft as “three thousand” and 1800 ft as “one thousand eight hundred,” adding “on (QNH)” if necessary.
Expressions of height measured from the 1013.2 hPa datum must always include the words “flight level.”
Pre-flight Altimeter Check
If an accurate QNH is available and the aircraft is at a known elevation, pilots must check the accuracy of the aircraft’s altimeter before takeoff. In order of priority, the pilot should use the following elevations for the check:
- threshold, or
- airfield reference point elevation.
Note: 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, such as the parking position and then the runway threshold (to determine altimeter serviceability).
With an accurate QNH set, a VFR altimeter(s) should read the site elevation to within 100 ft (110 ft at test sites above 3300 ft) to be considered serviceable by the pilot. If an aircraft equipped with two VFR altimeters continues to fly with one altimeter reading 100 ft (110 ft) or more in error, the faulty altimeter must be marked as unserviceable and the error noted in the maintenance release.
VFR altimeters are not allowed 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 automatic terminal information service, a tower, or an automatic remote-reporting aerodrome sensor. Area or forecast QNH should 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.