General Procedures
The Transition Altitude within the United Kingdom is 3000ft except in or beneath that airspace specified below.
Vertical positioning of aircraft when at, or below, any Transition Altitude will be expressed in terms of Altitude. Vertical positioning at, or above, the Transition Level will normally be expressed in terms of Flight Level; in these circumstances, when descending through the Transition Layer, vertical position will be expressed in terms of Altitude and when climbing, in terms of Flight Level. It should not be assumed that separation exists between the Transition Altitude and Transition Level.
Flight levels are measured with reference to the Standard Pressure datum of 1013.2 mb. In the UK consecutive Flight Levels above the Transition Level are separated by pressure intervals corresponding to 500ft in the ISA; at and above FL 200, by pressure levels corresponding to 1000ft (except in an active TRA where 500ft pressure levels are applied up to FL 245). FL 195 will not be allocated as a cruising level.
Civil aircraft using military aerodromes must conform to military procedures.
QNH and temperature reports for certain aerodromes are given in MET broadcasts and can also be obtained from ATS Units. These QNH values are rounded down to the nearest whole millibar but are available at certain aerodromes in tenths of millibars for landing aircraft on request.
Altimeter Setting Regions
Altimeter Setting Regions (ASR). To make up for any lack of stations reporting actual QNH, the UK has been divided into a number of ASRs for each of which the National Meteorological Office calculates the lowest forecast QNH (Regional Pressure Setting) for each hour. These values are available hourly for the period H+1 to H+2 and may be obtained from all aerodromes having an Air Traffic Service, from the London, Manchester and Scottish ACCs, or by telephone. A chart containing the ASR boundaries can be found in the AIP at ENR 6.1.
The ASRs are listed below, together with the MET Office Codes in parenthesis.
Skerry (01) Holyhead (07) Chatham (12) Orkney (17)
Portree (02) Barnsley (08) Portland (13) Marlin (18)
Rattray (03) Humber (09) Yarmouth (14) Petrel (19)
Tyne (04) Scillies (10) Cotswold (15) Skua (20)
Belfast (05) Wessex (11) Shetland (16) Puffin (21)
Airspace within all Control Zones (CTRs), and within and below all Terminal Control Areas (TMAs), Control Areas (CTAs) except Airways and the Daventry and Worthing Control Areas, during their notified hours of operation, do not form part of the ASR Regional Pressure Setting system.
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