Originally Posted by Genghis the Engineer
Above transition level (certainly above FL50 in the UK), I'd use QNE (1013) for traffic avoidance. Below TL, I'd use RPS (regional pressure setting, commonly but slightly incorrectly called regional QNH) for terrain avoidance.
QNE, slightly incorrectly called 1013...
QNE is not a pressure setting. QNE is the reading in feet on an altimeter with the sub-scale set to 1013.2 mb when the aircraft is at aerodrome or touchdown elevation. During conditions of exceptionally low atmospheric pressure it is not possible to set QFE or QNH on some aircraft altimeters. In these circumstances an aerodrome or runway QNE can be requested.
1013.2 is the Standard Pressure Setting. Flight Levels are measured with reference to the standard pressure setting of 1013.2 mb. In the UK, consecutive Flight Levels above the transition level are separated by pressure intervals corresponding to 500 ft; at and above FL200, by pressure intervals corresponding to 1000 ft.
The Regional Pressure Setting is a forecast of the lowest QNH value within an ASR. The values which are made available hourly for the period H + 1 to H + 2, are given in whole millibars.
UK Altimeter Setting Procedures are
here.
Originally Posted by Cusco
If you're squawking 7000 Mode C (as you do, don't you?) your altitude on the radar controller's screen will be based on FL/1013, so it makes eminent good sense if you're above TL whether you're farting about or not to use FL /1013 on your altimeter.
The aircraft altitude encoder transmits Mode C data based on Standard Pressure Setting 1013.2. The radar processors will convert the readout to indicate altitude or flight level depending on the Transition Altitude and the QNH entered into the radar data processors.