pressure-altitude-derived-level information
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Join Date: Nov 2009
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pressure-altitude-derived-level information
hi all,
I have problem with the change of chapter 8 of ICAO doc 4444.
Formerly there used to be written (radar derived level information) and now it is changed to (pressure-altitude-derived-level information).
As far as I know, pressure-altitude is shown to the pilot when the standard pressure of 1013.25 hpa is set on the altimeter. Does it mean that the level information which is shown to us on our radar screens (SSR) is based on 1013.25 only? I mean what about the case of QNH? We receive level information of a/c who are flying on AREA QNH not 1013.25 hpa!
I have problem with the change of chapter 8 of ICAO doc 4444.
Formerly there used to be written (radar derived level information) and now it is changed to (pressure-altitude-derived-level information).
As far as I know, pressure-altitude is shown to the pilot when the standard pressure of 1013.25 hpa is set on the altimeter. Does it mean that the level information which is shown to us on our radar screens (SSR) is based on 1013.25 only? I mean what about the case of QNH? We receive level information of a/c who are flying on AREA QNH not 1013.25 hpa!
Join Date: Apr 2009
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The change is only to accommodate new technology's such as ADS-B.
Aircraft have always only sent pressure alt based on 1013.25 and the correction for QNH is applied by the ATS System being used.
So you should see no difference for level readouts on radar.
Aircraft have always only sent pressure alt based on 1013.25 and the correction for QNH is applied by the ATS System being used.
So you should see no difference for level readouts on radar.
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All Mode C and Mode S Pressure Altitude reports from transponders are referenced to 1013.25mB regardless of QNH in the region or zone the aircraft is flying. This ensures that aircraft remain vertically separated regardless of pressure variations along routes. As such, above transition level, everyone uses Flight Levels.
Below transistion level, the transponder still uses 1013.25mB, no matter what the pilot has the QNH set to. Many (majority of?) display systems used by ATC will convert the 1013.25 referenced Flight Levels into an Altitude based upon a QNH setting, displaying the altitude to the controller which should be close to what the aircraft is flying to.
This change to ICAO, as mentioned below, is to accommodate other future sources of altitude / Flight Level information that may be "radar derived" but are not from pressure altitude source.
If any of the above isn't quite correct, I'm sure someone will correct me, after all I am only an engineer
RS
Below transistion level, the transponder still uses 1013.25mB, no matter what the pilot has the QNH set to. Many (majority of?) display systems used by ATC will convert the 1013.25 referenced Flight Levels into an Altitude based upon a QNH setting, displaying the altitude to the controller which should be close to what the aircraft is flying to.
This change to ICAO, as mentioned below, is to accommodate other future sources of altitude / Flight Level information that may be "radar derived" but are not from pressure altitude source.
If any of the above isn't quite correct, I'm sure someone will correct me, after all I am only an engineer
RS
