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Brian
Moderators: Guru's, The Ministry
I'm sorry he askedPeteP wrote: ↑03 Jul 2017, 08:02Ahhh, altimetry - my favourite subject, Mike.![]()
You're right when you say that the aircraft's altitude may be different from its flight level. In simple terms, altitude is defined as the vertical distance above sea level and a flight level is the vertical distance above the 1013.2 hPa pressure line. So, if the sea level pressure is 1013.2, then the aircraft's altitude and flight level - in this case, 6000ft and FL60 - will be the same. If the pressure at sea level is higher than the 1013.2, then FL60 will be higher than than 6000ft and conversely, if the seal level pressure is lower than 1013, FL 60 will be lower than 6000ft.
To give an example using the (very rough) rule of thumb that 1 hPa (used to be millibars) = 30 ft -
Sea level pressure 1003 - FL60 is at 5700ft amsl (the 1013 hPa pressure datum is 300ft below sea level)
Sea level pressure 1013 - FL60 is at 6000ft amsl (the 1013 hPa pressure datum is at seal level)
Sea level pressure 1023 - FL60 is at 6300ft amsl (the 1013 hPa pressure datum is 300ft above sea level)
This can make a big difference when operating in areas such as the London TMA where controllers are using both altitudes and flight levels - if the pressure is 1013 or above then 6000ft is separated by the minimum required 1000ft from FL70 but if the pressure is below 1013, the minimum usable flight level (to give at least 1000ft) becomes FL80.
Pete
Hi Eric,