Originally Posted by
BuzzBox
The TL is FL110 with QNH 980 or above, FL120 with QNH 979 or below. It's to maintain a vertical separation of at least 1000ft between aircraft using QNH and those using QNE (1013) when the QNH is very low, eg during the passage of a typhoon.
If you recall, a pressure difference of 1 hPa is approx 30ft. Ergo, if the QNH is 980, the vertical separation between an aircraft at the TL and one at the TA is (1013 - 980) hPa x 30 ft/hPa = 1000 ft (approx). If the QNH is less than 980, the separation between an aircraft at 9,000 ft on QNH and one at FL110 on QNE would be less than 1,000 ft. Hence a higher TL is used when the QNH is very low.
Thanks.
I'm just curious why the more complicated procedure instead of the straight forward single transition level that is used in much of the world.
As we all know...FL 180 in North America is the transition level, if the altimeter is below standard, then FL 170 not available from ATC. I'm sure both methods achieve the same thing.
Or the Philippines which has a space between the transition level and altitude like Hong Kong but still only uses FL130 for transition level.