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Hello Capt Lim,
Just another quick query that may not be so interesting to the general public but I am rather curious about....
My question is simply about the setting of the altimeter. I am aware that at flight level, the altimeter is set to ISA (1013/29.92).
Am I correct in saying that it is then set to QNH at 18000ft during the descent?
Most importantly, what I am curious about is, at which point in the approach do you set the altimeter to the airport height (QFE) to be able to know how high from the runway you are to complete an accurate touchdown? How accurate is it?
Also at which point after take off do you normally reset to local QNH?
Hope you can take time to answer, Happy flying as always.
Rob Hirons,
Marseille, France.
Hi Robert,
The QNH is always set when passing the transition level. The transition level varies from region to region. The transition level in the USA is 18000 feet and in France, it is given by the ATC, depending on the barometric pressure for that particular day. You are only correct if you are descending for a landing in any US Airports, whereby you set the QNH only passing through 18,000 feet. In France, it is different. After take off, you set the QNH after passing the transition altitude fixed at 4000 feet. On the descent, you would be notified on that day what the transition level is and you would then set the
QNH at, say 6000 feet.
QFE is used mainly in the Russian Federation, China and certain regions of the Middle East and Africa for transit flights. QFE is also used for circuit flying, especially for training purposes. It is rarely used in civil aviation today. On the approach, you set the QFE when you are given the reading by the Control Tower as you establish contact with them. If you are doing circuits and landings training, you set the QFE on the ground.
How accurate is it? When on the ground at the airport giving the QFE, the altimeter shows 0 feet. It is accurate as long as the QFE remains steady but you know the barometric pressures do change from time to time. The Control Tower will usually update the pilots when the QFE changes.
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