A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2020 Sep 25, 12:59 -0700
Gary wrote: Yep, but "flight levels" in the U.S. and in Europe and (I believe) in the rest of the world, are still spaced one-thousand FEET apart to provide safe separation in altitude between aircraft flying in clouds or above the "transition altitude," which in the U.S. is 18,000 feet and at various altitudes in other parts of the world, and 5,000 FEET over the ocean. One hundred meters is too close and one thousand meters is too far appart which would limit flight capacity. Below the "transition level" you have concerns with bumping into mountains so the various countries set their "transition altitudes" to be high enough to clear all their terrain. When flying in the flight levels all aircraft set their altitimeters to "QNE", 29.92 INCHES of mercury or 1,013.2 millibars (depending on the setting scale in your altimeter,)
It all gets very complicated (E.g. above FL290, Semi-Circular FLs are separated by 2000’ and only odd FLs are allocated), and procedures differ around the World. They're best looked up in current flight information documents, which is why first officers walk around carrying such huge black briefcases. It’s not just machismo or to keep the centre of gravity in the correct place. Wikipedia takes a reasonable crack at it, see Wikipedia-Flight Level.
In the UK Transition Altitude (which applies in the climb) can be as low as 3000ft AMSL which can be a bit of a problem flying in a high-performance aircraft. It’s also frequently below your Safety Altitude (that altitude below which you must not descend unless in visual contact with the ground or under positive radar control), so you need to convert that to a Minimum Safe Flight Level. Examiners are never short of question material. Transition Level (which applies in the descent) depends on the surface pressure, but it’s calculated so that it’s always a small amount higher above the ground than the Transition Altitude to save flight level traffic getting mixed up with altitude traffic. DaveP