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I commented on the previous question "Airspace Question" and thought I would ask this question. What is the difference between a flight level and a MSL altitude. For example, the difference between FL180 and 18,000ft. There is a regulation in Part 91 that deals specifically with this difference. Can you cite the regulation? |
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Mean Sean Level is based on the local barometric pressure, while the Flight Levels are defined using the international standard altitude (isa) of 29.92. I believe Flight Levels are used for Class A because the primary concern above 18,000 ft isn't terrain avoidance, but keeping very fast aircraft from slamming into each other. The regulation that deals with flight levels is Part 91.121 (Altimeter Settings). It looks like the lowest usable flight level isn't always 180, if the pressure is lower and you have to set your altimeter lower than 29.92, the lowest usable flight level increases. There's also an adjustment that has to be made to convert your altitude in MSL to the Flight Level altitude when you make the transition based on the current altimeter setting. |
I am stumped
@Sean C. What's the major event that happens as you enter Class A airspace at 18,000ft? I think this is an important concept as Flight Levels start at a much lower altitude in the rest of the world. Most of England has a FL50 (5000ft msl). On most US instrument plates, you will see the words, "Transition Level FL180" meaning the same I am getting at here.