Lowest usable flight level when local area of jurisdiction altimeter is 29.91 to 28.92

Lowest usable flight level when local area of jurisdiction altimeter is 29.91 to 28.92

• A. FL200
• B. FL195
• C. FL190
• D. FL180

Answer: (C)

Flight levels are based on standard pressure, but if the local altimeter setting can deviate by up to about 1 inHg from standard, the actual altitude corresponding to a given flight level can differ by roughly 1000 ft. Since flight levels are spaced by 1000 ft, you must choose a level high enough that even in the worst case this potential 1000 ft error doesn’t put you into the space of the next lower level. Here, the local altimeter range is 29.91 to 28.92 inHg, a maximum difference of about 1.00 inHg from standard, equating to roughly 1000 ft. Therefore, the lowest usable flight level is the one that remains safely above the next lower level when this offset is applied, which is 19,000 ft (FL190). Going any lower could allow the worst-case error to intrude on the space of the lower level.

Flight levels are based on standard pressure, but if the local altimeter setting can deviate by up to about 1 inHg from standard, the actual altitude corresponding to a given flight level can differ by roughly 1000 ft. Since flight levels are spaced by 1000 ft, you must choose a level high enough that even in the worst case this potential 1000 ft error doesn’t put you into the space of the next lower level. Here, the local altimeter range is 29.91 to 28.92 inHg, a maximum difference of about 1.00 inHg from standard, equating to roughly 1000 ft. Therefore, the lowest usable flight level is the one that remains safely above the next lower level when this offset is applied, which is 19,000 ft (FL190). Going any lower could allow the worst-case error to intrude on the space of the lower level.