If the altimeter is 28.89, what is the lowest usable flight level?

If the altimeter is 28.89, what is the lowest usable flight level?

• A. 190
• B. 180
• C. 200
• D. 210

Answer: (A)

Flight levels are based on the standard pressure setting of 29.92 inHg. When the local altimeter setting is lower than that, the altitude that corresponds to a given flight level is actually lower in true terms, roughly by 1000 ft for each inch of mercury difference. Here, 29.92 minus 28.89 equals about 1.03 inHg, which is roughly a 1,030 ft difference. That means the lower flight levels are effectively shifted downward, so to maintain required separation from terrain and airspace floors you use the next higher level. The lowest usable level under these conditions is FL190 (19,000 ft).

Flight levels are based on the standard pressure setting of 29.92 inHg. When the local altimeter setting is lower than that, the altitude that corresponds to a given flight level is actually lower in true terms, roughly by 1000 ft for each inch of mercury difference. Here, 29.92 minus 28.89 equals about 1.03 inHg, which is roughly a 1,030 ft difference. That means the lower flight levels are effectively shifted downward, so to maintain required separation from terrain and airspace floors you use the next higher level. The lowest usable level under these conditions is FL190 (19,000 ft).