What is the lowest usable flight level when the local area altimeter is between 28.91 and 27.92?

• A. FL200
• B. FL180
• C. FL250
• D. FL360

Answer: (A)

The concept here is how flight levels are tied to a standard pressure reference and how local pressure (QNH) affects what levels you can safely use. Flight levels assume standard pressure (29.92 inHg). When the local altimeter setting is low, the difference between the indicated flight level and the true altitude becomes larger, so to ensure adequate obstacle clearance you raise the minimum usable flight level.

With the local area altimeter between 28.91 and 27.92 inHg, the minimum safe flight level is raised to FL200. In these conditions, using any lower flight level could compromise vertical separation from terrain or obstacles, so FL200 is the lowest usable option.