For an altimeter setting between 29.91 and 28.92, what is the lowest usable flight level?

The lowest usable flight level for an altimeter setting between 29.91 and 28.92 is FL190. This is determined based on the altimeter settings established for safe altitude management in air traffic control and for pilots in flight.

When the altimeter setting is below 29.92 inches of mercury (Hg), the minimum flight level requires adjustment as lower altimeter settings indicate lower atmospheric pressure.

According to the regulations and practices used within the aviation industry, specifically the FAA guidance and procedures, flight levels are structured to ensure vertical separation of aircraft. The minimum usable flight level for altimeter settings between 29.91 and 28.92 is established at FL190 to maintain this necessary separation and safety margin. When the altimeter settings drop below 29.92, pilots must operate at higher flight levels to account for potential terrain and to prevent loss of separation between different aircraft.

In this specific scenario, FL190 is the appropriate response because it provides the necessary buffer and ensures compliance with altitude reporting and safety requirements when operating under those specific altimeter conditions.