What altimeter setting should a pilot use when descending through FL 150?

When descending through FL 150, a pilot should use QNH as the altimeter setting. QNH is the altimeter setting that relates to sea level pressure and is used when flying below the transition altitude, which is typically the altitude at which pilots switch from using flight levels (FL) to using actual altitude in feet.

At flight levels (above the transition altitude), altimeters are typically set to 1013.2 hPa (standard pressure) to ensure a common reference for aircraft altitudes. However, when descending below this altitude – in this case, when descending through FL 150 – the pilot needs to switch to QNH to accurately determine their height above sea level, especially when approaching an airport or flying in controlled airspace.

Using QNH provides a correct altitude reading that correlates directly with the elevation of the terrain and any air traffic control requirements. This is crucial in ensuring safe navigation and adequate vertical separation from obstacles and other aircraft. Thus, QNH is the appropriate setting for altimeters during this phase of descent.