When do you switch to local QNH during descent?

Switching to local QNH during descent is crucial for maintaining accurate altimeter settings and ensuring safe altitude separation. The correct answer reflects the standard practice in aviation operations regarding the transition level.

The transition level is the lowest flight level available for use above the transition altitude, which is the altitude at which aircraft switch from using the altimeter setting based on local atmospheric pressure (QNH) to the standard pressure setting of 1013.25 hPa (or 29.92 inHg). Below the transition altitude, aircraft should set their altimeters to the local QNH to reflect actual air pressure conditions on the ground. This ensures that pilots maintain appropriate altitude levels during descent, preventing any risk of collision with terrain or other aircraft.

Implementing this practice at the transition level helps pilots avoid misjudging their altitude above sea level due to pressure changes, thereby enhancing overall safety. In contrast, the other options do not align with standard aviation procedures for switching altimeter settings. For instance, switching at 10,000 feet or when entering controlled airspace may not accurately reflect regional atmospheric pressure changes, which could lead to safety issues in altitude awareness.