How is the height of an aircraft generally measured using QNH?

The correct choice is based on how QNH, which stands for "aeronautical pressure" setting, is used in aviation. QNH is a pressure setting that allows pilots to adjust altimeters to reflect the height of an aircraft in relation to Mean Sea Level (MSL). When pilots set their altimeter to the QNH value reported by air traffic control or taken from a local weather station, they are effectively calibrating their instruments to measure altitude above MSL.

This is essential because MSL provides a standard reference point that is universally applicable, regardless of local terrain variations. Knowing the altitude in relation to MSL helps ensure safe navigation, especially in mountainous or varied landscapes, where flying at a specified height above MSL is critical for avoiding terrain and ensuring compliance with aviation regulations.

Other measurement methods, such as altitude Above Ground Level (AGL) or flight levels (FL), serve different purposes. AGL is more context-specific and varies based on the height of the terrain, while flight levels are used in high-altitude flight and are measured in hundreds of feet above the standard pressure setting of 29.92 inHg, not directly related to QNH.