Aircraft altitude is defined as the:

• A. The actual height above mean sea level at which the aircraft is flying.
• B. The altitude above ground level.
• C. The altitude of the cockpit floor.
• D. The altitude an aircraft maintains during cruise.

Answer: (A)

Altitude is the vertical distance of an aircraft above mean sea level. This standard reference is what pilots use for most communications and flight planning, read from the altimeter with the local pressure setting applied. It differs from height above ground level, which would measure height relative to the terrain directly beneath the aircraft and can change with terrain. The cockpit floor height isn’t the basis for describing how high the aircraft is in air navigation. While pilots may talk about cruising at a specific altitude or flight level, that describes a target value rather than the basic definition itself. Remember that, in practice, terms like indicated altitude and true altitude come into play, but the fundamental concept remains: altitude is height above mean sea level.