True altitude is best defined as the aircraft's height above

• A. The local terrain
• B. Mean Sea Level
• C. The air mass
• D. The ground level

Answer: (B)

True altitude is the actual vertical distance of the aircraft above a fixed reference, mean sea level. This standard reference lets pilots and air traffic control compare positions consistently regardless of terrain or location, which is essential for obstacle clearance and proper separation. The altimeter is adjusted to reflect height above MSL when set correctly to the local pressure (QNH), so the indicated altitude aligns with the true altitude under standard conditions. Temperature deviations can cause true altitude to differ from the indicated value, but the reference point remains mean sea level. The other options describe heights relative to the terrain, the air itself, or the local surface, which are not fixed global references.