Minimum Descent Altitude (MDA) is expressed in which datum on instrument approaches?

Minimum Descent Altitude (MDA) is expressed in which datum on instrument approaches?

• A. MSL
• B. AGL
• C. Both AGL and MSL
• D. Local height

Answer: (A)

Minimum Descent Altitude is the lowest altitude you may descend to on a non-precision instrument approach, and it is always stated in feet above mean sea level. This datum is used because the altimeter setting you rely on (the local QNH) gives you altitude relative to sea level, so the published minimums apply consistently regardless of the airport’s elevation. Using MSL avoids tying the minima to the terrain height at each airport, which would vary widely and could compromise obstacle clearance. For example, if the field elevation is 421 ft MSL and the published MDA is 540 ft MSL, you may descend to 540 ft MSL. That equals about 119 ft above the airport (roughly 119 ft AGL) at that point. You must remain at or above the MDA until you have the required visual references to land.

Minimum Descent Altitude is the lowest altitude you may descend to on a non-precision instrument approach, and it is always stated in feet above mean sea level. This datum is used because the altimeter setting you rely on (the local QNH) gives you altitude relative to sea level, so the published minimums apply consistently regardless of the airport’s elevation. Using MSL avoids tying the minima to the terrain height at each airport, which would vary widely and could compromise obstacle clearance.

For example, if the field elevation is 421 ft MSL and the published MDA is 540 ft MSL, you may descend to 540 ft MSL. That equals about 119 ft above the airport (roughly 119 ft AGL) at that point. You must remain at or above the MDA until you have the required visual references to land.