Definition
The lowest altitude, expressed in feet above mean sea level (MSL), to which a pilot is authorized to descend on a non-precision instrument approach when no electronic glideslope is provided. The aircraft may not descend below the MDA unless the required visual references for the runway environment are in sight and the aircraft is in a position to make a normal landing.
Plain English
On an approach without a vertical guidance beam, this is the lowest altitude you're allowed to fly down to while looking for the runway. If you can't see the runway clearly by the time you reach this altitude, you cannot go any lower — you level off and fly along at that altitude until either you spot the runway or it's time to go around.
Context Anchor
Seen on instrument approach charts, especially in the profile view and minimums information for nonprecision approaches and some circling procedures.
Derivation
The phrase is built from plain English, but each word is doing real work. 'Minimum' means the floor — you cannot go lower. 'Descent' tells you it applies during the descent portion of the approach. 'Altitude' (from Latin 'altus,' meaning 'high') refers to height above mean sea level, not above the ground. So MDA is a hard floor, measured from sea level, that limits how far down you may descend during the approach.
Why Pilots Care
It sets the hard limit for how low you can go without visual contact with the runway, directly affecting whether you can land or must execute a missed approach.
Grounding Statement
Think of MDA as a hard floor on the approach until the pilot has the required outside visual cues to safely continue lower.
Intuition Check
MDA is not a target to descend through. It is a published lower limit: descend to it only when authorized, and do not go below it unless the required landing conditions are met.
Example Sentence 1
The approach chart showed an MDA of 1,240 feet, so the pilot leveled off at that altitude and continued toward the missed approach point while looking for the runway.
Example Sentence 2
Once we reached MDA with the airport in sight we continued the landing; if the runway had remained hidden we would have gone missed at the MAP.