Definition
The lowest altitude, expressed in feet above mean sea level (MSL), to which descent is authorized on final approach or during circle-to-land maneuvering when executing a non-precision instrument approach. The aircraft must not descend below the MDA unless the required visual references for the intended runway are in sight and the aircraft is in a position to make a normal landing.
Plain English
On a non-precision instrument approach, the lowest altitude you are allowed to fly down to while looking for the runway. You stay at or above this altitude until you can clearly see the runway environment and are set up to land normally.
Context Anchor
Seen on instrument approach charts, especially for non-precision approaches and circling approaches.
Derivation
Minimum (Latin minimus, 'least'), Descent (Latin descendere, 'to climb down'), and Altitude (Latin altitudo, 'height'). Together: the least height to which you may climb down. The wording is literal — it is the floor of your descent until you can see what you need to see.
Why Pilots Care
Sets the exact altitude at which a missed approach must begin if the runway environment is not visible, protecting against controlled flight into terrain.
Analogy
Think of MDA as a hard floor during the approach. You may descend down to it, but you do not go through it unless the landing area is safely in sight and you can continue normally.
Intuition Check
MDA is not a target that you must always reach, and it is not permission to keep descending. It is a lower limit: descend no lower than MDA unless the landing requirements are met.
Example Sentence 1
The approach chart showed an MDA of 1,240 feet, so the pilot leveled off there and began scanning for the runway environment.
Example Sentence 2
Reaching MDA without the required visual references means immediate execution of the missed approach procedure.