Definition
The lowest altitude, expressed in feet above mean sea level, to which a pilot is authorized to descend on a non-precision straight-in instrument approach until the required visual references for the runway are in sight and a normal landing can be made. The aircraft must not descend below this altitude unless those visual references are seen and other landing requirements are met.
Plain English
On an instrument approach lined up with the runway, this is the lowest altitude you're allowed to fly down to using your instruments alone. You stay at or above it until you can actually see the runway environment well enough to land normally.
Context Anchor
Seen on instrument approach procedures and in cold temperature correction tables when checking whether the published approach altitude needs to be corrected for very cold air.
Derivation
"Straight-in" means the final approach is aligned with the runway, so no circling maneuver is required. "Minimum Descent Altitude" comes from "minimum" (lowest allowed), "descent" (going down), and "altitude" (height above sea level). Together: the lowest height you may descend to on a straight-in non-precision approach.
Why Pilots Care
Sets the safe limit for continuing the approach; descending below it without visual contact risks hitting obstacles.
Grounding Statement
Think of this as the lowest safe altimeter reading for that direct approach unless you can clearly continue to the runway and land.
Intuition Check
Straight-in does not mean you must fly a perfectly straight line from far away; it means the approach is intended to lead directly to a runway landing instead of circling to another runway. Minimum does not mean a target altitude to beat; it is a lower limit you must not go below unless landing can safely continue.
Example Sentence 1
Cleared for the VOR Runway 27 approach, the pilot descended to the straight-in minimum descent altitude of 1,240 feet and held that altitude until the runway came into view.
Example Sentence 2
Apply the cold temperature correction to the straight-in minimum descent altitude before starting the final descent.