Minimum Descent Altitude (MDA) is the lowest altitude a pilot may descend to on a non-precision instrument approach. The pilot must have the runway environment clearly in sight at this altitude to continue descending and land.
How It Works#
A non-precision approach is an instrument approach procedure that provides lateral (left-right) guidance but no vertical (up-down) glidepath. Examples include the VOR approach, NDB approach, and LNAV approach. Because there is no electronic glidepath to follow, the procedure uses an MDA instead of a Decision Altitude (DA), which is the equivalent limit used on precision approaches.
The pilot descends to the MDA and then flies level at that altitude. This level segment is sometimes called the "dive and drive" phase. The aircraft holds the MDA while the crew scans ahead for the runway environment, which includes the runway itself, approach lighting, or other visual references listed in 14 CFR §91.175.
If the required visual references are not in sight by the Missed Approach Point (MAP), the pilot must execute a missed approach. The MAP is a specific geographic fix or a timing point, depending on the procedure. Flying below the MDA without the runway environment in sight is a serious violation and a significant safety risk.
MDA values are published on the instrument approach procedure chart for each airport and runway. The value is expressed in feet MSL (mean sea level, the standard aviation altitude reference). Different aircraft categories (A, B, C, D) may have different published MDAs based on approach speed.
Example in Aviation#
A Cessna 172 pilot is conducting a VOR approach to a small regional airport in IMC (instrument meteorological conditions, meaning clouds and reduced visibility). The published MDA for that approach is 1,400 feet MSL. The pilot descends to 1,400 feet, levels off, and flies toward the MAP.
At the MAP, the pilot spots the approach lighting system and the runway threshold ahead. Because the runway environment is in sight and the conditions meet the visibility minimums, the pilot continues descending and lands normally. If the runway had remained hidden in cloud, the pilot would have immediately climbed and flown the published missed approach procedure.
Why It Matters#
MDA is a hard floor. Descending below it without visual contact is not a judgment call. It is one of the clearest bright lines in instrument flying, and understanding it is essential for any pilot training toward an instrument rating.
For aviation enthusiasts and students, grasping the difference between MDA and DA clarifies the fundamental split between non-precision and precision approaches. That distinction shapes how procedures are designed and why some approaches require better weather minimums than others.
Key Takeaways#
- MDA is the lowest altitude allowed on a non-precision instrument approach.
- The pilot must fly level at the MDA until the runway environment is visible.
- Descending below the MDA without visual references is prohibited.
- The Missed Approach Point marks the last chance to identify the runway environment.
- MDA values are published in feet MSL on the approach chart, by aircraft category.