Decision Altitude vs Minimum Descent Altitude

DA vs MDA

What is the difference between a DA and an MDA? A DA or a Decision Altitude is used on a precision or precision like approach. This is when a glide slope or glide path is used to descent from the final approach fix inbound. Like on an ILS. The DA is minimums for these types of approaches and is the altitude the pilot must make the decision to continue descent to land or go missed. An MDA or Minimum Descent Altitude is used on a non-precision approach and is the lowest altitude a pilot can descent to on that approach (the minimums) before hitting the Visual Descent Point or the Missed Approach Point. The Visual Descent Point is the earliest point on the approach the pilot can descend to land if runway environment is in sight, and the Missed Approach Point is the last point the pilot can descend to land without having to go missed. So, the pilot must maintain the MDA until at least the VDP if the runway is in sight or until the MAP if not.

 

So, when can you descend below?

According to 14 CFR 91.175…

  1. The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers
  2. The flight visibility is not less than the visibility prescribed in the standard instrument approach being used
  3. At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
    1. The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.
    2. The threshold
    3. The threshold markings
    4. The threshold lights
    5. The runway end identifier lights
    6. The visual glideslope indicator
    7. The touchdown zone or touchdown zone markings
    8. The touchdown zone lights
    9. The runway or runway markings
    10. The runway lights

 

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