Topic summary
Decision altitude
Extracted from the Wikipedia article Instrument approach.
Decision height or altitude
In a precision approach, the decision height (DH) or decision altitude (DA) is a specified lowest height or altitude in the approach descent at which, if the required visual reference to continue the approach (such as the runway markings or runway environment) is not visible to the pilot, the pilot must initiate a missed approach. (A decision height is measured AGL (above ground level) while a decision altitude is measured above MSL (mean sea level).) The specific values for DH and/or DA at a given airport are established with intention to allow a pilot sufficient time to safely re-configure an aircraft to climb and execute the missed approach procedures while avoiding terrain and obstacles. While a DH/DA denotes the altitude at which a missed approach procedure must be started, it does not preclude the aircraft from descending below the prescribed DH/DA.