Topic of the Day: Approach Minimums
Is this on the test? You bet!
Decision Altitude (DA)
Is a specified altitude on a precision approach at which a missed approach must be initiated if the required visual references to continue the approach have not been established.
Decision altitude is charted in feet above mean sea level and is read on the altitude tape.
Decision Height: (DH)
Is a specified altitude on a precision approach, charted in height above threshold elevation, radio altitude above ground level at which a decision must be made either to continue the approach or to execute a missed approach.
Decision height is read on the radio-altimeter.Decision heights are normally associated with CATII and CATIII approaches.
Alert Heights (AH)
The FAA and ICAO define alert height as the height above a runway, based on airplane fail operational systems, above which a CAT III approach must be discontinued and a missed approach initiated if a failure occurs in one of the redundant parts of the flight control or related aircraft systems, or if a failure occurs in any one of the relevant ground systems.Above Alert height, if lost system redundancy results in a downgrade of the airplane’s capability, the crew must execute a missed approach.Alert height is read on the radio-altimeter.
The Minimum Descent Altitude (MDA) or Minimum Descent Height (MDH) is a specified altitude or height in a Non-Precision Approach or Circling Approach below which descent must not be made without the required visual reference.
MDA is referenced to mean sea level and MDH is referenced to the aerodrome elevation or to the threshold elevation if that is more than 2 m (7 ft) below the aerodrome elevation. An MDH for a circling approach is referenced to the aerodrome elevation.
The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path. In the case of a circling approach the required visual reference is the runway environment.
