The Decision Altitude (DA) or Decision Height (DH) is a crucial point during a Precision Approach. It marks the altitude at which a pilot must decide to initiate a Missed Approach if they cannot see the necessary visual references to continue landing safely. According to ICAO Annex 6, DA is measured relative to mean sea level, while DH is based on the elevation of the runway threshold.
For Category II and III approaches, pilots rely on a radio altimeter to determine DH, making it impossible to use a barometric altimeter for these operations. As a result, the minima is expressed as DH only. However, for approaches with a DH of 200 feet or higher, the radio altimeter may not provide reliable readings due to terrain variations. In these cases, a barometric altimeter is used, allowing for the minima to be expressed as either DH or DA.
The required visual reference refers to the visual aids or approach area that must be visible long enough for the pilot to assess the aircraft’s position and trajectory concerning the desired flight path. In Category III operations, this visual reference is specific to the procedure being followed. To simplify, both terms can be combined and abbreviated as “decision altitude/height” or “DA/H.”
Pilots must start a Missed Approach at the DA/H unless the required visual reference has been established. Calculating the DA/H considers that the aircraft might descend below this point during a Missed Approach. For further details on calculating DA/H, refer to Aerodrome Operating Minima (AOM).
