This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
DA/DH
Decision Altitude/Decision Height
Decision Altitude (DA) and Decision Height (DH) are the altitudes or heights at which a pilot flying a precision or approach with vertical guidance must decide to either continue the approach to landing or initiate a missed approach. If adequate visual reference has not been established by the DA/DH - meaning the pilot cannot see the runway environment clearly enough to land safely - a missed approach must be immediately executed. Decision Altitude is referenced to mean sea level (MSL) and is used for approaches based on barometric altimetry, while Decision Height is referenced to the runway threshold elevation and is typically used for ILS approaches with radio altimeter guidance.
The specific DA/DH for an approach is published on the instrument approach chart and depends on the ILS category, the aircraft’s equipment, the operator’s approved minimums, and the airport’s ground infrastructure. CAT I ILS approaches typically have a DA/DH of 200 ft, while CAT II approaches may have a DH as low as 100 ft, and CAT III approaches can have DH values below 50 ft or no DH at all (for fully automatic landings). Below the DA/DH, the approach must not be continued unless sufficient visual reference is maintained. The required visual references are defined in the relevant regulations and typically include runway lighting, threshold markings, or approach light systems.