For a CAT II ILS approach in the C-17, how is the decision height (DH) determined?

In a CAT II Instrument Landing System (ILS) approach for the C-17, the decision height (DH) is determined based on the radar altimeter setting. The radar altimeter provides precise measurements of height above the terrain, which is crucial for safe landings in low-visibility conditions, as it directly measures the altitude above the ground rather than relying on atmospheric pressure changes that can affect barometric altimeters.

Utilizing the radar altimeter allows pilots to establish a specific DH that accounts for the actual height above the runway when executing the approach. This is particularly important in CAT II operations where very low visibility conditions exist, and maintaining accurate height information is vital for making timely and safe decisions regarding landing.

While other altimeter systems are useful for various flight operations, they do not provide the level of precision needed for determining the DH in CAT II approaches. The radar altimeter’s capacity to deliver accurate height information regardless of external atmospheric conditions makes it the preferred method for determining DH during these critical phases of flight.