Unicom

September 2009 Issue

Decisions, Decisions

I am a longtime subscriber to Aviation Safety and recommend your publication to my clients as the best aviation magazine out there. However, in your August issue, the article on IFR GPS had what I believe were a few errors in an otherwise informative article. The author describes DH (decision height) as a height above sea level and DA (decision altitude) as "above runway." This explanation is likely to confuse people trying to understand this concept because it is the opposite of the correct definition. Instead, DH is an agl altitude and DA based on msl. The FAA describes it similarly, using a few more words, as follows: "Decision Altitude/Height (DA/H): A specified altitude or height (A/H) in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. Note 1: Decision altitude (DA) is referenced to mean sea level (MSL) and decision height (DH) is referenced to the threshold elevation." The other statement with the potential to cause confusion was, "a WAAS box will automatically suspend when you pass the missed approach point," which might lead one to think that a non-WAAS GPS will not. In fact both non-WAAS and WAAS GPS navigators suspend when passing the MAP.

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