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Aero-Tips 02.04.06

There’s been a subtle yet dramatic change in philosophy regarding descent to Minimum Descent Altitude (MDA) in non-precision approaches. Most IFR pilots have been taught to descend expeditiously to MDA, then fly level to the Missed Approach Point (MAP) while looking for the runway. Call this the "dive and drive" method.

In recent years there’s been a move to teach (and evaluate) a shallow, constant descent from the Final Approach Fix (FAF) or last step-down to MDA, similar to that associated with an Instrument Landing System (ILS) glideslope. Let’s call this the "glide" method.

Advantage of the Glide

The biggest advantage of the glide method is that it makes descent below MDA less likely, because there is no duration at which the flight is exposed to flight at this altitude—hit MDA and either proceed visually or immediately miss the approach. It allows for a "stabilized" approach. Modern GPSs can compute the precise rate of descent to arrive at MDA at the MAP given actual wind conditions (although they cannot predict changes in winds during descent).

Advantage of Dive and Drive

By getting the airplane to MDA sooner, hopefully below the bases of clouds, the dive and drive method permits visual acquisition of the runway sooner, with a greater chance of landing success when the field is at minimums.

Disadvantages

Biggest disadvantage of the glide method is that it puts the airplane at 500 feet AGL or higher at the MAP…often over the runway threshold, midfield or even at the far end of the runway. A "glide method" approach to near minimums may require circling to safely land—meaning in practice minimums may be higher in a glide method approach than a traditional "dive and drive" approach.

Sure, there’s a bigger chance the undisciplined pilot will "bust minimums" in the dive and drive method, but with practice and attention this can easily be avoided. 

Aero-tip of the day: Recognize the advantages and disadvantages of opposing philosophies toward non-precision approach descents—and choose the method that best fits conditions.

FMI: Aero-Tips