July 1, 2011
When IFR GPS broke onto the scene nearly two decades ago, it caused a sea change in the way approaches could be flown into airports too small to have an ILS. Rather than conducting an approach referencing a relatively inaccurate VOR or NDB signal, every airport in the country could be approached with localizer accuracy. What the first GPS did for lateral navigation, WAAS has done for vertical navigation. Yet, pilots still aren’t taking full advantage of WAAS capability, and some training providers are teaching procedures that hobble a major safety benefit of WAAS.
The issue is the ability of WAAS-capable Garmin navigators to create vertical guidance as part of most GPS approaches without published WAAS minimums. This advisory vertical guidance turns an LNAV-only approach into an “LNAV+V” approach. Where “+V” vertical guidance is available, the autopilot is fully capable of coupling to the vertical path, as if the approach was a full-blown LPV or ILS. Yet many pilots conduct this type of approach in the same manner they would a VOR or NDB—setting the MDA in the altitude selector and using vertical speed mode to reach MDA. The presence of vertical guidance on an LNAV approach doesn’t change the MDA nor make it an official APV (approach with vertical guidance) in the eyes of the FAA. However, there’s no reason not to use the same procedure to fly the approach as if it were an LPV, coupling vertically via the glidepath mode, and never leveling off at MDA (assuming runway in sight).
Flying an LNAV+V in the manner of a precision rather than nonprecision approach offers several safety benefits. The most important is that the aircraft can be on speed and configured to land outside the FAF, with no further configuration or power changes required. Wherever the pilot may be when visual contact with the runway is acquired, the aircraft will be on a stable path to the touchdown zone. In contrast, a pilot who dives down to the MDA at a high rate of descent may visually acquire the runway before the aircraft is in a position to resume a descent. Deciding when it’s safe to leave MDA can require quick thinking at a time when the aircraft is very close to terrain—not a desirable situation. Because level-off at MDA is not required, the altitude preselector can be set to the level-off altitude of the missed approach procedure once the FAF is passed. Forgetting to set the missed-approach altitude after level-off at MDA is a common error made by transitioning jet pilots, and beginning the missed without the correct altitude set can lead to undesirable autopilot behavior or an ATC violation.
There is a trick to turning an MDA into a DA. Built into a published DA is consideration that if a pilot looks up at DA, the aircraft will still be traveling downwards while the pilot makes the decision to go missed, adds power, and transitions to a climb. An MDA is what it says—a minimum altitude below which the pilot musn’t go unless landing. Add padding to the MDA so that even if it takes a few seconds to get the airplane climbing, MDA will not be violated.
A light jet flying on a three-degree descent path will be descending about 550 fpm, or a little more than nine feet per second. Assuming a worst-case scenario of five seconds to transition from descent to climb and rounding up, 50 feet results. So adding 50 feet to the published MDA results in the new functional DA—the altitude at which the missed approach will commence, and which should be bugged as “minimums.”
Neil Singer is a Master CFI and a mentor pilot in Cessna and Embraer jets.
Safety and Education,
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