Page 14 - Volume 10 Number 8

leveled problems can occur. Most common with WAAS units is not a GPS navigation failure, but a downgrade in capabilities. WAAS operations require an increasingly precise level of GPS guidance for the various types of WAAS approaches. When a WAAS receiver determines it cannot meet the tolerances of, say, a Localizer Precision with Approved Vertical Guidance (LPV) procedure, it will then determine if it can meet the tolerances of a lesser type of GPS approach. If it can, it will “downgrade” and advise the pilot of the highest tolerances it is capable of. Most common would be a downgrade from LP or LPV capabilities to LNAV-only capabilities. In many such cases, the same approach can still be flown, but the higher LNAV minimums must be respected (due to the less precise lateral guidance and the lack of approved vertical guidance). That assumes, of course, the approach in question publishes both LP(V) and LNAV minima. If not, it is likely that a separate approach exists with LNAV minimums. Plus, while many WAAS avionics incorporate for-reference-only vertical guidance into LNAV procedures (LNAV+V), both approved and reference-only vertical guidance is removed whenever the system determines a downgrade is required. It is imperative that WAAS GPS users brief each approach with the possibility of a signal downgrade in mind (Figure 2). Such downgrades will often not be detected until the Final Approach Fix (FAF) becomes the active waypoint and the GPS unit alerts the pilot of the downgrade
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Figure 2: The RNAV (GPS) RWY 25 approach into Rochelle, IL (KRPJ) is a typical WAAS-type approach with multiple sets of minimums. While LPV minimums are published, if the WAAS service level is downgraded for any reason, the most common result would be a “Downgrade to LNAV” message. The approach could still be flown, using the published LNAV minimums, but no vertical guidance should be expected.
(and/or the Course Deviation Indicator (CDI) sensitivity is annunciated as “LNAV” versus the expected “LP” or “LPV”). This is not the time to re-brief the approach and fly it as a downgraded procedure. Better to abandon the approach and start fresh, briefing the downgraded or alternate approach procedure to be used subsequently. In the rare event that a downgrade or RAIM-induced failure occurs inside the FAF, the IFR-certified GPS will continue to function to the best of its ability for five minutes thereafter to give the pilot an opportunity to safely initiate a missed approach procedure.
To practice dealing with such procedures, find a knowledgeable CFI and/or simulator instructor who’s well versed in creating such scenarios with the equipment in your aircraft. Most WAAS-enabled GPS units (whether independent, part of an FMS unit or incorporated into an integrated avionics system) allow WAAS features to be manually de-selected. An instructor knowledgeable in your navigation system should be familiar with a variety of ways to force degraded capabilities and ways in which you can best utilize the capabilities which remain.
Jason Wondra
Michael Williams
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12 • KING AIR MAGAZINE
AUGUST 2016