FAA Instrument Rating Airplane (IRA)Navigation SystemsHard
An aircraft is flying an RNAV (GPS) approach using LPV minima when the WAAS receiver displays a loss of WAAS augmentation while GPS RAIM remains available. What should the pilot expect the system to do, and how should the approach be flown?
- AThe system automatically reverts to LNAV minima; the pilot must fly to the LNAV MDA rather than the LPV DA
- BThe autopilot will automatically fly a coupled go-around
- CThe system continues to provide LPV vertical guidance using RAIM in place of WAAS
- DThe approach must be immediately discontinued and a missed approach executed
Show answer & explanationAnswer & explanation
Correct answer: A. The system automatically reverts to LNAV minima; the pilot must fly to the LNAV MDA rather than the LPV DA
When WAAS signal augmentation is lost but basic GPS RAIM remains, the receiver automatically reverts to a lower level of service, typically LNAV, which provides lateral guidance only. The pilot must then use the LNAV MDA rather than continuing with vertical (LPV) guidance to a DA.
Why the other options are wrong
- B. Autopilot reversion behavior is unrelated to WAAS/RAIM status; no automatic go-around is triggered.
- C. RAIM alone cannot substitute for WAAS vertical guidance integrity.
- D. A missed approach is not automatically required; the approach may continue using the lower service level.
WAAS Service Level Reversion
WAAS-enabled GPS receivers can lose augmentation while retaining basic GPS RAIM, causing automatic reversion from LPV/LNAV+V to LNAV-only minima.
- LPV provides both lateral and vertical guidance using WAAS
- Loss of WAAS augmentation drops service to LNAV (lateral only)
- Pilot must transition to LNAV MDA, not continue descent on vertical guidance
Memory trick: 'Lose WAAS, lose the glide—go lateral, find your MDA guide.'