FAA Airline Transport Pilot (ATM)Navigation, Air Traffic Control and ProceduresHard
An RNAV (GPS) approach publishes LPV, LNAV/VNAV, and LNAV minimums. During the approach, the aircraft's WAAS-based vertical guidance becomes unavailable, but GPS lateral RNP performance remains valid. Which minimums must the crew now use?
- AUse the LNAV-only minimums and associated MDA
- BContinue to LPV minimums using backup vertical guidance
- CImmediately execute the published missed approach
- DRevert to the circling approach minimums
Show answer & explanationAnswer & explanation
Correct answer: A. Use the LNAV-only minimums and associated MDA
Loss of WAAS vertical guidance means the aircraft can no longer use LPV or LNAV/VNAV lines of minima, which depend on vertical guidance. Since lateral RNP guidance remains valid, the crew should continue using LNAV-only minimums with a Minimum Descent Altitude rather than a Decision Altitude.
Why the other options are wrong
- B. LPV requires functioning WAAS vertical guidance, which is no longer available.
- C. A missed approach is not required simply because vertical guidance is lost if lateral RNP remains valid.
- D. Circling minimums are unrelated to the loss of vertical guidance and require a different approach maneuver.
RNAV (GPS) Lines of Minima
RNAV (GPS) approaches may publish multiple lines of minima (LPV, LNAV/VNAV, LNAV, Circling) depending on the type of vertical and lateral guidance available to the aircraft.
- LPV uses WAAS for both lateral and vertical guidance
- LNAV/VNAV uses baro-VNAV or WAAS for vertical guidance
- LNAV uses GPS lateral guidance only, with an MDA
- Loss of vertical guidance drops the approach to LNAV-only minimums
Memory trick: 'No vertical? Go LNAV — lateral survives alone.'