FAA Instrument Flight Instructor (FII)IFR Procedures and ApproachesMedium
A pilot is planning an instrument flight to an airport where the only available instrument approach is an RNAV (GPS) approach. The pilot checks the NOTAMs and finds that the WAAS signal is unreliable for the estimated time of arrival. What impact does this have on the approach capability?
- AThe pilot must cancel the IFR flight plan, as no approved vertical guidance will be available.
- BThe pilot must plan to use LNAV minimums, as LPV and LNAV/VNAV will be unavailable.
- CThe pilot may use LNAV/VNAV minimums, as they do not solely rely on WAAS.
- DThe pilot can still use LPV minimums, but with reduced accuracy.
Show answer & explanationAnswer & explanation
Correct answer: B. The pilot must plan to use LNAV minimums, as LPV and LNAV/VNAV will be unavailable.
LPV and LNAV/VNAV minimums on RNAV (GPS) approaches are dependent on WAAS for their precision and approval. If the WAAS signal is unreliable, these higher precision minimums will not be available. The pilot must then revert to LNAV minimums, which are non-precision and do not require WAAS, or plan an alternate strategy.
Why the other options are wrong
- A. While a lack of precision approaches might necessitate an alternate, the RNAV (GPS) approach usually has LNAV minimums which are still available without WAAS.
- C. LNAV/VNAV minimums are often dependent on WAAS, or Baro-VNAV if the aircraft is equipped. If WAAS is unreliable, and Baro-VNAV is not an option, then LNAV/VNAV minimums would be unavailable.
- D. LPV minimums are entirely dependent on WAAS. Reduced accuracy would likely lead to the system annunciation changing to a less precise mode, or even a loss of navigation.
WAAS Reliability Impact on GPS Approaches
The reliability of the Wide Area Augmentation System (WAAS) signal directly affects the availability of higher precision RNAV (GPS) approach minimums. Unreliable WAAS means LPV and WAAS-dependent LNAV/VNAV minimums are unavailable, and pilots must plan to use LNAV minimums.
- LPV and LNAV/VNAV (WAAS-dependent) require reliable WAAS.
- Unreliable WAAS means these minimums are not available.
- LNAV minimums do not require WAAS and remain available.
Memory trick: WAAS is the Key for Precision GPS.