A pilot is flying an RNAV (GPS) approach. The approach chart shows LNAV minimums with a Minimum Descent Altitude (MDA) of 800 feet MSL and a required visibility of 1 SM. The aircraft is equipped with a non-WAAS, IFR-approved GPS receiver. The pilot is aware that RAIM availability is predicted to be intermittent during the approach time. What is the pilot's best course of action?
- AProceed with the approach, as LNAV minimums do not require WAAS and RAIM is only a prediction.
- BRequest vectors from ATC to the final approach course, as this will mitigate the need for continuous RAIM.
- CSelect an alternate approach procedure that does not rely on GPS, or divert to an airport with suitable non-GPS approaches.
- DMonitor RAIM status closely; if RAIM fails during the approach, execute a missed approach.
Show answer & explanationAnswer & explanation
Correct answer: C. Select an alternate approach procedure that does not rely on GPS, or divert to an airport with suitable non-GPS approaches.
If RAIM is predicted to be intermittent or unavailable, the GPS receiver cannot guarantee the accuracy required for the approach. LNAV minimums, even without WAAS, still rely on RAIM for integrity monitoring. Therefore, the pilot should not plan to use that approach and must select an alternative or divert.
Why the other options are wrong
- A. LNAV minimums do require RAIM for integrity, even if not WAAS. Intermittent prediction means it's unreliable.
- B. Vectors might help with initial positioning, but the integrity of the GPS signal for the approach itself still relies on RAIM.
- D. Waiting for RAIM to fail during the approach is reactive and unsafe. Pre-flight prediction of intermittency requires a proactive alternative plan.
RAIM Prediction and RNAV Approaches
Receiver Autonomous Integrity Monitoring (RAIM) checks GPS signal integrity. If pre-flight RAIM prediction indicates intermittent or unavailable status for an RNAV approach, the approach should not be planned or conducted as the primary procedure.
- RAIM is crucial for non-WAAS GPS integrity.
- Predicted intermittency or unavailability means the approach is effectively 'NA'.
- Pre-flight check is mandatory for IFR GPS approaches.
- Pilot must have an alternative if RAIM is insufficient.
Memory trick: RAIM: 'Reliability Assured, If Monitored' – check BEFORE you take off.