A crew is planning an RNAV (GPS) approach to an airport with no other approach available due to weather, using a non-WAAS GPS receiver. Before departure, dispatch reviews the RAIM prediction and finds that RAIM will be unavailable for 10 minutes surrounding the estimated arrival time at the destination. What is the appropriate course of action?
- ADepart as planned and use ATC radar monitoring in place of RAIM during the predicted outage period
- BDepart as planned since RAIM outages only affect en route GPS navigation, not approaches
- CSelect an alternate arrival time or an approach not reliant on GPS, since the RNAV (GPS) approach cannot be flown without RAIM availability
- DPlan to fly the approach anyway, since the GPS receiver will automatically revert to dead reckoning during the outage
Show answer & explanationAnswer & explanation
Correct answer: C. Select an alternate arrival time or an approach not reliant on GPS, since the RNAV (GPS) approach cannot be flown without RAIM availability
RAIM (Receiver Autonomous Integrity Monitoring) provides GPS integrity assurance required to fly an RNAV (GPS) approach with a non-WAAS receiver; if RAIM is predicted to be unavailable at the estimated time of arrival, the approach cannot be legally or safely flown using GPS during that window. The crew must adjust timing or use an alternate non-GPS-dependent approach.
Why the other options are wrong
- A. ATC radar monitoring is not an approved substitute for RAIM integrity requirements on a GPS approach.
- B. RAIM outages directly affect the integrity monitoring needed for GPS approaches, not just en route navigation.
- D. Non-WAAS GPS receivers do not have dead-reckoning backup integrity monitoring for approach use.
RAIM (Receiver Autonomous Integrity Monitoring)
RAIM is a GPS receiver function that cross-checks satellite signals to detect navigation errors, required for non-WAAS GPS approaches; if predicted unavailable, the approach must not be flown using GPS.
- RAIM prediction should be checked during preflight planning for GPS approaches
- WAAS-equipped receivers generally do not require a RAIM check since WAAS provides integrity
- If RAIM is lost in flight, the pilot must revert to another approved means of navigation
Memory trick: No RAIM, no claim to fly the approach.