RNAV Approach with a GPS Integrity Alert in Mountainous Terrain
You are flying a single-pilot IFR flight in a pressurised single-engine turboprop at FL200 over mountainous terrain. The aircraft has a GNSS receiver approved for LNAV/VNAV and LPV approaches and a single GPS/WAAS installation; there is no DME and no ILS at the destination. While being vectored for an RNAV (GPS) RWY 14 approach with LNAV/VNAV minima, the GPS annunciator changes from 'LNAV/VNAV' to 'LNAV' and a moment later shows 'INTEGRITY' with the navigation flag. ATC clears you to descend to 6,000 ft and intercept the final approach course. The minimum safe altitude in the sector is 7,200 ft. The missed approach requires a climbing right turn to 9,000 ft. You are in IMC; the nearest VOR is 40 NM away and unusable below 8,000 ft.
1. What has the GPS integrity alert done to your navigation capability, and how should that change your response to the clearance?
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An integrity alert means the receiver can no longer guarantee the position it is computing is within the required containment bound. It is not a small degradation: the system has effectively withdrawn the navigation service for the level of RNP the procedure requires. The downgrade to LNAV first signals that vertical guidance is no longer valid; the integrity flag then removes lateral assurance as well.
Consequently you cannot accept a clearance that depends on that navigation: intercepting a final approach course and descending to 6,000 ft in a sector whose MSA is 7,200 ft would put you below safe terrain clearance with unreliable position information. The correct response is to advise ATC immediately that you have lost GPS integrity and cannot continue the RNAV approach, maintain terrain-safe altitude (at or above 7,200 ft, ideally the last assigned safe level), and request either vectors to a safe area, a different approach, or a climb to a level where you can re-establish navigation. Do not descend on an unverified position in mountains.
2. ATC offers radar vectors to the nearest VOR, which is 40 NM away and unusable below 8,000 ft. How do you assess that option?
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Treat the offer as a possible but not immediate solution. To use the VOR you must first climb to at least 8,000 ft, which is also above the 7,200 ft sector MSA, so the climb is terrain-safe and improves your options. But the VOR is 40 NM away, so you would spend time and fuel in IMC with degraded primary navigation; you need to confirm that the aircraft's other systems (VOR receiver, ADF, radar, and dead reckoning from the last good fix) can carry you there, and that ATC will provide monitoring and vectors.
Ask yourself: is the VOR approachable in the weather? What are its minima and whether it is an on-airport aid? Is there a nearer ILS-equipped aerodrome? If ATC can vector you on radar to an ILS, that is usually a better plan than a distant VOR. Accept the VOR only if it is the best available and you have briefed the approach, the missed approach and the fuel implications. Meanwhile, climb and stay at a safe level.
3. Draft the essential content of your transmission to ATC and the follow-up actions you would take while the situation develops.
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Transmission, concise and priority-ordered:
'Mayday' is not yet warranted unless safety is compromised; use urgency if needed. A suitable call: '[Callsign], GPS integrity failure, unable RNAV approach, request vectors and climb to 8,000 ft, request ILS at [alternate].'
Include: your position and level, the nature of the failure (loss of GPS integrity, no vertical guidance), what you cannot do, what you want, and your intentions if no clearance is available.
Follow-up actions:
- Climb to a terrain-safe level; do not descend below the sector MSA.
- Cross-check position with all available means: VOR/DME if in range, radar vectors, dead reckoning, visual references if any.
- Brief the diversion or the alternate approach before you need it.
- Manage fuel and consider declaring an emergency if the situation deteriorates or fuel becomes critical.
- Notify company/operations if required by your SOP.