GNSS RAIM outage with a single VOR/DME backup
You are pilot-in-command of a PA-28 planning a VFR cross-country from Isfahan to Shiraz, about 180 NM, at 9,500 ft. The aircraft has one VOR/ILS receiver, one DME and a panel-mounted GPS. A NOTAM says GPS RAIM outage from 10:00 to 12:00 UTC; your planned departure is 10:15 and ETA 11:30. The direct route is defined by GNSS waypoints and the controlled-airspace clearance you want requires RNAV 5. A VOR/DME airway lies about 20 NM south of the direct track and is within coverage above 8,000 ft. Weather is VMC. You have no IRS or second VOR. Your briefing requires a positive position fix at least every 30 min and an approved backup for any GNSS-based route.
1. Can you depart as planned under the RAIM outage? Explain the integrity issue and the backup you would need.
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No, not as planned on the GNSS direct route. RAIM is the receiver's ability to detect a satellite fault and warn the pilot; during a published RAIM outage the GPS may still show a position but cannot provide the integrity required for RNAV 5. A single VOR/DME is a useful backup, but the route must be re-planned onto the VOR/DME airway, not flown as the original GNSS waypoint route. Check that the VOR and DME are serviceable, that coverage exists above 8,000 ft along the airway, and that you can obtain a positive fix at least every 30 min. Because you have only one VOR, you should also identify a fallback: delay until after 12:00, use radar vectors, or choose a route where dead reckoning can be cross-checked by DME. Departing and hoping the GPS remains usable is not an acceptable plan.
2. If the GPS fails at 10:40 while you are 60 NM from Isfahan and 120 NM from Shiraz, outline your navigation and position-fixing plan.
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First fly the aircraft and maintain the last assigned altitude and heading. Tune and identify the Isfahan VOR/DME if still within range, or the next VOR/DME ahead. Take a radial and DME cut to establish a positive fix, plot it on the chart, and compare it with your dead-reckoning position. From the fix, compute the wind using your heading, TAS and track made good, then turn toward the VOR/DME airway about 20 NM south. Use the VOR radial to track and DME to measure progress, with 5-10 min drift checks. Advise ATC that GPS has failed and give your last confirmed fix, altitude and intention. If no VOR/DME fix is possible, request radar vectors or climb for better VOR reception; if neither is available, divert to a suitable aerodrome rather than continuing on unverified dead reckoning.
3. What would you do if you cannot obtain a VOR/DME fix and ATC asks for your position? Include lost-comms and emergency considerations.
Read the worked answer
Tell ATC honestly: 'Unable to obtain a position fix, request radar vectors or assistance.' Give your last known fix, time since that fix, heading, altitude and estimated groundspeed. If radar coverage is available, use it to re-establish the flight. If not, climb to improve VOR/DME line-of-sight and try another station; do not enter controlled airspace without a clearance. If you become uncertain of your position near controlled airspace or terrain, declare an urgency condition and squawk 7700 if an emergency exists. For lost communications, squawk 7600, continue on the last acknowledged clearance or the published lost-comms procedure, and land at a suitable aerodrome as soon as practical. The priority is to avoid blundering into controlled airspace or terrain while trying to solve the navigation problem alone.