GNSS RAIM failure inbound to Christchurch: using VOR/DME
You are 28 NM southwest of Christchurch on a VFR flight, cleared into controlled airspace. Your GNSS suddenly annunciates “RAIM unavailable” and the moving map freezes. You still have a serviceable VOR receiver. You set the OBS to 030° and select the TO indicator; the CDI shows a full-scale left deflection and the DME reads 28 NM. Your magnetic heading is 030°M and TAS is 100 kt. Seven minutes later the DME reads 18 NM. You have 60 L usable fuel, burn 30 L/h, and require a 30-minute reserve. Christchurch is busy and ATC expects you to report position. You are not IFR rated and the aircraft has a single VOR/DME. You must use the VOR/DME, not the failed GNSS, to establish your radial, track and ETA, and decide what to tell ATC.
1. With OBS 030, TO, and CDI left, what radial are you on, and what heading change is needed to track to the station?
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The TO indication means the selected course 030° is the course you would fly to the station. If the CDI were centred with TO, you would be on the 210° radial, because the bearing to the station would be 030°.
A left CDI deflection means the selected course is to the left of your actual bearing to the station; equivalently, the station is to the left of the selected course. Your actual bearing to the station is therefore less than 030°, about 020° in this example. You are on the reciprocal radial of about 200°, 28 NM from the station.
To track to the station you should turn left toward about 020°M and then keep the CDI centred. Do not chase full-scale deflection; make a positive heading change, then correct with smaller changes.
2. Using the DME readings, calculate groundspeed and ETA to the VOR. What fuel is required?
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DME has decreased from 28 NM to 18 NM in 7 minutes, so you have covered 10 NM along the slant range:
Groundspeed = 10 × (60/7) = 85.7 kt, approximately 86 kt. Because DME measures slant range, the true groundspeed is slightly higher, but 86 kt is a reasonable planning figure.
Remaining DME distance is 18 NM. ETA = 18/85.7 × 60 = 12.6 min, about 13 minutes.
Fuel burn is 30 L/h = 0.5 L/min. Fuel to the VOR = 0.5 × 12.6 = 6.3 L. Add the 30-minute reserve (15 L) and you need about 21.3 L from the present position. With 60 L usable, fuel is not a limiting factor for this leg.
3. What are the operational risks of continuing with a RAIM warning, and what should you report to ATC?
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RAIM unavailable means the receiver cannot assure the integrity of the GNSS position. The displayed position may be outside the required accuracy, or the receiver may not have enough satellites to detect a faulty signal. You should not use the GNSS as the primary means of navigation into controlled airspace. Revert to the VOR/DME, confirm the radial and distance, and cross-check with the chart and visual features.
Tell ATC clearly: “GNSS RAIM warning, navigating on VOR/DME,” and give your radial and DME distance. If you are not certain of your position, say so and request assistance rather than pressing on. Continue to maintain VFR and terrain clearance; a RAIM warning is not automatically an emergency, but it does require a change in navigation method and honest position reporting.