Fluctuating VOR/DME on a Wellington approach
You are single-pilot IFR in a Cessna 182T conducting the VOR/DME RWY 16 approach at NZWN. The weather is wind 190/25G35, visibility 4 km, and low cloud. At 8 DME and 2,500 ft, cleared for the approach, the VOR needle swings plus or minus 10 degrees and the DME jumps between 5.2 and 6.8 NM. You have a GNSS moving-map receiver, but the approach is a VOR/DME procedure and you are not approved for an RNAV overlay. The missed approach requires climbing to 4,000 ft on the VOR radial. Thunderstorm activity is west of the aerodrome. ATC has offered no delay and asks whether you can continue. You must decide whether the guidance is usable, what to do with the GNSS, and how to handle ATC.
1. Can you continue the VOR/DME approach? Give your reasoning.
Read the worked answer
No. A VOR needle swinging plus or minus 10 degrees and a DME jumping 1.6 NM are outside normal approach stability. The VOR/DME is the only approved lateral and vertical guidance for this procedure; a GNSS moving map may be used for situational awareness but does not make it an RNAV approach. At 8 DME and 2,500 ft you still have time to discontinue. The gusty wind and thunderstorm activity can cause needle oscillation, but whatever the cause, you cannot rely on the course guidance. Request vectors to a holding pattern, a different approach, or diversion. Do not descend on a fluctuating needle.
2. What would you do with the GNSS and ATC?
Read the worked answer
Use the GNSS for situational awareness only, not as the primary approach aid unless the chart and your approval allow it. Tell ATC the VOR/DME is unreliable and request either radar vectors for a different approach or clearance to a holding fix or alternate. If you are already inside the final approach fix, fly the published missed approach using the VOR only if you can identify it reliably; otherwise request vectors and climb. The missed approach climb to 4,000 ft must be assured. Because the VOR is unreliable, the safest missed approach is ATC vectors with terrain avoidance. Do not accept a clearance that depends on the VOR until it is stable.
3. What minimum evidence would make you continue?
Read the worked answer
Continue only if the VOR is stable within about plus or minus 5 degrees, the DME is consistent, the approach is stabilised by the final approach fix, and the missed approach is assured. With a single VOR receiver and no approved alternate guidance, a swinging needle is a no-go. If a second VOR receiver or approved GNSS overlay were available and agreed, you could cross-check, but this aircraft does not have that. The decision should be made before the final approach fix; after it, a go-around is required unless the needle is steady and the required visual reference is in sight.