Inbound on the 045 Radial with a RAIM Gap
You are 40 nm from a VOR/DME station, cleared to track inbound on the 045 radial at 6,500 ft. TAS is 120 kt, the wind at altitude is 270°/30 kt, and the heading indicator is aligned. Your GNSS is your only area navigation source because the aircraft's second VOR receiver is unserviceable and deferred. The destination approach is RNAV (GNSS) with LNAV minima. Your pre-flight RAIM prediction for the destination shows an outage from 17:20 to 17:32; you are overhead your present position at 16:55 and your ETA without wind correction would be about 17:15. NOTAMs show the destination VOR is serviceable but unmonitored, and the NDB is off the air. There is 25 minutes of holding fuel above final reserve.
1. State the track, compute the heading to steer and the groundspeed, and give your revised ETA.
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Track. The 045 radial runs outward from the station on a magnetic bearing of 045°. Flying inbound on that radial means tracking towards the station, so the track is the reciprocal: 225° M. The common error is to steer 045°.
Wind components. Angle between wind direction (270°) and track (225°) = 45°, from the right.
- Crosswind = 30 × sin 45° = 21.2 kt
- Headwind = 30 × cos 45° = 21.2 kt (the wind is ahead of the beam)
Wind correction angle. sin WCA = 21.2 / 120 = 0.177 → WCA ≈ 10° into wind, i.e. to the right. Heading = 225 + 10 = 235° M.
Groundspeed. 120 × cos 10° − 21.2 = 118.2 − 21.2 ≈ 97 kt.
ETA. 40 nm at 97 kt = 0.412 h = 24.7 min. From 16:55, ETA ≈ 17:20.
The lesson: the no-wind ETA of 17:15 was 5 minutes optimistic, and that 5 minutes is the difference between arriving before the RAIM outage and arriving inside it.
2. What does the CDI/HSI show as you pass overhead, and why is station passage a poor moment to start a turn?
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As the aircraft enters the cone of confusion directly over the station, the received signal geometry collapses: the CDI becomes erratic, swings through full deflection, and the TO/FROM flag flips. It is not a failure - it is the expected loss of usable guidance.
Why not to turn on it:
- The needle gives no reliable left/right information overhead, so any turn started on the indication is started on noise.
- TO/FROM reversal is ambiguous for several seconds; misreading it sends you out on the wrong radial.
- Overhead, small heading errors translate into large radial errors a few miles out, and the needle then saturates rather than guiding you.
Instead: note the time of station passage (the flip to FROM if you were inbound with a TO indication), hold heading and allow for drift, and fly the outbound leg by time and heading until the needle settles. Then re-identify the station, set the new OBS value, and intercept positively. Confirm the station ident throughout - an unmonitored VOR is still usable but the identification is on you, not on the maintenance organisation.
3. Given the RAIM gap and the ETA you calculated, what is your decision?
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The ETA of 17:20 coincides with the start of the 17:20–17:32 outage, so the GNSS approach cannot be flown as planned. RAIM prediction must cover the approach plus the 15 minutes after ETA; 17:20 falls inside the gap, and at 17:15 you are also inside the 15-minute buffer.
Options, in order of preference:
- Plan a conventional approach. The destination VOR is serviceable, so if the aircraft has one working VOR receiver and a published VOR approach exists, brief and fly that instead. 'Unmonitored' is acceptable with a note that signal quality is not assured; you must positively identify it.
- Absorb the delay with holding, using the 25 minutes of holding fuel, and shoot the GNSS approach after 17:32 with the buffer satisfied - but confirm you still land with final reserve intact.
- Divert to an aerodrome with a non-GNSS approach before the fuel picture degrades.
What you must not do is arrive assuming RAIM will be there. With a single deferred receiver as the only area navigation source, the RAIM gap is a planning fact, not a surprise, and the pre-flight prediction was the tool that told you.