GNSS Failure Over the Sahel on a Divert
You are flying VFR from Kano (KAN) to Maiduguri (MIU), 280 nm, planned track 075°T, TAS 105 kt, wind 090°/20 kt, variation 2°E. About 200 nm from Kano, the GNSS receiver suddenly shows “no fix” and does not recover after a restart. Maiduguri weather is deteriorating: visibility 4 km in dust haze with scattered cloud at 1,200 ft. You decide to divert to Gombe (GMO), which is 95 nm away on a chart track of 210°T and reports 10 km visibility. You have 22 US gal remaining and burn 9 US gal/h. The aircraft has a magnetic compass, a heading indicator, a watch, and a current chart. There is no VOR/DME coverage along the diversion track. You must build a dead-reckoning heading and estimate before turning, because there is no radar service and the terrain is featureless in haze.
1. Calculate the heading to steer for Gombe in °T and °M, showing the wind triangle reasoning.
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Track is 210°T. The wind is from 090° at 20 kt; relative to the track, the wind is 120° off the nose, so the tailwind component is 20 cos 60° = 10 kt and the crosswind component is 20 sin 60° = 17 kt from the right. Drift angle = (17 / 105) × 60 ≈ 9.7°, so crab left by about 10°. True heading = 210° − 10° = 200°T. With 2°E variation, magnetic heading = 200° − 2° = 198°M. Expected groundspeed = 105 + 10 = 115 kt. I would round the heading to 200°M and check the drift after 15–20 minutes using a map feature or the sun’s known bearing, because the wind forecast may be wrong.
2. Estimate the time and fuel to Gombe and decide whether the diversion is within your reserves.
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Time = 95 nm / 115 kt = 0.83 h, or about 50 minutes. Fuel = 0.83 × 9 = 7.5 gal. Fuel remaining on arrival at Gombe = 22 − 7.5 = 14.5 gal, or about 1.6 hours at 9 gal/h. That leaves a reasonable margin for a missed approach, a go-around, or a further diversion if Gombe weather changes. The main risk is not fuel but navigation: with no GNSS and no radio navaid, a 10° heading error over 95 nm would put me about 16 nm off track. I would therefore note the time abeam each identifiable feature, and if I have not identified Gombe by the revised ETA plus 10 minutes, I would climb, conserve fuel, and call ATC for assistance rather than press on.
3. What would you do if the wind turns out to be 090°/35 kt rather than 20 kt? Recalculate the key numbers.
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With 35 kt, crosswind = 35 sin 60° = 30 kt and tailwind = 35 cos 60° = 17.5 kt. Drift = (30 / 105) × 60 ≈ 17°, so true heading = 210° − 17° = 193°T, or 191°M. Groundspeed = 105 + 17.5 = 122 kt. Time = 95 / 122 = 0.78 h, or 47 minutes, and fuel = 7.0 gal. The heading change is significant - about 7° from the earlier plan - so I would not simply apply the original 200°M. I would re-establish drift using a feature or a timed turn and then update the ETA. The fuel difference is small, but the track-keeping difference over 95 nm is large.