Coastal Fog TEMPO and an Unusable Night Alternate at Walvis Bay
A Cessna 182 pilot plans a night VFR flight from Eros to Swakopmund on the coast, departing 20:30 with an ETA of 22:20. Fuel on board is 3 h 10 min; planned flight time 1 h 50 min, and the fixed reserve is 45 minutes. The only alternate considered is Walvis Bay, 18 nm south. The 18:00 TAF for Walvis Bay reads: 18012KT 9999 SCT012 TEMPO 1820/1906 0400 FG BKN002. Swakopmund aerodrome has no runway lighting and no night ATC. The pilot has a night rating, 12 hours of night time, and has never landed at either field. The area forecast warns of a coastal low with stratus spreading inland after midnight. There is no ATC at either field at night; Windhoek information is available on the ground.
1. Interpret the TAF: what does the TEMPO mean here, what are the timing implications, and what does it do to the planning for a 22:20 arrival?
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Decode. Valid 1800Z on the 18th to 1800Z on the 19th. Prevailing: wind 180° at 12 kt, visibility 10 km or more, scattered at 1,200 ft. From 2000Z on the 18th to 0600Z on the 19th - 22:00 to 08:00 local, UTC+2 - a TEMPO for 400 m visibility in fog and broken cloud at 200 ft.
What TEMPO means. Temporary fluctuations expected to last less than an hour at a time and, in total, for less than half the period. It is not the prevailing condition, but it is a real and locally frequent one: roughly a one-in-three chance of 400 m and a 200 ft ceiling at any given moment in the window.
Planning effect. 400 m and a 200 ft ceiling is far below night VFR minima and below any straight-in or circling minimum available at an unmanned field. An ETA of 22:20 falls inside the window's first minutes. The destination forecast is therefore unusable as planned, and the flight needs an alternate forecast to stay above alternate minima for the entire relevant period - which the same TAF does not provide.
2. Work the fuel and alternate numbers and state whether the plan is legal and safe.
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Fuel. 3 h 10 min usable against 1 h 50 min planned and a 45-minute fixed reserve leaves roughly 35 minutes of discretionary holding.
Alternate. Walvis Bay is 18 nm, about 10–12 minutes - but the only alternate is covered by the same TEMPO, so at 22:20 it cannot be forecast above alternate minima. Swakopmund cannot serve at all: no runway lighting means no night operation, and there is no ATC.
Conclusion. There is no legally usable and no practically usable alternate for an arrival between 22:00 and 06:00. The 45-minute reserve would have to cover holding, a diversion and an approach at a field where fog is forecast to thicken, and the area forecast's coastal low makes early clearance unlikely. Add the pilot's 12 hours of night time and total unfamiliarity with both fields, and the plan fails on both regulatory and airmanship grounds. The correct action is to replan before departure - earlier arrival or daylight - not to launch and hope.
3. Assume the flight departs. State the in-flight triggers and the actions at each, and explain the trap in 'going to have a look'.
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Pre-briefed triggers:
- 40 nm from the coast - if the aircraft/ATIS reports or a visible stratus edge indicate fog, turn back while 45 minutes of reserve is still intact.
- Bingo point - the point at which returning to Windhoek still lands with reserve, computed before flight. Crossing it commits the flight to landing with less than reserve.
- On arrival - if the runway lights are not clearly visible by 5 nm, or the aircraft is not stabilised by 500 ft, go around and divert; no second look.
The trap. 'Going low to have a look' over an unlit field at night, in a coastal fog environment, with no horizon and rising ground, is the classic CFIT sequence: the pilot descends to keep the ground in sight, loses the horizon entirely and flies into terrain. In fog over an unlit field there is nothing to see at any height that is safe.
The only realistic night diversion is a lit aerodrome with ATC, and the fuel must be protected for it before the decision is needed.