Evening arrival at Kisumu: reading TEMPO, PROB and the alternate's TAF
At 1630Z you are planning a night IFR flight from Nairobi to Kisumu (elevation 3,780 ft), ETA 1930Z, with Eldoret as destination alternate. The forecasts are:
TAF HKKI 131100Z 1312/1412 11008KT 9999 SCT025 BECMG 1318/1320 16012KT 8000 -SHRA BKN018 TEMPO 1319/1323 32015G30KT 3000 TSRA FEW015CB BKN018 PROB30 1321/1323 1200 +TSRA
TAF HKEL 131100Z 1312/1412 12006KT 9999 SCT020 BECMG 1319/1321 4000 BR BKN005 TEMPO 1320/1324 2000 -RA BKN004
Kisumu has only VOR and NDB approaches, with MDA 4,400 ft and visibility 2,400 m. Eldoret has an ILS with minima of 7,450 ft (500 ft above aerodrome elevation) and 1,200 m. Company rules require the alternate to be above its minima for one hour before and one hour after ETA.
1. Decode the Kisumu TAF and state the most likely conditions at and around your ETA.
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Base: surface wind 110°/08 kt, visibility 10 km or more, scattered cloud at 2,500 ft.
BECMG 1318/1320: between 1800Z and 2000Z the wind becomes 160°/12 kt with 8,000 m in light rain showers and broken cloud at 1,800 ft.
TEMPO 1319/1323: temporarily, between 1900Z and 2300Z, wind 320°/15 kt gusting 30 kt, visibility 3,000 m in thunderstorm rain, few CB at 1,500 ft, broken at 1,800 ft.
PROB30 1321/1323: 30% probability between 2100Z and 2300Z of visibility 1,200 m in heavy thunderstorm rain.
At an ETA of 1930Z the aircraft is inside the TEMPO window. The 3,000 m is above the 2,400 m minima, but the forecast has a 210° wind shift (110° to 320°), a 30 kt gust, a CB and thunderstorm rain. The PROB30 is not a small add-on: 1,200 m is half the required visibility and is well inside the arrival window. A 30 kt gust front on final also means wind shear and a real chance of a missed approach.
2. Is Eldoret a legal destination alternate for a 1930Z ETA? Justify with the numbers.
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No. The controlling group is BECMG 1319/1321: between 1900Z and 2100Z the conditions become 4,000 m in mist with broken cloud at 500 ft AGL. At Eldoret's elevation that is a cloud base of about 7,400 ft, which is below the 7,450 ft approach minima. Visibility is not the problem (4,000 m beats 1,200 m); cloud base is. TEMPO 1320/1324 then offers BKN004 - 400 ft AGL, still worse.
The company window is 1830Z to 2030Z. The becoming group starts as early as 1900Z, and the TEMPO starts at 2000Z, so for most of the window Eldoret may be at or below minima. Even the earliest reading is marginal, and an alternate is there to be usable precisely when things are going wrong at the destination. A forecast at or below minima does not qualify.
Conclusion: the filed plan has no legal alternate. It needs a second alternate with a forecast comfortably above minima (or a delay until the Kisumu weather behind the gust front improves).
3. What operational plan do you brief, assuming a legal alternate is found?
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Brief the approach as a convective arrival, not a routine VOR/NDB.
- Extra fuel for one hold and a diversion. A missed approach at Kisumu is likely, and the NDB/VOR approach in a 30 kt gust front is not a place to press on.
- Weather avoidance: request radar vectors around the CB if any radar is available; at night, a cell on the nose is not something to pick a way through visually.
- Approach gates: if the wind shear is reported or the gust spread on final exceeds the aircraft's limit, go around early - before the MDA, not at it.
- If the MDA is reached without the required visual reference, execute the missed approach and divert. Do not attempt a second approach into a TEMPO TSRA; the fuel and the alternate window will not support it.
- Keep the alternate in the decision loop: if the alternate's weather is trending towards its TEMPO group, the diversion decision has to be made earlier, not later.
- Pass the actual conditions to the following traffic; on a night like this the PIREP is worth as much as the TAF.