A Line of Cumulonimbus on the Diani Track
You are planning a VFR flight from Wilson to Ukunda: 240 nm, track 135 degrees magnetic, TAS 110 kt, in a C172 with 110 L usable fuel and a cruise burn of 34 L/h. Planned departure 10:00 local (07:00Z). Forecast wind at 7,500 ft: 250/30. Forecasts issued at 05:00Z:
WILSON 100500Z 1006/1106 09010KT 9999 FEW025 BKN080 TEMPO 1008/1012 VRB15G30KT 3000 TSRA FEW018CB BKN020
UKUNDA 100500Z 1006/1106 12008KT 9999 SCT025 BECMG 1008/1010 05015G28KT 5000 SHRA SCT018CB
Pilot reports place a line of cumulonimbus over the coastal ridge and the Shimba Hills, tops above 30,000 ft, moving west at 10 kt. The aircraft has a GPS and one VOR, no weather radar and no oxygen. Between Voi and the coast there are few diversion airfields.
1. Interpret the two forecasts. What weather do you expect en route and at the destination, and when?
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First convert: Kenya is UTC+3, so 10:00 local is 07:00Z and a 2.5-hour sector arrives at about 09:30Z.
Wilson (departure). 09010KT, 10 km or more, few at 2,500 ft, broken at 8,000 ft - a normal Nairobi morning. The TEMPO 1008/1012 is the important part: temporarily, between 08:00Z and 12:00Z (11:00-15:00 local), variable wind 15 gusting 30 kt, visibility 3,000 m in thunderstorm rain, few CB at 1,800 ft and broken at 2,000 ft. That is an afternoon convective pattern and it matters for the return leg: you need to be back at Wilson before about 11:00 local, or you will be arriving into a thunderstorm.
Ukunda (destination). 12008KT, 10 km, scattered at 2,500 ft initially - fine. But BECMG 1008/1010 means a permanent change between 08:00Z and 10:00Z (11:00-13:00 local) to wind 050 at 15 gusting 28 kt, visibility 5,000 m in rain showers, and scattered cumulonimbus at 1,800 ft. Your 09:30Z arrival falls inside that transition window: a wind shift of about 70 degrees, gusts to 28 kt, showers and CB in the vicinity.
En route. The pilot reports are the decisive information: a line of CB with tops above 30,000 ft. You cannot overfly it without oxygen, and a 172 will not climb above it anyway. It is moving west at 10 kt - towards you - so any gap in the line is closing while you fly towards it. Expect the classic sea-breeze convergence over the coastal ridge: building cumulus from late morning, then CB.
Overall: departure fine, arrival marginal and deteriorating, return leg actively bad.
2. Compute the heading, ground speed, leg time and fuel required, and state whether the flight is viable as planned.
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Wind triangle. Track 135 degrees M, wind 250/30, TAS 110 kt.
- Angle between track and wind direction: 250 - 135 = 115 degrees, so the wind is from the left, largely across the track.
- Crosswind component = 30 x sin 115 = 30 x 0.906 = 27 kt from the left.
- Headwind component = 30 x cos 115 = 30 x (-0.423) = 13 kt headwind.
- Wind correction angle: sin WCA = 27 / 110 = 0.245, so WCA = 14 degrees into wind. Heading = 135 - 14 = 121 degrees M.
- Ground speed = 110 x cos 14 - 13 = 106.7 - 13 = about 94 kt.
Time and fuel.
- Leg time = 240 / 94 = 2.55 h = 2 h 33 min.
- Cruise fuel = 2.55 x 34 = 87 L.
- Add 5-6 L for taxi, climb and the richer climb mixture: about 93 L.
- Add the 45-minute reserve (26 L): about 119 L required.
Verdict: not viable as planned. Usable fuel is 110 L, so you are about 9 L short before you even consider a diversion, a hold, or the extra fuel burned at low level while dodging build-ups. Note also that the 30 kt wind is a headwind in this direction - a pilot who assumes 110 kt groundspeed will be 40 minutes out in his fuel planning.
Options: a fuel stop en route (for example at Voi or Malindi, accepting a later and more convective arrival), or fewer passengers and bags with the same fuel, or a different day. If you take the fuel stop, you must then rebuild the plan with the destination BECMG and a genuine alternate.
3. You are 40 nm from Ukunda at 7,500 ft with 75 minutes of fuel remaining. The CB line is 15 nm ahead, its bases obscured, and there is no obvious gap. Decide and justify.
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Decision: divert now, while you still have options - do not press on towards the line.
The numbers. 40 nm at about 94 kt is roughly 26 minutes to the destination. You have 75 minutes of fuel, so about 50 minutes of discretionary endurance - enough for one diversion decision, not for a hold followed by a diversion. The CB line is 15 nm ahead (about 10 minutes) and moving west at 10 kt towards you, so the gap, if any, is closing at roughly 100 kt of closing speed.
What I will not do:
- Climb over it. Tops above 30,000 ft, no oxygen, and a 172's service ceiling far below the cloud tops.
- Fly under it. Bases obscured means you cannot see what is beneath, and the surface gust front and microburst risk is exactly where you would be.
- Penetrate it. Hail, severe turbulence, icing and lightning - and a 172 has no weather radar to pick a soft spot.
- Hold and wait. You do not have the fuel to hold and still have a robust alternate.
What I will do:
- Turn away from the line immediately - towards the last confirmed clear area behind me - and reduce power to conserve fuel.
- Tell ATC my position, fuel state and intention, and ask for the latest actual weather at the nearest airfields clear of the line.
- Land at the nearest suitable aerodrome clear of the line (Voi, Malindi or Mombasa, depending on what the actual reports show). A precautionary landing at a diversion airfield is a non-event; a CB penetration is not.
- If no airfield is clearly clear of the line, the answer is still the same: land, refuel, wait for the line to pass, then continue VFR.
The forecast supports this: the destination is forecast to go to a 70-degree wind shift, 28 kt gusts and showers, which is the outflow from exactly this convection. Arriving there as the outflow hits, with minimum fuel, is the scenario that produces accidents.