Typhoon Squall Line and the Island Alternate
An IFR turboprop is at FL240 from Tokyo to Naha, ETA 0900Z. Fuel: trip 1,600 kg, alternate 500 kg, holding 400 kg, final reserve 300 kg; total 3,200 kg. Destination TAF: ROAH 120500Z 1206/1312 18018G32KT 5000 SHRA SCT008 BKN015 TEMPO 1206/1210 2500 +SHRA FEW005 BKN008CB; BECMG 1210/1212 22010KT 9999 NSW SCT020. Alternate ROIG, Ishigaki, is 210 nm away; its TAF is fine. At 0800Z, Naha ATIS gives 17020G35KT, visibility 3,000 m, heavy rain, BKN008, CB overhead. Airborne radar shows a squall line across the final approach. The aircraft burns about 800 kg/h at FL240 and 600 kg/h in the hold. The crew has already diverted once and is concerned about fuel. The ILS minima are 200 ft and 550 m; the runway is wet. The crew must decide whether to continue to Naha, hold, or divert now.
1. Assess the destination forecast and fuel plan. Is the current alternate fuel sufficient? Show the calculation.
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The TEMPO period 1206/1210 covers the 0900Z ETA. At Naha, the forecast at that time includes +SHRA, FEW005 BKN008CB, and 2,500 m visibility. That is close to or below the approach minima and certainly indicates convective, gusty conditions. A destination alternate is therefore required, and the crew must be prepared not to land at Naha.
ROIG is 210 nm away. At a reasonable turboprop ground speed of 250 kt, the diversion takes about 210/250 = 0.84 h, or 50 minutes. At 800 kg/h, that is about 670 kg, plus an approach allowance, say 100 kg, so roughly 770 kg is needed. If the ground speed is 280 kt, the time is 45 minutes and the fuel is about 600 kg plus approach. The planned alternate fuel of 500 kg is therefore short by roughly 100–270 kg. It may cover the diversion only if the actual ground speed is high and the approach is short.
After trip fuel, 1,600 kg remains. Required alternate 770 + holding 400 + final reserve 300 = 1,470 kg, leaving about 130 kg. A 40-minute hold at 600 kg/h uses 400 kg, so only one such hold is possible before diverting. The final reserve must not be planned for use.
2. At 0800Z, what decision should the crew make: continue, hold, or divert?
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At 0800Z the destination is already reporting heavy rain, a CB overhead, visibility 3,000 m, and a squall line across the final approach. The forecast TEMPO covers the ETA. This is not a situation for pressing on to 'have a look' without a solid alternate plan.
The safer decision is to divert now to ROIG while the fuel margin is known and the weather there is fine. If the crew decides to continue, they should set a hard limit: one hold of no more than about 20–30 minutes, with the diversion fuel and final reserve untouched. Any further delay, a failure of the destination weather to improve, or a low-fuel state should trigger an immediate diversion. Do not attempt an approach below minima, and do not use final reserve to reach the alternate.
Because the aircraft has already diverted once, the crew should also consider the nearest suitable airport rather than only the planned alternate. If fuel becomes critical, declare 'minimum fuel' or 'Mayday fuel' as appropriate and ask ATC for priority.
3. Identify the main weather and approach hazards at Naha and explain how they affect the approach decision.
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The main hazards are embedded cumulonimbus, a squall line, heavy rain, gusty surface wind, possible windshear/microburst, and a wet runway. The gust spread is at least 15 kt (20G35), and the squall line lies across the final approach, so the crew should expect sudden speed and glide-path changes, especially below 1,000 ft.
On approach, use the weather radar to avoid the red/magenta cells, request a deviation if needed, and do not penetrate a storm cell. Fly the ILS with a stabilised approach; if the aircraft becomes unstable, or if windshear is encountered, go around immediately. The wet runway increases landing distance and reduces braking action. The 200 ft/550 m minima are not a target; they are the lowest legal point, and the crew must not descend below them without the required visual reference. Given the convective activity, a go-around is a normal outcome, not a failure.