A Tropical Storm Between Two Islands: Two Aerodromes and One Weather System
A moderate tropical storm is centred 260 nm north-west of Mauritius, moving south-east at 10 kt. The forecast track takes the centre over Réunion within 14 hours, and 60 nm west of Mauritius 8 hours later. A scheduled night flight FIMP–FMEE departs at 22:40Z with 148 passengers and 35 minutes' planned flight time. Forecasts at the ETA (23:15Z): FMEE wind 340°/25 kt gusting 45 kt, visibility 3,000 m in heavy showers, TEMPO thunderstorm, SCT 800 ft cumulonimbus; FIMP wind 090°/22 kt gusting 38 kt, visibility 8 km, showers, BKN 1,500 ft. The two aerodromes are 130 nm apart. Company fuel figures: trip 900 kg, contingency 5% (45 kg), destination alternate 900 kg, final reserve 1,200 kg, no discretionary fuel. The commander signs the flight plan; no company restriction applies to this sector.
1. Assess whether FIMP remains a valid destination alternate for this flight, and identify what is missing from the fuel plan.
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Alternate validity
For planning purposes the alternate must be forecast to be at or above the applicable planning minima at the ETA ±1 hour, with the approach aid and the runway serviceable. On paper FIMP meets that: 8 km visibility and a 1,500 ft cloud base comfortably exceed the ILS CAT I planning minima, and the wind is within limits. So the plan is legal.
The real problem
The storm is moving from the north-west towards the south-east, so the sequence is unfavourable: Réunion is affected first (centre overhead within 14 hours, i.e. close to the outbound ETA) and Mauritius is affected next, close to the time the crew would want to divert there. The two aerodromes lie 130 nm apart - inside the same synoptic system, in the same warning area, with the same convective environment and the same possibility of a closed aerodrome. They are not independent alternates in any operational sense, even though the regulation does not require independence as a formal test.
What the fuel plan lacks
There is no hold fuel and no second alternate or 'island reserve'. Given a 35-minute sector, a realistic plan needs enough fuel to hold for a defined period at the destination, then divert to FIMP and still hold there - or to turn back to FIMP with a full reserve intact. The 900 kg alternate fuel covers the diversion flight only; it does not cover a missed approach at FMEE, a hold, or a second approach at FIMP in a gusting crosswind.
2. State the go/no-go decision and the conditions under which you would release the flight.
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Decision
On the plan as presented, do not depart. The flight is only 35 minutes, but the exposure is not the en-route leg: it is arriving at Réunion at the peak of the system with a gusting crosswind and 800 ft cumulonimbus, then possibly diverting into an alternate that will be deteriorating as the storm moves towards it. A legal fuel plan that leaves no room to hold, miss and divert is not a safe plan.
Conditions for release
- Add sufficient discretionary fuel for a defined holding period at FMEE plus a diversion to FIMP plus a further approach there; that is roughly 30 minutes holding (about 1,150 kg at 2,300 kg/h) plus a second-approach allowance.
- Confirm from the aerodrome operator, not just the TAF, that both aerodromes will remain open and that rescue and fire-fighting cover is not reduced during the warning.
- Check that the destination alternate crosswind is within limits for the expected gusts, and that the approach aid is serviceable.
- Alternatively, delay departure until after the centre has passed Réunion, when the wind and shower activity behind the system are forecast to ease.
What is not acceptable is departing on the minimum plan and relying on the flight being short. A 35-minute sector is exactly the case where the crew has the least time to recover from a wrong arrival decision.
3. Explain why gusts of 45 kt at Réunion do not by themselves close the aerodrome, and what other meteorological elements in this forecast matter more for the approach decision.
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Gusts
A 45 kt gust on a 25 kt mean wind is a 20 kt gust increment - large but typical of convective and cyclonic conditions. Gusts describe short-period fluctuations, so the decision is based on the steady wind (for the crosswind component and for the approach speed increment) plus the gust factor applied to the crosswind limit and the turbulence. The runway in use may be aligned reasonably with 340°; the crosswind and the resulting speed increment are what matter, not the raw gust number. Gusts also mean windshear and a fluctuating indicated airspeed on short final, which changes the go-around decision at the minima.
What matters more
- SCT 800 ft cumulonimbus with TEMPO thunderstorm: convective cloud this low implies embedded cells on the approach path, microburst risk, and heavy rain that can reduce visibility to well below the reported 3,000 m in seconds.
- TEMPO validity: a TEMPO group is not a forecast of continuous conditions, so the reported weather at the ETA may sit anywhere between the prevailing and the TEMPO values; brief excursions below minima are compatible with the forecast.
- The trend: the storm's approach means conditions deteriorate after the ETA, so the alternate will be less usable later, not more.
A commander who bases the decision on the 3,000 m prevailing visibility and ignores the 800 ft cumulonimbus and the TEMPO group has read the forecast as a description of an average afternoon rather than as a warning of a convective environment.