Thunderstorm at destination and an alternate below minima
An evening IFR flight is planned from A to B, ETA 1740Z. B has one runway and an ILS to 200 ft. The 1600Z TAF for B is: 1700-1900 24018G32KT 9999 SCT025 BKN040 TEMPO 1720-1800 25025G45KT 4000 TSRA BKN012CB. The TAF for alternate C, 40 minutes away, is: 1700-1900 18006KT 0600 FG OVC002 BECMG 1830 2500 BR BKN008. The operator's alternate minima for C are 600 ft ceiling and 1,600 m visibility. The aircraft has weather radar, but its antenna stabilisation is deferred under the MEL with a note 'avoid use in moderate or greater turbulence'. Fuel to C is 40 minutes; final reserve is 30 minutes. The destination report at 1700Z is 2,500 ft scattered, 5,000 ft broken, 8 km, with a CB 15 NM northwest. The crew expects to arrive just after the TEMPO period. The dispatcher notes that C's BECMG is 'only a forecast' and releases the flight with C as the only alternate.
1. Is C usable as the only alternate at ETA 1740Z, and what does the TEMPO at B mean for the arrival?
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C is not usable as the only alternate at 1740Z. Its forecast at that time is 600 ft ceiling and 600 m visibility in fog, below the operator's alternate minima of 600 ft and 1,600 m. The BECMG 1830 forecast improvement occurs after the ETA, and a forecast improvement cannot be used to satisfy the alternate requirement at the time the aircraft would need to divert. The dispatcher's comment that it is 'only a forecast' cuts both ways: it is not a guarantee, and the current TAF is the basis for planning.
At B, the TEMPO 1720-1800 includes the ETA. TEMPO means temporary fluctuations, but TSRA with a broken CB at 1,200 ft and visibility 4,000 m can produce wind shear, microbursts, hail and a sudden loss of visibility. The 1700Z report is better, but it is an observation, not a forecast for 1740Z. Arriving 'just after the TEMPO period' is an expectation, not a clearance. The crew must plan to hold or divert if a cell is over the airport.
2. How does the deferred radar antenna stabilisation affect the strategy?
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Without antenna stabilisation, the radar display can be misleading in turbulence because the antenna tilt changes with aircraft attitude, causing the beam to paint different levels and possibly miss a cell or show false gaps. The MEL note also says to avoid use in moderate or greater turbulence, which is exactly the environment near a CB. The crew cannot rely on the radar to thread a narrow gap between cells.
The strategy should therefore be more conservative: keep a wider lateral margin from convective cells, use ATC weather and pilot reports, and avoid penetrating any area of moderate or greater precipitation. If the destination is covered by a cell, do not attempt a close-in diversion around it; hold clear or divert early. The radar is still useful for gross avoidance, but it is not a precision tool in this configuration.
3. State the dispatch and in-flight decision.
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The flight should not be dispatched with C as the only alternate. Add a second alternate D with weather at or above the operator's minima and enough fuel to reach it, or delay until C's improvement is forecast to occur before ETA. The fuel plan must protect the final reserve; the 30-minute final reserve is not holding fuel. If the destination TEMPO is active, carry holding fuel or plan to divert before reaching the final reserve.
In flight, obtain an updated destination and alternate forecast, use the radar and ATC information to stay clear of the CB, and set a decision point before the destination. If B is not clearly stable by the final approach fix, or if a cell is over the airport, go around and divert to D. Do not use C just because it is the planned alternate. If fuel becomes tight, declare minimum fuel or urgency early. The human-factors trap is the expectation that the TEMPO will have passed and the belief that a later BECMG can be used as an alternate; both should be resisted.