Convective line on the approach and a tightening fuel state
It is 21:40 local. You command a twin turboprop on a 1 h 50 min IFR night flight. The destination TAF gives wind 240/18 G 32 kt, visibility 8 km, few cloud 600 ft, scattered 2,000 ft, with TEMPO 2,000 m thunderstorms and rain, scattered CB 1,200 ft. The destination has a CAT I ILS (DA 250 ft, RVR 800 m). Your alternate 35 minutes away is forecast 10 km, broken 3,000 ft, with an ILS. Fuel on departure: taxi 40 kg, trip 900 kg, contingency 90 kg, alternate 400 kg, final reserve 300 kg. You are 45 nm from the destination in IMC. The weather radar shows a broken line of cells across the final approach area, with one gap 12 nm north of the field. Fuel remaining is 950 kg; you burn 500 kg/h in cruise and 420 kg/h in the hold.
1. Assess whether the planned alternate is acceptable and explain how the TEMPO group must be treated in your planning.
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Alternate requirement: the destination forecast contains TEMPO thunderstorms with CB, so you cannot plan on the destination being usable at ETA; a destination alternate is required. The "two separate runways" exemption does not help, because it requires the forecast to be above the applicable minima for the whole relevant period, and a TEMPO TS group is not.
Is the alternate legal? An alternate must be forecast, for the period from 1 hour before to 1 hour after the estimated time of arrival, to be at or above its operating minima. The alternate forecast of 10 km and broken 3,000 ft is far above the ILS minima, so it is acceptable on weather. Two further checks: it must be reachable with the planned alternate fuel, and it must be open and have the approach serviceable - worth confirming with the operator at this hour.
Treating TEMPO: TEMPO means conditions expected to occur for less than one hour at a time and in total for less than half the period, but there is no guarantee they avoid your ETA. Plan as if the destination may be at 2,000 m visibility with CB on the approach at the moment you arrive, and treat the destination as a bonus rather than the plan. Brief the approach with a firm go-around trigger based on the actual conditions on the day.
2. With 950 kg remaining at 45 nm, decide between holding for the gap to move over the field and diverting now. Show the fuel numbers and set a decision point.
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If you land at the destination: 45 nm at about 300 kt groundspeed is roughly 9 minutes, about 75 kg, plus 80 kg for the approach and landing, so landing fuel is roughly 795 kg. That must still cover alternate 400 kg plus final reserve 300 kg, leaving only about 95 kg of margin. The aircraft is already near the edge of its planned fuel.
Cost of holding: 420 kg/h = 7 kg/min.
- 10 minutes: 70 kg - landing fuel about 725 kg, which still covers 400 + 300 = 700 kg, but with only 25 kg spare.
- 20 minutes: 140 kg - landing fuel about 655 kg, below the 700 kg needed to divert and hold the final reserve.
Decision: a hold longer than about 10 minutes makes the diversion itself marginal, and the weather is a broken line of CB with a gust front, not a single cell that will clear predictably. Divert now to the ILS alternate. State the intention to ATC, request the diversion, and keep the destination as an option only if it becomes clearly available before you leave the hold. If fuel ever falls below the alternate plus final reserve, declare a fuel emergency (MAYDAY FUEL) rather than pressing on quietly. Never let the gap in the line become an unplanned visual approach into a thunderstorm at night.
3. You are on short final at the destination and the conditions are deteriorating. What are the signs of gust-front windshear and what is your response?
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Signs to watch for on final:
- A sudden increase in headwind and airspeed, then a rapid reversal to a tailwind and a sink - a microburst gives performance better than normal before it becomes much worse.
- An abrupt wind shift or increase in gustiness, a temperature drop of a few degrees, a rapid pressure change (altimeter or VSI), and heavy rain or virga near the touchdown zone.
- A roll cloud or visible precipitation shaft under a cell; PIREPs from preceding traffic; a predictive windshear warning in the cockpit.
- A go-around or unstabilised approach by the aircraft ahead.
Response:
- At the first sign, go-around immediately - do not try to fly through it and do not chase the glideslope.
- Apply maximum take-off thrust, pitch to the go-around attitude and hold it; do not fight the flight director.
- Keep the configuration unchanged until clear of the shear and a positive climb is confirmed above 400 ft, then clean up.
- Advise ATC, and do not attempt a second approach into the same cell; proceed to the alternate.
With CB and a 32 kt gust spread on the field, the sensible decision is to divert before the approach rather than after a windshear encounter at night.