ATS flight-plan level, alternate and endurance
A business jet is planned from Nice (LFMN) to Shannon (EINN). The planned track is 315° magnetic. The route distance is 780 NM. The crew intends to file FL310, but the CFMU advises that FL310 is not available and offers FL300. Forecast wind at FL310 is 270°/60 kt; at FL300 it is 270°/50 kt. TAS is 440 kt and cruise fuel flow 2,200 kg/h. The semi-circular rule applies. The destination forecast at ETA is 25012KT 9999 SCT030, but there is a PROB30 TEMPO 2500 -RA BKN008. Planning minima for Shannon is 600 ft ceiling and 3 km visibility. The destination alternate is Cork, 90 NM away, with fuel 450 kg. Final reserve is 600 kg, taxi 150 kg, contingency 5% of trip fuel, and additional fuel 200 kg. The crew must file an ATS flight plan and decide the level and alternate.
1. Choose the correct cruising level under the semi-circular rule and calculate ground speed, time and fuel for FL300 versus FL310. Which level should be filed?
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Track 315° lies in the 180–359° semicircular band, so even flight levels are required. FL310 is odd and therefore not correct; FL300 is even and compliant. The CFMU offer also removes FL310.
FL300: wind 270/50, track 315. Tailwind component ≈ 50 cos45° = 35 kt; crosswind ≈ 35 kt; drift ≈ arcsin(35/440) ≈ 4.6°. GS ≈ 440 cos4.6° + 35 ≈ 474 kt. Time = 780/474 = 1.646 h = 98.8 min; fuel = 2,200 × 1.646 = 3,621 kg.
FL310 would give about 480 kt and 3,576 kg, only about 45 kg less, but it is not fileable. File FL300.
2. Decide whether a destination alternate must be filed, and compute the endurance to enter on the ATS flight plan.
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The TEMPO condition gives BKN008 (800 ft) and 2500 m visibility. The visibility is below the 3 km planning minimum, even though the ceiling is above 600 ft. Because the TEMPO condition affects the ETA, a destination alternate is required. File Cork.
Trip fuel 3,621 kg; contingency 5% = 181 kg. Total fuel = taxi 150 + trip 3,621 + contingency 181 + alternate 450 + final reserve 600 + additional 200 = 5,202 kg. Take-off fuel = 5,052 kg. Flight endurance = 5,052/2,200 = 2.30 h = 2 h 18 min; total fuel endurance = 5,202/2,200 = 2 h 22 min. Enter the figure required by company/ATS procedure, normally the total endurance, and ensure it covers trip, alternate, contingency and final reserve.
3. If the EOBT slips by 45 minutes, what revisions are needed?
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A 45-minute slip exceeds the 30-minute threshold normally used for ATS flight-plan updates, so the crew must revise the EOBT with ATC/CFMU, not simply depart late. Re-check the destination TAF and alternate weather for the new ETA, because the TEMPO condition may move. Recompute fuel only if the delay changes taxi fuel, holding, or the alternate; the trip fuel itself is unchanged. If a CTOT is issued, the crew must comply with it and update the flight plan accordingly. The ATS endurance and alternate entry must still match the revised fuel state.