Christchurch to Dunedin: fuel, alternate and a passenger deadline
You are planning NZCH–NZDN in a Cessna 172S (53 US gal usable, 9.5 gal/hr at 65% power, TAS 110 kt). Track distance is 178 nm. The forecast wind is 250/35, giving an average 30 kt headwind. The Dunedin TAF includes TEMPO 2000 ft and 5 km, so company SOP requires an alternate; NZTU is 92 nm from NZDN with a forecast 15 kt tailwind component. Company minima are: 45 min final reserve, 10% contingency on trip and alternate fuel, and 1.5 gal for taxi/run-up. The aircraft currently has 40 gal on board. The VNC shows high terrain on the direct track and a minimum en-route altitude of 7,500 ft; you plan 8,500 ft. The aircraft's service ceiling is 14,000 ft. There is a suitable fuel stop at NZTU, but it adds 40 min and a landing fee. The passenger is a vet who must reach Dunedin for an emergency.
1. Calculate the fuel required under company SOP and decide whether the flight can depart with 40 gal.
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Use the forecast ground speeds, not TAS.
- Trip: GS = 110 - 30 = 80 kt. Time = 178/80 = 2.23 hr. Fuel = 2.23 x 9.5 = 21.2 gal.
- Alternate NZDN–NZTU: GS = 110 + 15 = 125 kt. Time = 92/125 = 0.74 hr. Fuel = 0.74 x 9.5 = 7.0 gal.
- Contingency: 10% x (21.2 + 7.0) = 2.8 gal.
- Final reserve: 45/60 x 9.5 = 7.1 gal.
- Taxi/run-up: 1.5 gal.
Total required = 21.2 + 7.0 + 2.8 + 7.1 + 1.5 = 39.6 gal.
With 40 gal on board the mathematical margin is 0.4 gal, about 2.5 min of cruise fuel. That is not a defensible margin: a 5 kt stronger headwind raises the trip fuel by roughly 1.3 gal and makes the flight non-compliant. The CAA day-VFR minimum may be lower than company minima, but the operator's 45 min reserve and contingency are binding here. Conclusion: do not depart on the present plan; the flight is either non-compliant or so close to the limit that normal forecast error would breach reserve.
2. What is the best operational solution: fuel stop at Timaru, delay, or reduce payload? Explain.
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Reducing payload does not solve the problem because fuel, not weight, is the limiting item; carrying less fuel would breach the reserve. A fuel stop at Timaru is the robust solution. Recalculate the first leg NZCH–NZTU, roughly 85 nm: at 80 kt GS, time = 1.06 hr, trip fuel = 10.1 gal; add 45 min reserve (7.1), 1.5 taxi and about 1.0 contingency = 19.7 gal. With 40 gal available there is ample margin. Refuel at Timaru, then plan NZTU–NZDN with the current wind and a fresh alternate assessment. A delay is acceptable only if the forecast headwind decreases enough to restore a realistic margin; the TEMPO conditions also increase the chance of a diversion. The passenger deadline creates pressure, but it does not change the fuel mathematics. Brief the passenger that a short refuelling stop is the safe way to protect the reserve, and record the decision in the fuel/navigation log.
3. What would you record in the navigation and fuel log, and what would you monitor in flight?
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Record the calculated trip fuel, alternate fuel, contingency, final reserve, taxi fuel, total required and actual fuel on board. Record the wind used, the resulting ground speeds, and the decision to stop at Timaru or delay. In flight, compare actual ground speed and fuel flow with the plan at each waypoint. A simple rule: if actual fuel burn exceeds plan by more than about 5%, or if the ETA slips by more than 10 min, recalculate the reserve immediately. Monitor the Dunedin TEMPO conditions and the Timaru weather as the alternate. If the headwind increases or a diversion looks likely, land at Timaru with the reserve intact rather than pressing on to Dunedin. The log should show the time of each fuel check and the remaining fuel, so the decision is auditable.