Reroute, Headwind, and a Deteriorating Destination
You are flying a Cessna 172 from Enugu (ENU) to Port Harcourt (PHC), 120 nm, at 105 kt TAS and 9.5 US gal/h. Planned groundspeed is 110 kt. Usable fuel is 50 US gal and you depart with 40 gal after taxi. The planned alternate is Owerri (QOW), 35 nm from PHC, about 0.4 h. You plan 45 minutes final reserve and 15 minutes contingency. Twenty minutes after departure, ATC reroutes you around traffic. The reroute adds 25 minutes and the actual groundspeed drops to 90 kt. At that point you have flown 30 nm. Port Harcourt then reports visibility 2,000 m in haze with broken cloud at 800 ft. Owerri reports 10 km visibility and scattered cloud at 2,500 ft. You must decide whether to continue to PHC, divert to QOW, or return to ENU, and you must show the fuel arithmetic.
1. Calculate the fuel remaining at Port Harcourt if you continue, and compare it with the fuel required for the alternate and reserves.
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After 20 minutes at 9.5 gal/h, fuel used is 3.2 gal, leaving 36.8 gal. The remaining distance is 90 nm. At 90 kt that is 60 minutes, plus the 25-minute reroute, so 85 minutes to PHC. Total flight time is 105 minutes = 1.75 h. Fuel used in flight = 1.75 × 9.5 = 16.6 gal. Fuel at PHC = 40 − 16.6 = 23.4 gal (ignoring the small taxi allowance already used). Required after landing: alternate 0.4 h = 3.8 gal, reserve 0.75 h = 7.1 gal, contingency 0.25 h = 2.4 gal, total 13.3 gal. The surplus is about 10.1 gal, or 64 minutes. Fuel alone does not require a diversion.
2. Decide whether to continue to PHC, divert to QOW, or return to ENU. Justify the decision using the weather and the fuel figures.
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I would not continue to PHC under VFR. Broken cloud at 800 ft and 2,000 m visibility mean I cannot maintain the required VFR cloud separation and visibility, and there is no guarantee of a visual approach. The fuel calculation shows I have enough to reach PHC, but “enough fuel” does not make an illegal or unsafe VFR arrival acceptable. QOW is the better choice because it is close, has 10 km visibility and scattered cloud at 2,500 ft, and is already the planned alternate. Returning to ENU is safe but unnecessary unless QOW weather also deteriorates. The decision point is now, while I still have 23.4 gal, not after flying another 60 nm toward a destination I cannot legally enter.
3. From your position 30 nm from ENU, estimate the time and fuel to reach QOW. State any assumption you make.
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The remaining distance to PHC is 90 nm, and QOW is 35 nm from PHC. Assuming QOW lies roughly along the same route, the distance from the present position to QOW is about 90 − 35 = 55 nm. At the current 90 kt groundspeed, time = 55 / 90 × 60 = 37 minutes. Fuel = 37/60 × 9.5 = 5.9 gal. Fuel on arrival at QOW would be about 36.8 − 5.9 = 30.9 gal, leaving a large reserve. The assumption matters: if QOW is off track rather than on the direct line, the distance could be 60–70 nm, so I would verify the track and distance from the chart before turning. I would also confirm QOW weather by radio before committing.