Fuel, altitude and a 170 NM leg that does not add up
You are planning the club Cessna 172S (MTOM 1,157 kg, basic empty mass 745 kg) on a 170 NM VFR leg. On board will be the pilot (85 kg), two passengers (80 kg and 80 kg) and 15 kg of baggage. The tanks hold 95 litres of the 201 litres usable, and fuel is available at the departure airfield. The AFM gives a TAS of 110 kt. At 4,500 ft the forecast is a 20 kt headwind with a fuel flow of 34 l/h; at 2,500 ft it is an 8 kt headwind with 36 l/h. Your alternate is a 20-minute flight from the destination. You intend to apply the Part-NCO fuel policy: taxi 5 minutes, trip fuel, 5 % contingency, alternate fuel and a 45-minute final reserve. Your first choice is 4,500 ft, which gives a smoother ride above the thermals.
1. Calculate the fuel required for the 4,500 ft plan and compare it with the 95 litres on board.
Read the worked answer
Work component by component; the order matters because the reserve must survive the whole flight.
- Trip: TAS 110 kt minus 20 kt headwind = 90 kt ground speed. 170 / 90 = 1.89 h, i.e. about 1 h 53 min. Fuel = 1.89 x 34 = 64.2 l.
- Contingency: 5 % of trip fuel = 3.2 l.
- Alternate: 20 min at 34 l/h = 11.3 l.
- Final reserve: 45 min at 34 l/h = 25.5 l.
- Taxi: 5 min at 34 l/h = 2.8 l.
Total = 64.2 + 3.2 + 11.3 + 25.5 + 2.8 = 107.0 litres.
On board: 95 l. That is about 12 litres short, so the flight as planned is not legal and, more importantly, not safe: with 95 l you would land with roughly 33 l remaining instead of the 36.8 l needed for alternate plus reserve, and any delay at the destination eats the difference.
Check the uplift is possible: 745 + 85 + 80 + 80 + 15 = 1,005 kg, leaving 152 kg to MTOM, equivalent to about 211 l - more than the tanks hold, so fuel is the only limit, not mass. Take at least 110 l.
2. Compare 4,500 ft with 2,500 ft on time and fuel, and state which profile you would file.
Read the worked answer
At 2,500 ft: TAS 110 minus 8 = 102 kt ground speed. 170 / 102 = 1.667 h, i.e. 1 h 40 min. Trip fuel = 1.667 x 36 = 60.0 l; contingency 3.0 l; alternate 11.3 l; reserve 25.5 l; taxi 2.8 l. Total = 102.6 l.
So the lower level saves about 4.4 l and 13 minutes despite the higher fuel flow, because the headwind component is 12 kt weaker and ground speed is what burns the clock.
Which to file:
- File 2,500 ft - it is better on both counts, and the smoother air at 4,500 ft is a comfort argument, not a planning one.
- But recognise what this does not fix: 102.6 l is still more than the 95 l in the tanks. Neither level makes the flight legal without refuelling.
- After refuelling to 110 l, the margin at 2,500 ft is about 7.4 l - around 13 minutes of flight, which is thin for a leg with a forecast wind. That margin, not the smoothness, should drive the cruise-level decision.
3. Forty minutes after departure (fuel flow 34 l/h) ATC reroutes you, adding 12 minutes, and the headwind is 5 kt stronger than forecast. Assess the fuel situation in flight.
Read the worked answer
Recompute against what is actually left, not against the plan.
- Fuel used: 40 min at 34 l/h = 22.7 l. Remaining from 110 l = 87.3 l.
- Trip fuel still needed: 64.2 - 22.7 = 41.5 l, plus the 12-minute reroute (6.8 l) = 48.3 l.
- Add contingency 3.2 l, alternate 11.3 l and final reserve 25.5 l = 88.3 l.
That is 1 litre more than you have. The stronger headwind then makes it worse: 5 kt of extra headwind on 90 kt is roughly 5 % more time, about 2.5 l.
Decision options, in order of preference:
- Reduce power to a lean-of-peak or lower cruise setting and accept a longer flight at a reduced flow; a 10 % cut in flow easily restores the margin.
- Cancel the alternate only if the destination weather genuinely allows a visual arrival with no alternate required - not because fuel is tight.
- Divert now to an airfield on track where fuel is available, rather than pressing on and discovering the shortfall overhead the destination.
The exam point is that the final reserve is not a buffer to be spent; it is the fuel that keeps you airborne while you find somewhere to land after the plan has already failed.