Double the Planned Burn, Halfway to Yola
You are flying a club Cessna 172 from Kaduna to Yola, a planned distance of 246 nm. Usable fuel is 144 L, cruise consumption 34 L/h, TAS 108 kt, and the forecast wind 090/20 gives an expected 14 kt tailwind. Taxi and run-up use 4 L. You apply a 30-minute contingency and a 45-minute final reserve; no destination alternate is required because the forecast is CAVOK for ±2 hours. One hour and ten minutes after take-off the totaliser shows 82 L used, the left gauge reads 30 L and the right gauge 32 L. The right gauge has been dropping visibly faster since about 40 minutes. The engine runs smoothly with normal EGT and fuel pressure. An updated wind gives a headwind of 8 kt, reducing ground speed to 100 kt. The destination is still 140 nm ahead, and there is an aerodrome with a 1,600 m runway 30 nm off track.
1. Reproduce the planned fuel calculation and state the departure surplus in litres and minutes.
Read the worked answer
Ground speed = 108 + 14 = 122 kt. Time = 246 ÷ 122 = 2.02 h.
- Trip fuel: 34 × 2.02 = 68.6 L
- Taxi/run-up: 4.0 L
- Contingency 30 min: 34 × 0.5 = 17.0 L
- Final reserve 45 min: 34 × 0.75 = 25.5 L
- Total required: 115.1 L
Usable fuel 144 L gives a surplus of 28.9 L, about 51 minutes at cruise burn. That is a normal, legal plan - but note how thin it is: the surplus is little more than the contingency plus a bit. There is no alternate fuel in the plan at all, because the destination forecast made one unnecessary. The whole plan therefore depends on the burn rate and the wind both being right.
2. Using the in-flight figures, decide whether the destination can be reached with the final reserve intact.
Read the worked answer
Fuel remaining = 144 − 82 = 62 L (consistent with the gauges: 30 + 32).
On the planned burn: 140 nm at 100 kt = 1.4 h; 34 × 1.4 = 47.6 L, leaving 14.4 L ≈ 25 minutes - below the 45-minute reserve of 25.5 L. Already not legal.
On the observed burn: (82 − 4 taxi) ÷ 1.167 h = 66.9 L/h, almost double the plan. At that rate the 140 nm needs 93.7 L, and 62 L gives only 0.93 h (56 minutes) of endurance against 84 minutes required. The destination is unreachable.
The transition from a comfortable plan to an unreachable destination happened without any single dramatic event, which is exactly how fuel incidents occur. The correct decision is made now, not in 20 minutes: divert to the aerodrome 30 nm off track. At 100 kt that is 0.3 h; at the observed 66.9 L/h it costs about 20 L, leaving roughly 42 L (about 38 minutes) on landing - comfortably above reserve.
3. What does the asymmetric gauge behaviour mean, and what should you actually do in the next ten minutes?
Read the worked answer
The totals agree with the totaliser, so the fuel is not 'missing'; what is abnormal is the rate and the split. One tank draining faster, combined with double the planned consumption, points to a leak or transfer between tanks, or a gauge/sender error. You cannot distinguish these in flight, so you must plan on the pessimistic interpretation.
Actions, in order: confirm the fuel selector is on BOTH (a selector left on one tank explains the split), lean the mixture and reduce power to cut consumption, and cross-check the gauges against elapsed time every 5–10 minutes rather than trusting a single reading. Then declare PAN PAN (a MAYDAY if the engine falters), tell ATC your fuel state, endurance and diversion, and land at the nearest suitable aerodrome. Do not press on to a destination with a 45-minute reserve that you no longer hold, and do not delay the diversion to 'see how it goes' - a leak does not improve, and the reserve exists precisely to buy the time you are now spending.