Fuel Planning: Fitting a Fourth Leg Into a Fixed Fuel Budget
A crew is planning a three-leg survey sortie in a twin-engine aircraft with 320 gal of usable fuel. Planned cruise fuel flow is 52 gal/hr, except for leg 2, which must be flown at low altitude where flow rises to 58 gal/hr. The legs are 1.2 hr, 1.8 hr, and 0.9 hr. Company fuel policy requires landing with a reserve equal to 45 minutes at cruise flow plus 10% of the fuel actually planned for the trip. Winds are as forecast and no alternate is required. After the briefing, the mission commander asks whether a fourth leg of 1.1 hr at 52 gal/hr can be added to collect additional survey data, since the aircraft will be airborne anyway.
1. Compute the fuel required for the three-leg plan and state how much fuel would remain above the required reserve on landing.
Read the worked answer
Trip fuel
- Leg 1: 1.2 hr x 52 gal/hr = 62.4 gal
- Leg 2: 1.8 hr x 58 gal/hr = 104.4 gal
- Leg 3: 0.9 hr x 52 gal/hr = 46.8 gal
- Total trip fuel = 213.6 gal
Reserve
- 45 min at cruise flow = 0.75 x 52 = 39.0 gal
- 10% of trip fuel = 0.10 x 213.6 = 21.4 gal
- Total reserve = 60.4 gal
Result
- Required fuel = 213.6 + 60.4 = 274.0 gal, inside the 320 gal available.
- Fuel at landing (reserve untouched) = 320 - 213.6 = 106.4 gal.
- Margin above reserve = 320 - 274.0 = 46.0 gal, about 0.88 hr of cruise flight.
The three-leg plan is comfortably legal; the interesting question is what that 46 gal margin will actually buy.
2. Does the proposed 1.1 hr fourth leg fit within policy? Show the comparison.
Read the worked answer
With the fourth leg
- Extra fuel: 1.1 hr x 52 gal/hr = 57.2 gal
- New trip fuel = 213.6 + 57.2 = 270.8 gal
- New reserve = 39.0 + (0.10 x 270.8) = 39.0 + 27.1 = 66.1 gal
- Total required = 270.8 + 66.1 = 336.9 gal
Available usable fuel is 320 gal, so the plan is short by about 17 gal.
The trap is checking only the extra leg (57.2 gal) against the 46 gal margin. Adding trip fuel also enlarges the 10% reserve term, which is why the plan fails by 17 gal rather than needing only 57.2 gal. Answer: no, the fourth leg as briefed does not fit.
3. What is the longest fourth leg, at 52 gal/hr, that still satisfies the reserve policy? Express it in minutes and say whether it is worth flying.
Read the worked answer
Let t be the extra hours at 52 gal/hr. Trip fuel becomes 213.6 + 52t, and the policy requires:
1.10 x (213.6 + 52t) + 39.0 <= 320
- 234.96 + 57.2t + 39.0 <= 320
- 57.2t <= 46.04
- t <= 0.805 hr, i.e. about 48 minutes
So a 45-minute leg fits with roughly 6 gal of slack (0.05 hr x 57.2 gal/hr equivalent, about 3 gal of trip fuel plus its 10%). A 50-minute or 1.1 hr leg does not.
Judgement point: the answer is not 'yes or no' but a number. A commander told only '1.1 hours is too long' may shorten to 1.0 hour and still be illegal; the usable figure is about 48 minutes, and a conservative 45 minutes leaves margin for the low-altitude flow being slightly worse than forecast.