Fuel, MRW and the Refuelling Decision
A Brunei operator plans a flight in a turboprop with MTOM 6400 kg, MLM 6400 kg, MZFM 5400 kg and MRW 6500 kg. Basic empty mass 3900 kg at arm 3.80 m. Crew 150 kg at arm 3.20 m. Payload is 1200 kg distributed with a combined moment of 4200 kg·m. The planned trip fuel is 700 kg; the pilot also wants 400 kg of reserve and 300 kg of extra fuel for a possible diversion, all carried in tanks at arm 4.00 m. The ramp is short, and the pilot proposes refuelling to the maximum possible before boarding passengers.
1. Determine the maximum fuel that can be carried without exceeding MTOM, MZFM or MRW, and state which limit is binding.
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Fixed masses
- ZFM = 3900 + 150 + 1200 = 5250 kg (MZFM 5400 kg - margin 150 kg)
- So payload/crew are fixed; fuel is the only variable.
Fuel limits from each mass limit
- From MTOM: fuel ≤ 6400 − 5250 = 1150 kg
- From MRW: fuel ≤ 6500 − 5250 = 1250 kg
- From MZFM: no fuel is in ZFM, so MZFM does not limit fuel directly - but it limits payload.
Binding limit: MTOM, giving a maximum fuel of 1150 kg.
Check against planned fuel
- Planned total fuel = 700 + 400 + 300 = 1400 kg - this exceeds the 1150 kg MTOM-derived maximum by 250 kg. The flight as planned is not legal; 250 kg of fuel must be offloaded, or payload reduced. Note MRW (1250 kg) is not binding here because MTOM is lower relative to the fixed mass.
2. The pilot decides to offload 250 kg of the extra fuel. Compute the take-off mass and CG.
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Revised fuel
- Total fuel = 1400 − 250 = 1150 kg at arm 4.00 m
- TOM = 5250 + 1150 = 6400 kg - exactly MTOM (legal, zero margin)
CG calculation
- Empty: 3900 × 3.80 = 14 820
- Crew: 150 × 3.20 = 480
- Payload moment: 4200 (given)
- ZF moment = 14 820 + 480 + 4200 = 19 500; ZF CG = 19 500 / 5250 = 3.714 m
- Fuel: 1150 × 4.00 = 4600
- TO moment = 24 100; TO CG = 24 100 / 6400 = 3.766 m
Interpretation: The CG is aft of the ZF CG because fuel is carried aft of the ZF centroid (4.00 m > 3.71 m). The aircraft is at MTOM with no margin - any last-minute passenger or baggage addition would breach MTOM. Good practice is to plan a small margin below MTOM rather than operating exactly at the limit, because taxi fuel and last-minute changes are common.
3. Explain why MRW did not govern this flight and describe a scenario in which MRW would become the binding limit.
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Why MRW did not govern
- MRW (6500 kg) is the maximum mass at which the aircraft may be manoeuvred on the ground; it is normally ≥ MTOM because it includes taxi fuel.
- Here MTOM (6400 kg) is 100 kg below MRW, so the take-off mass limit is reached first. Since fuel was the variable and MTOM − fixed mass = 1150 kg, that figure was lower than MRW − fixed mass = 1250 kg.
When MRW would bind
- If the aircraft were loaded with a large fuel load for a long taxi or a long sector, the ramp mass could reach MRW before MTOM. For example, if fixed mass were 5250 kg and fuel 1250 kg, ramp mass = 6500 kg = MRW, while TOM after 100 kg taxi burn = 6400 kg = MTOM - both limits met simultaneously.
- If MTOM were higher than MRW (some aircraft have MRW below MTOM), MRW would always bind first.
- The candidate should compute all three fuel-derived maxima (MTOM − fixed, MRW − fixed) and take the lowest; MZFM limits payload, not fuel. In this case MTOM governed, but the check must be done every time because the binding limit changes with payload and fuel distribution.