The Commercial Fuel Plan - ATPL 033 — Flight Planning and Monitoring

The previous lesson supplied the machinery; this one builds the plan the machinery serves. For commercial flights the basic fuel scheme of the operational requirements (CAT.OP.MPA.181) prescribes the plan component by component, each with its own recipe, and the exam tests every recipe separately and then the sum. The components come in a fixed order: taxi, trip, contingency, alternate, final reserve, additional, extra — and this lesson walks the stack from the bottom up.

Taxi fuel

The first fuel spent never leaves the ground: the taxi fuel covers engine start, the APU and taxiing to the runway, and it is determined from the manual's usage tables in the light of the local conditions at the departure aerodrome: a home base with a two-minute taxi and a hub with rush-hour queues to the holding point burn very different amounts before brake release.

Trip fuel

The trip fuel is the planned burn from take-off to landing at the destination (or, when the plan is redone in flight, from the point of in-flight replanning). It is priced segment by segment from the manual's tables, and the segments are:

Take-off and climb — from brake release to the initial cruising level, on the expected departure routing.
Cruise — including any planned step climbs or en-route descents.
Descent — from top of descent to the point the approach begins.
Approach and landing — down to the runway at the destination.

What the trip fuel does not contain is just as testable: the missed approach at the destination is not trip fuel. The trip ends at a planned landing; the moment the landing fails, the next component takes over.

Contingency fuel

The trip fuel is a forecast, and forecasts miss. Contingency fuel exists for the unforeseen deviations that push the real burn past the planned one:

The aeroplane itself — this airframe's consumption deviating from the book figures.
The meteorology — winds and temperatures turning out worse than forecast.
The routing — ATC assigning levels or tracks other than the planned ones.

The amount is the higher of two figures: 5 per cent of the planned trip fuel (on in-flight replanning, 5 per cent of the trip fuel for the remainder of the flight), or the fuel to fly for 5 minutes at holding speed at 1,500 ft above the destination in ISA. On a long sector the percentage wins comfortably; on a short hop 5 per cent of a small trip can shrink below the 5-minute figure, and the holding branch becomes the floor. Compute both, keep the larger.

Alternate fuel

When the destination refuses the landing, the plan must already have paid for the escape. The alternate fuel covers the whole diversion, and it starts where the trip fuel stopped: with the missed approach at the destination, then the climb to the diversion cruising level, the cruise, the descent, and the approach and landing at the destination alternate. Three cases share the component:

One alternate — the standard case: fuel for the full diversion just described.
Two alternates required — when conditions at the destination demand a second one, the alternate fuel is based on the alternate requiring the greater amount.
No alternate required — where the rules permit planning without a destination alternate, the component becomes the fuel to fly for 15 minutes at holding speed at 1,500 ft above the destination in ISA.

Final reserve fuel

The previous lesson set the floor; here it takes its place in the stack. The final reserve fuel is the last protection against fuel exhaustion, the quantity every plan intends to land with intact: 30 minutes for turbines, 45 for pistons, at holding speed at 1,500 ft above the aerodrome in ISA. It is calculated with the holding fuel flow at the estimated arrival mass at the destination alternate (or at the destination, where no alternate is required), because a light aeroplane holds cheaper than a heavy one and the reserve is priced where it would actually be burned.

Additional fuel

Additional fuel is the component most plans carry none of, because it is a supplement, required only if the rest of the plan falls short of one specific emergency: an engine failure or a loss of pressurisation at the most critical point of the route, whichever of the two demands more fuel. From that point the aeroplane must be able to descend as necessary, proceed to a fuel en-route alternate, hold there for 15 minutes at 1,500 ft in ISA, and make an approach and landing. Depressurisation is often the harder case, since the rest of the flight must then be flown low, where the engines drink. The test is a comparison: price the scenario, compare it with the fuel the standard plan would have on board at the critical point, and load only the shortfall — if the standard plan already covers it, additional fuel is nil.

Extra fuel and the cost of carrying it

Everything so far is required; extra fuel is chosen. The commander or the operator may add it for anticipated delays or predictable operational constraints: a destination that always holds at rush hour, forecast thunderstorms sitting on the arrival. How much extra is possible is a question of margins, and the most restrictive one governs: the room left in the tanks above the planned block fuel, the room under the maximum take-off mass, and the room under the landing-mass limit at the destination.

Extra fuel is not free to transport: every kilogram carried raises the aircraft mass, and a heavier aeroplane burns more to fly the same route. The fuel penalty of carrying fuel is quoted as a percentage of the carried mass per hour. Carry 1,000 kg of extra fuel for 4 hours at a penalty of 4 per cent per hour and the burn rises by 1,000 × 0.04 × 4 = 160 kg: only 840 kg of the loaded thousand actually arrives. The arithmetic runs both ways: to arrive with a wanted quantity, more than that quantity must be loaded, a truth that decides whether carrying cheap fuel outbound ever pays — the economics of that trade, tankering, belong to the next lesson.

The total and the completed plan

The total is the sum of the stack, and the sum has two names the mass-and-balance course already uses: the whole quantity on board at start-up is the block fuel, and block fuel minus taxi fuel is the take-off fuel, the amount at brake release. Completing the fuel section of the navigation plan then distributes the total along the route: each leg gets its planned burn from the tables, the burns accumulate waypoint by waypoint, and the plan states the planned fuel remaining at every waypoint. Those printed figures are what the in-flight fuel checks will later be measured against, which is what makes the completed plan an instrument rather than paperwork.