In-Flight Fuel Management

Fuel planning ends when the aircraft leaves the stand, and fuel management takes over. However carefully the block fuel was calculated, the flight will meet winds, levels, routings and delays that differ from the plan. In-flight fuel management is the discipline of noticing those differences early, while they are still cheap to solve, and of acting on them before the final reserve is threatened.

The rules make it an explicit duty. Under EASA rules the in-flight fuel management policy is one of the three parts of an operator's fuel scheme (CAT.OP.MPA.185 for commercial air transport), Part-NCO has its own equivalent for private flights (NCO.OP.185), and SERA.11012, with its guidance material, covers the two radio calls that tell ATC about a fuel problem. Fuel exhaustion is rarely a matter of arithmetic alone. In 1978 United Airlines Flight 173, a DC-8, held near Portland, Oregon, for about an hour while its crew dealt with a landing gear indication, and ran out of fuel. The first officer and flight engineer mentioned the fuel state but never pressed the point.

Fuel on board and usable fuel

Fuel on board (FOB) is the total quantity in the tanks, shown in modern aircraft by a fuel quantity indicating system; on the Airbus A320 it appears on the engine and warning display. Only usable fuel counts for planning and for decisions in flight. Unusable fuel is the small quantity that cannot be relied on to reach the engines in all attitudes; it is part of the basic empty mass, not of the fuel figure.

An indication is not the same as knowledge. The float-type gauges of many light aeroplanes read directly from the tanks but are often inaccurate at intermediate levels. Pilots therefore confirm the contents before flight visually or with a calibrated dipstick, then track fuel in flight by time and known consumption, using the gauges as a cross-check. A fuel totaliser subtracts measured fuel flow from a starting quantity entered by the pilot. It is precise but cannot see the tanks, so a wrong starting figure or a leak passes unnoticed. The robust practice compares three sources: gauges, totaliser and fuel log.

Airliners apply the same logic. On the A320, for example, the fuel used by each engine is counted from engine start, so fuel on board plus fuel used should stay consistent with the fuel on board at departure. A growing discrepancy, or fuel on board falling faster than the fuel flow explains, is the classic sign of a leak, and the manufacturer's fuel leak procedure then applies.

Endurance and safe endurance

Endurance is the time the aircraft can fly on its usable fuel: usable fuel divided by fuel flow. With 133 litres usable at 38 litres per hour it is 3 h 30 min. Endurance is a time, so the wind does not change it; the wind changes the range, the distance that can be flown in that time. The fuel endurance in hours and minutes is also the E/ entry in Item 19 of the ICAO flight plan, the supplementary information passed to search and rescue services if the aircraft becomes overdue.

Safe endurance is the endurance less the reserve that must remain on landing. An aeroplane with 136 litres usable burning 32 litres per hour has an endurance of 4 h 15 min; keeping the 30-minute day VFR reserve intact leaves a safe endurance of 3 h 45 min. Safe endurance is the figure used for the point of safe return (see point of equal time and point of no return), and it is the right figure for any decision in flight. An aeroplane that took off with 120 litres, burns 30 litres per hour and finds its destination closed after exactly 2 hours has 60 litres left; protecting a 15-litre reserve, it can fly for 1.5 hours more, not 2.

The speed flown changes both figures. For the longest time airborne, while holding for a runway to reopen, a propeller aeroplane flies at its minimum power speed and a jet near its minimum drag speed. For the greatest distance into a headwind, both fly somewhat faster than their still-air best range speed, because less time is spent in the wind; with a tailwind, somewhat slower.

In-flight fuel checks

The commander must ensure that fuel checks are made at regular intervals. At each check the usable fuel remaining is recorded and evaluated to:

• compare actual consumption with planned consumption;
• check that the remaining fuel is sufficient to complete the flight; and
• determine the fuel expected to remain on arrival at the destination.

An airline operational flight plan prints the planned fuel remaining against each waypoint, and the crew write the actual time and fuel beside it; a private pilot's navigation log does the same in its fuel column. The check is only useful if it is made against the planned figure for that point.

EASA material adds a decision rule. If a check shows that the fuel expected on arrival at the destination is less than the alternate fuel plus the final reserve fuel, the commander must take into account the traffic and operational conditions at the destination, along the diversion route and at the alternate before deciding whether to continue or divert. Whatever the decision, the flight is conducted so as to land with at least the final reserve. Part-NCO puts the same idea for private flights: the usable fuel remaining must never be less than the fuel needed to reach an aerodrome where a safe landing can be made with the final reserve intact. If the planned alternate can no longer be reached with that margin, the plan changes, for example by diverting early to a nearer aerodrome.

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Fuel consumption monitoring

A single check shows whether the flight is on plan; the trend shows why not. A steady excess over the planned burn may come from a stronger headwind than forecast, a lower level than planned, a heavier aeroplane, an off-schedule speed, a configuration problem or an airframe that burns more than the book figure. Groundspeed worked out at each fix gives the revised ETAs: crossing a VOR at 10:12 and a point 30 NM further on at 10:24 means 150 kt, so the next 70 NM will take 28 minutes. Comparing the actual wind with the flight plan wind shows whether the rest of the plan is still realistic. In aircraft with a flight management system the fuel prediction pages do this continuously, but their predictions rest on the entered wind, weights and fuel, and must be checked against the log.

At operator level, a fuel consumption monitoring programme records the actual burn of each aircraft against the planning figures over many flights. Such data, analysed statistically with the approval of the authority, can allow an operator to carry less contingency fuel than the basic 5 per cent of trip fuel. The same data reveal individual airframes whose performance has deteriorated.

Bingo fuel and diversion decisions

Bingo fuel is a fuel figure, worked out in advance, at which the crew will start the diversion or return whatever the prospects at the destination. The term is military slang for the fuel state at which an aircraft must break off and return to base; it is not a term of the civil fuel rules or of standard ATC phraseology, in which only MINIMUM FUEL and MAYDAY FUEL have defined meanings. In informal civil use it is the fuel required to fly from the aircraft's position to the chosen alternate and land with the final reserve, plus any margin the crew decide on. Setting it before the pressure builds turns a judgement made under stress into a comparison of two numbers.

Holding is where it matters most. Before entering a hold at a destination with delays, the crew work out how long they can hold before they must leave for the alternate, tell ATC how much holding time they have, and commit to leaving at that point. The Airbus A320's hold page shows the last exit time and fuel for this purpose. The trap is plan continuation: the destination seems about to reopen, so the crew hold a few minutes more, then a few more, until the diversion is made with minimum fuel or not at all. Stating the decision point aloud, and treating it as binding, is the countermeasure.

An in-flight diversion is decided by fuel, weather, runways and facilities together (see alternate aerodromes and managing non-normal situations and diversion). An early diversion costs little: it leaves a choice of aerodromes and normal procedures. A late one may leave a single aerodrome, priority handling and an emergency.

Minimum fuel declaration

When the crew are committed to landing at a specific aerodrome, and any change to the existing clearance may result in landing with less than the planned final reserve fuel, they declare MINIMUM FUEL to ATC, in the sense given by the guidance material to SERA.11012. It is an advisory, not an emergency. It asks for no priority and receives none; ATC tells the crew of any expected delay, so that they can decide before the situation becomes an emergency. A MINIMUM FUEL call does not make a plan acceptable that would breach the final reserve: if a nearer aerodrome can still be reached with the reserve intact, the right action is to divert there.

The FAA's AIM (5-5-15) uses the same term with a similar meaning: on reaching the destination the aircraft cannot accept any undue delay. It is not an emergency and does not imply a need for traffic priority. A pilot who needs priority declares an emergency for low fuel and reports the fuel remaining in minutes.

MAYDAY FUEL

MAYDAY MAYDAY MAYDAY FUEL is a distress call. SERA.11012 requires a fuel distress to be declared with MAYDAY, preferably spoken three times, followed by the word FUEL, and the fuel management rules set the trigger, which is met when the calculated usable fuel predicted to remain on landing at the nearest aerodrome where a safe landing can be made is less than the planned final reserve fuel. The test is a prediction, not the quantity in the tanks now: the call is made as soon as the calculation shows that the reserve will be used, not when it is reached. A piston-engined commercial aeroplane that would land with 35 minutes of fuel against its 45-minute final reserve is already in distress. The declaration gives the flight priority.

The two calls form a sequence. MINIMUM FUEL says that the reserve is at risk if anything changes; MAYDAY FUEL says that it will be used whatever happens. Using the defined words, rather than phrases such as "short of fuel", removes any doubt about what the controller should do (see distress and urgency communications and fuel planning and fuel reserves).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.