Fuel calculation

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The fuel calculation (engl. Fuel calculation ) for the fueling to the fuel amount of an aircraft is performed by the pilot or by the flight dispatcher .

The amount of fuel required for the flight is determined by flight planning software, e.g. B. Lufthansa Systems LIDO or Jeppesen Jetplanner, calculated and listed in the Operational Flight Plan (OFP) with the respective individual values.

The relevant weight and fuel data are entered into the flight management computer (FMC) via the Multi Control Display Unit (MCDU) during flight preparation . Among other things, this reads out the actual amount of fuel available in the aircraft and, in addition to calculating the fuel, provides accurate data on the remaining fuel quantities to be expected during the flight.

The fuel calculation is part of the flight planning subject (EASA learning target 033.03.00.00) for the training of professional pilots and flight service advisors (FDB).

Legal basis

With the Flight Planning and Fuel Management Manual (Doc 9976), the ICAO provides its member states with guidelines for national legislation.

In most European countries that operate their aircraft in accordance with EASA specifications, the calculation of the amount of fuel required for a flight in accordance with EU VO 965/2012 Air Operations, Section CAT.OP.MPA.150 Fuel policy .

There are three possible calculation variants for a flight plan:

  1. Basic Procedure.
  2. Reduced Contingency Fuel (RCF) Procedure - a special procedure that can be applied to your own needs.
  3. Predetermined Point (PDP) Procedure - a special procedure that must be based on the condition of an Isolated Aerodrome.

Definitions and Abbreviations

Fuel is measured in units of mass (kg or lb ) because the volume fluctuates greatly with temperature changes, especially since the temperature of the fuel is subject to large fluctuations (60 ° C in summer at the gate ; -50 ° C near the pole at FL 330).

  • Taxi Fuel - the amount of fuel required to run the auxiliary power unit, start the engines and taxi to the runway.
  • Trip Fuel - the amount of fuel for take-off, climb, travel and descent, as well as for the approach and landing at the destination airport (Destination Aerodrome)
  • Contingency Fuel - the amount of fuel for unforeseen influences during the cruise. The EASA defines several conditions depending on the duration of the flight and monitoring by the aircraft operator (operator).

The higher value between option A and option B must be selected - within option B, however, the lowest value may be used, provided it is not less than option A (MINCONT).

Option A: The MINCONT. The legal minimum is an amount of fuel that can circle five minutes at 1500 feet above the destination airport (Destination Aerodrome). The expected landing weight is important for the calculation.

or

Option B: 5% of the triple fuel

3% of the triple fuel with the addition of an en-route alternate aerodrome.

20 minutes flight time, provided the aircraft operator has a so-called fuel consumption monitoring program and has this procedure approved.

Statistically recorded method, provided the aircraft operator has sufficient statistical data and has this procedure approved.

  • Alternate Fuel - the planned amount of fuel that is required for an onward flight from the Destination Aerodrome to the Alternate Aerodrome .
  • Final Reserve Fuel - the absolute minimum of (usable) reserve fuel. This reserve fuel corresponds to the amount of fuel required to fly a holding pattern in the form of a racetrack pattern at an altitude of 1500 feet above the ground of the alternate airfield with the estimated landing mass for a duration of 30 min / jet, 45 min / prop .
  • Additonal Fuel - the amount of fuel that is listed, for example, in the event that the flight planning takes place without an alternate airport, whereby 15 minutes must be added to the Final Reserve Fuel. In addition, the additional fuel is also necessary when special procedures are used in fuel planning, such as the Predetermined Point (PDP) procedure.
  • Extra Fuel - the planned amount of fuel, which is provided for separate decisions or adjustments by the crew.
  • Block Fuel - planned (expected) fuel quantity - which is composed of all individual fuel quantities (Taxi, Trip, Contingency, Alternate, Final, Additional, Extra) and together forms the complete tank quantity. Within EASA, this amount of fuel is also referred to as ramp fuel
  • Take-Off Fuel - planned amount of fuel which will be reached before the start, after the taxi fuel has been used. This amount of fuel is significant, especially with regard to the Maximum Take-Off Weight (MTOW)
  • Tank Capacity - the amount of fuel available in the tanks due to the design of the aircraft
  • Dry operating weight - empty weight with equipment, non-usable fuel and crew (without cargo and usable fuel)
  • Payload - the weight of all passengers (PAXe) including luggage and hand luggage, air freight (cargo), mail (excluding fuel and empty weight). The Dry Operating Weight (DOW) and the Payload (PL) together result in the Zero Fuel Weight (ZFW)
  • Zero Fuel Weight - weight with payload and empty tanks (without usable fuel)
  • Take-Off Weight - planned take -off weight at the Departure Aerodrome
  • Landing Weight - planned landing weight at the Destination Aerodrome
  • Gross Weight (GW) - total weight of the aircraft including all weights. DOW + PL + current fuel weight
  • Desired Cruise Speed ​​- planned cruise speed. The flight dispatcher can use several speeds for the calculation during flight preparation. While modern flight planning programs usually select an economical speed for cruising as the standard (ECON, LRC), it can be deviated from in individual cases and the speed can be adjusted manually. Among other things, when flying through the North Atlantic High Level Airspace (NAT HLA), the speed must be calculated in Mach . In addition, the specific desired airspeed can also be specified in True Airspeed (TAS). All adjustments affect fuel consumption.
  • Distance - in aviation, this is divided into Nautical Miles (NM) and Nautical Air Miles (NAM). While the distance, measured over the ground (nautical miles), from Frankfurt to Tenerife is always the same, it can differ significantly in the air due to the wind and the corresponding air mass movement (nautical air miles). For example, an airplane covers a significantly higher distance than the air in a headwind, while the opposite is the case on a return flight. The distance above ground does not change in either case. A simple calculation basis is the following: Nautical Air Miles (NAM) / True Airspeed (TAS) = Nautical Miles (NM) / Ground Speed ​​(GS).

Fuel reserves

The flight route, as already mentioned above, is calculated from the Departure Aerodrome to the Destination Aerodrome as Trip Fuel (TF). Conversely, this means that the fuel tanks would be empty after landing. For this reason, the various aviation authorities, such as the FAA or the EASA, have introduced various reserve rules.

The reserve fuel is divided into:

  • Contingency Fuel - as already explained in the Definitions.
  • Alternate Fuel - for the flight from the Destination Aerodrome to the alternate airport
  • Final Reserve Fuel - 30 Min - Reserve fuel for a possible flight on hold, whereby the consumption values ​​refer to an altitude of 1500 ft above the alternate airport.
  • Extra Fuel - additional amount of fuel which is refueled based on the decision of the crew.

Procedure for the fuel calculation

The fuel calculation is carried out from behind (backwards) - starting with the landing (at alternate airport C). In this way it is achieved that only the unconsidered weight of the currently calculated partial fuel quantity leads to a small calculation error. Previously used partial amounts of fuel no longer weigh anything. Only proportional reserves for this are not taken into account. Sub-quantities of fuel consumed later were calculated in advance and can be taken into account as weight.

Overhead calculation

For the sake of simplicity, a so-called overhead calculation is given. This means that the flight route from Departuer Aerodrome to Destination Aerodrome is carried out at cruising altitude and surcharges for the climb, as well as deductions for the descent, are carried out afterwards (see picture).

When the aircraft lands at the alternate airport and comes to a standstill on the runway, the “ideally” fuel tank is “almost” empty. Only the final reserve fuel is left in the tank ( jet : 30 minutes, prop : 45 minutes).

The fuel calculation is based on the assumption that the aircraft has to take off before landing at the alternate airport and can only then land. (Incidentally, to save fuel, you can fly in the holding loop at the speed for the lowest fuel consumption.) The flight altitude in the holding loop is assumed to be 1500 feet above the alternate airport. At this low altitude, the fuel consumption of turbine aircraft is extremely high. The fuel consumption for the Final Reserve Fuel is usually taken from the Airplane Flight Manual (AFM) or the Operations Manual (OM) Part B of the respective airline. In the respective section, the fuel data, using the example of a Boeing 737-800 , can be found in the chapter Performance Dispatch - Enroute - Holding Planning using a table.

The next calculation step goes back one step. The fuel requirement for the flight from the Destination Aerodrome to the Alternate Aerodrome is determined - the Alternate Fuel. For this purpose, the distance (NAM) between the two airports is determined. With this distance and the total weight (in our case: Gross Weight = Zero Fuel Weight + Final Reserve Fuel) you can determine the expected amount of fuel. For the Boeing 737-800, this can also be determined in the Enroute chapter under Short Trip Fuel and Time.

Up to this point we are with the fuel calculation overhead at destination airport B and the aircraft weight consists of the Zero Fuel Weight, the Final Reserve and the Alternate Fuel.

Next, the fuel requirement for the cruise (at cruise altitude including stepclimb) from the Departure Aerodrome to the Destination Aerodrome is calculated. To do this, the distance (along the planned flight route) is determined together with the respective wind components. Based on this distance including the influence of wind (NAM). Based on this distance, the respective fuel consumption and flight time can be determined in the chapter Enroute - Trip Fuel and Time Required based on the distance and the expected landing weight.

The contingency can in turn be determined on the basis of the trip fuel to be expected. While the definitions for this have already been explained in detail, in this case either the MINCONT, 5% TF, 3% TF, 20 min or a statistically recorded amount is filled up.

There are two ways to proceed after this step:

Protected

Within the calculation of the protected contingency fuel, this is again added to the gross weight (GW) and thus also to the expected landing weight (LW) and the consumption for the route is calculated again. This ensures that the additional consumption on the route is included in the calculation and that the contingency fuel is therefore completely available on the entire route

Unprotected

The contingency fuel is filled up here, but the additional consumption due to the additional weight is not included in the calculation.

Depending on whether the contingency fuel is calculated as protected or unprotected, a so-called take-off fuel (TOF) results with all individual fuel values. Together with the ZFW, the TOF results in the so-called Take Off Weight (TOW). This is the amount of fuel to be expected on the runway on which the Taxi Fuel will then be added - together this ultimately results in the Block Fuel.

  • Within the consideration of the integrated range (IR), step climbs (change in flight altitude due to the reduced weight after consumption of the fuel) and the respective changes for fuel consumption are then considered in this scenario and the trip fuel is adjusted.

Summary

GW over C + Alternate Fuel = GW over B;

GW over B + second TF + second contingency = preliminary TOW;

DOW + Load = ZFW;

ZFW + Holding Fuel = GW above C;

Special procedure

Reduced Contingency Fuel (RCF) Procedure

Reducedc Contingency Fuel ( RCF ) Procedure

Process description

The Reduced Contingency Fuel Procedure is a special procedure in flight preparation.

Two different flight plans with different framework conditions and destination airports are calculated, both of which are approached via the Decision Point (DP).

The legislature defines the requirements for the flight plan, here only in part, as follows:

  • Flight plan 1 from Departure Aerodrome to Commercial Destination

(1) The sum of:

(ii) trip fuel to the destination 1 aerodrome, via the decision point;

(iii) contingency fuel equal to not less than 5% of the estimated fuel consumption from the decision point to the destination 1 aerodrome;

  • Flight plan 2 from Departure Aerodrome to Optional Refuel Destination

(1) The sum of:

(ii) trip fuel to the destination 2 aerodrome, via the decision point;

(iii) contingency fuel equal to not less than the amount calculated in accordance with (a) (3) above from departure aerodrome to the destination 2 aerodrome;

particularities

In the method shown, only the contingency fuel from the decision point (DP) to the commercial destination is calculated in flight plan 1.

In flight plan 2, the usual contingency fuel rules are calculated for the entire route segment, from the departure aerodrome to the optional refuel destination.

Both flight plans are compared with each other and the highest fuel value is used.

The remaining fuel quantity must be checked above the decision point (DP). If this is not sufficient for the onward flight to the commercial destination, the optional refuel destination must be flown to.

Advantages and disadvantages

The RCF method is similar to the PDP method and necessarily has a decision point, in this case the Decision Point (DP).

It offers various possibilities to influence the amount of fuel:

  • Waiver of destination 1 or destination 2 alternate insofar as the trip time between DP and destination is less than 6 hours and the conditions of CAT.OP.MPA.180 (b) are met.   
  • Adjustment of the decision point (DP) to reduce the calculation of the 5% contingency for destination 1
  • Use of a nearby Destination 2 Aerodrome

The RCF procedure offers advantages in terms of reducing the actual take-off weight, the possible payload or the amount of fuel available. In which cases can the procedure be used sensibly?

  • Exceeding the max. Take-off weight (MTOM)
  • Exceeding the available fuel capacity (TCAP) of the aircraft
  • Increase in the payload insofar as the aircraft is already at the MTOM

In all three cases, depending on the aircraft type and consumption, the method used can create a buffer in the ton range.

However, the process not only offers advantages, but also disadvantages. 

While OFP 2 is only calculated up to the optional refuel destination with the applied contingency rule - often 3% or 20 minutes - OFP 1 is calculated at 5% up to the actually planned commercial destination from the decision point. As part of this planning, one accepts the possibility that the flight will not be carried out as planned, but instead has to stop at another airport for refueling in favor of a higher payload.

On a hypothetical route from Kuala Lumpur (WMKK) to Frankfurt am Main (EDDF) with an optional refuel destination in Prague (LKPR), the flight plan can theoretically be planned without an alternate aerodrome in Frankfurt am Main. At this point, at the latest, it becomes clear that only the final reserve of 30 minutes remains on arrival at the commercial destination.

In this case, a clear evaluation of the weather data and an accurate monitoring of the flight performance are extremely important.

Predertmined Point (PDP) Procedure

Predetermined Point (PDP) Procedure

Definition of the Isolated Aerodrome

The Pre-Determined Point Procedure is another special procedure in flight preparation. Compared to the RCF method, the dispatcher is not free to choose this. Rather, the location of the alternate airport determines the use: For the selection of alternate aerodromes and the fuel policy, the operator shall consider an aerodrome as an isolated aerodrome if the flying time to the nearest adequate destination alternate aerodrome is more than:

(a) for airplanes with reciprocating engines, 60 minutes; or

(b) for airplanes with turbine engines, 90 minutes.

In short, this requirement means that a Destination Aerodrome is isolated if the flight time to the next adequate alternative airport is greater than 90 minutes for turbine-powered aircraft.

This condition creates the obligation for the PDP procedure. In addition, the use of an Isolated Aerodrome is not the responsibility of the operator, but is subject to approval by the responsible aviation authority.

The use of an isolated aerodrome exposes the aircraft and passengers to a greater risk than to operations where a destination alternate aerodrome is available. Whether an aerodrome is classified as an isolated aerodrome or not often depends on which aircraft are used for operating the aerodrome. The competent authority should therefore assess whether all possible means are applied to mitigate the greater risk.

(a) Using an isolated aerodrome as destination aerodrome with aeroplanes requires the prior approval by the competent authority.

Process description

Two flight plans are also calculated as part of the PDP process. The main difference, however, is that neither an alternate fuel nor a final reserve fuel is shown in both plans, but instead both quantities are replaced by the additional fuel. The extract, only partially defined here, from the legal text:

(1) The sum of:

(ii) trip fuel from the departure aerodrome to the destination aerodrome, via the predetermined point;

(iii) contingency fuel calculated in accordance with (a) (3);

(iv) additional fuel if required, but not less than: (B) for aeroplanes with turbine engines, fuel to fly for 2 hours at normal cruise consumption above the destination aerodrome, this should not be less than final reserve fuel; other

(2) The sum of:

(ii) trip fuel from the departure aerodrome to the destination alternate aerodrome, via the predetermined point;

(iii) contingency fuel calculated in accordance with (a) (3);

(iv) additional fuel if required, but not less than:

(A) for airplanes with reciprocating engines: fuel to fly for 45 minutes; or

(B) for aeroplanes with turbine engines: fuel to fly for 30 minutes at holding speed at 1,500 ft (450 m) above the destination alternate aerodrome elevation in standard conditions, this should not be less than final reserve fuel;

In this context, the larger amount of the two flight plans is refueled, analogous to the RCF procedure. At the Predetermined Point, the crew must evaluate whether the remaining amount of fuel available is sufficient to be able to circle over the destination airport for another 2 hours at cruising altitude. If this is not the case, the alternative airport must be flown to.

Major incidents

  • LaMia flight 2933 , invalid flight schedule based on negligent fuel calculation
  • Hapag-Lloyd flight 3378 , additional fuel requirement due to extended landing gear was not taken into account, emergency landing in Vienna
  • "Gimli Glider" , incorrect conversion of fuel measurements

See also

Individual evidence

  1. ↑ Regulation (EU VO) No. 965/2012 , Air Operations on the EASA website.

Web links