The fuel carried by aircraft is highly regulated. It is commonly believed that an aircraft carries only the fuel that is required to fly from one airfield to another airfield. But this is not entirely true. Aircraft, under commercial operations, carry a lot more than that.

The fuel policy

The amount of fuel to be carried for a flight depends on local air regulations. The fuel policy must be written by the airline in their operations manual based on these regulations. Generally, the policy must be based on:

  • Fuel data provided by the aircraft manufacturer
  • Fuel data derived by the airline based on the aircraft age, performance, etc. on a fuel consumption monitoring program.
  • Fuel burn increase due to increased aircraft weight
  • Expected meteorological conditions in the routes operated by the airline.

The fuel for a commercial flight is divided into taxi fuel, trip fuel, contingency fuel, alternate fuel, final reserve fuel, additional fuel, and extra fuel.

Staff members refuel an Airbus A350-900 with SAF

The use of SAF is a growing trend in the airline industry. Photo: Getty Images.

Taxi fuel

Taxi fuel is the amount of fuel required to taxi the aircraft from the gate to the runway for takeoff. It also includes fuel burnt at the gate due to the operation of the Auxiliary Power Unit (APU).

Airbus A330

The taxi fuel is the fuel that is required to move the aircraft from the gate to the takeoff runway. Photo: Airbus

Trip fuel

This is the fuel required to fly from the destination aerodrome to the arrival aerodrome. It should include:

  • Fuel for takeoff and climb and cruise. It should also consider the departure route. Some airports have longer Standard Instrument Departures (SIDs), which may require uplifting of more fuel.
  • Fuel from Top of Climb (TOC) to Top of Descent (TOD)
  • Fuel from TOD to the initial approach fix, or where the approach for landing begins. Similar to the departure, this should consider the complexity of the Standard Instrument Arrival Route (STARs) at the airport.
  • Fuel to shoot the approach and make the landing at the destination.
Aer Lingus Airbus A330 in air

Trip fuel is the fuel required to get from origin to destination airport. Photo: Vincenzo Pace | Simple Flying

Contingency fuel

As the name suggests, contingency fuel is the fuel that is uplifted to account for unforeseen circumstances such as changes in the weather, route changes due to ATC constraints, holding, etc. The contingency fuel should be higher of:

  • 5% of planned trip fuel or, in the event of in-flight replanning, 5% of the trip fuel for the rest of the flight
  • Not less than 3% of the trip fuel. This is typically allowed by the regulators if the route has an en-route alternate.
Plane traveling through storm clouds in sky

Contingency fuel accounts for unexpected weather. Photo: Getty Images.

Alternate fuel

The alternate fuel is the fuel required to divert at the destination if the pilots fail to land at the destination for some reason. The alternate is chosen by the airline before the flight, and sometimes two or more possible alternates are provided in the flight plan. The fuel for alternate should include:

  • Fuel that is required for a missed approach at the applicable minima at the destination airfield to the published missed approach altitude. This should include the complete missed approach procedure
  • Fuel for the climb from missed approach altitude to the cruising altitude to the alternate aerodrome. This should consider the SIDs at the destination
  • Fuel to cruise from TOC to TOD. Here, the complexity of the STARs at the alternate aerodrome should be looked at
  • Fuel from TOD to the initial approach fix, or where the approach for landing begins
  • Fuel to perform an approach and landing at the alternate aerodrome
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A320neo in high altitude campaign flight

The alternate fuel accounts for a go around at the destination. Photo: Airbus

Final reserve fuel

The final reserve fuel is the absolute minimum fuel that is required for an aircraft to remain airborne safely. Ideally, pilots should not ever reach this fuel. If, for some reason, the pilots feel that their fuel load will drop to final reserve, a Mayday should be declared.

The final reserve for an aircraft with turbine engines is the fuel that is required to fly for 30 minutes at holding speed at 1500 ft above the alternate aerodrome. The aircraft weight to calculate this fuel should be not less than the weight that is estimated that it will be when it reaches the alternate. To be conservative, airlines can choose a higher weight as increments in weight increase fuel burn.

For an aircraft with piston engines, the fuel is that required to fly for 45 minutes. This difference is because turbine engines are more reliable than piston engines.

Additional fuel

The additional fuel is an interesting one. Not all airlines, or to be more accurate, not all routes, have additional fuel as a minimum fuel requirement. However, certain operations, such as ETOPS, require airlines to carry additional fuel when flying ETOPS routes. This is the fuel required to divert to an ETOPS alternate with engine failure and/or loss of pressurization.

This is the extra fuel carried at the discretion of the captain of the flight. The captain is authorized to carry more than the flight plan fuel based on his judgment. However, many times airlines require captains to give justifications for any extra fuel that is carried.

airbaltic cockpit with tablet

The captain decides the extra fuel required. Photo: airBaltic

Flexibilities given by regulators to airlines on fuel to be carried on board

Reduced Contingency Fuel (RCF) or Inflight Replanning

If you remember, we have previously said that a minimum of 5% of trip fuel must be carried as contingency fuel. It also stated that if the inflight replanning procedure is followed, you only need 5% required for the remaining part of the flight. The 5% may not be an issue for a short flight. However, for longer flights with higher trip fuels, the contingency fuel can skyrocket. So, how does RCF or inflight replanning work?

Let us say, you are planning to fly from destination A to B. And when the fuel is calculated, it shows that 80,000 kg of fuel is required to fly from A to B (trip fuel). Now, as a contingency of 5% of this trip fuel is a part of the minimum fuel to be legal you must carry an additional 4,000 kg of fuel (5% of 80,000 kg). As you can imagine, this is a lot of fuel.

Now, if you want to reduce this contingency amount, you could use the RCF or the inflight replanning procedure.

To follow this procedure, you can find an aerodrome that is closer to A than B. We will call this aerodrome C. You then find out that to fly from A to C, only 50,000 kg of fuel is needed. This means your contingency now falls to 2,500 kg.

So, you will initially make a flight plan from A to C to fly to aerodrome B (your original destination). This flight plan must include a decision point, maybe close to your TOD to aerodrome C. At this decision point, you make a quick fuel calculation to see whether you have enough fuel to reach aerodrome B from the decision point plus 5% of the fuel that is required to fly from the decision point to aerodrome B.

If it is determined that you have enough fuel as above, you will simply do an inflight replanning to destination B. The leeway given by the regulators is that if such a replanning inflight is done, at the decision point, you are only required to have 5% of the fuel that is needed to fly from the decision point to the destination (in our case aerodrome B).

Reduced Contingency Procedure

Photo: Anas Maaz – Simple Flying

So, how is the minimum fuel for such a procedure determined? It should be the greatest of a) or b)

a)

  • Taxi fuel
  • Trip fuel to aerodrome B, via decision point
  • The contingency of 5% from the decision point to aerodrome B
  • Alternate fuel
  • Final reserve
  • Additional fuel (if required)
  • Extra fuel

b)

  • Taxi fuel
  • Trip fuel to aerodrome C, via decision point
  • The contingency of 5% of the trip fuel required to fly from A to C
  • Alternate fuel
  • Final reserve
  • Additional fuel (if required)
  • Extra fuel
Brussels Airlines Airbus A330-342

Inflight replaning saves airlines a lot of money. Photo: Vincenzo Pace | Simple Flying

Predetermined Point (PDP) Procedure

When flying to places that are super isolated, an alternate aerodrome can be quite far from the destination. In these situations, the flight can only be routed via a predetermined point. The predetermined is the point where the fuel in the aircraft is only enough to reach either the destination aerodrome or the alternate aerodrome. So, before the PDP, pilots should make a calculation to make sure they have enough fuel to continue to the destination.

PDP

Photo: Anas Maaz – Simple Flying

To apply this procedure, the fuel should be the greatest of a) or b)

a)

  • Taxi fuel
  • Trip fuel from the departure aerodrome to the destination aerodrome via the predetermined point
  • Contingency fuel
  • Additional fuel: For turbine engines, this is fuel to fly 2 hours at normal cruise consumption above the destination aerodrome (should not be less than final reserve fuel)
  • Extra fuel

b)

  • Taxi fuel
  • Trip fuel from the departure aerodrome to the alternate aerodrome via the predetermined point
  • Contingency fuel
  • Additional fuel: For turbine engines, this is fuel to fly 2 hours at normal cruise consumption above the destination aerodrome (should not be less than final reserve fuel)
  • Extra fuel

How do pilots calculate the required fuel

The airline dispatch office prepares the flight plan, which specifies the minimum fuel required to fly from one aerodrome to the other. This fuel is based on current wind and temperature data. So, it is fairly accurate. However, pilots, many times, take extra fuel for their comfort, particularly when the weather is bad.

Two pilots discussing flight plan at desk

The pilots decide the fuel based on dispatch data. Photo: Airbus

To calculate how much they can carry, an underload can be calculated. The underload is the amount of weight that can be carried in the aircraft without exceeding any limiting weights with passengers and cargo loaded. For an airliner, there are three main limiting weights. The Maximum Takeoff Weight (MTOW), the Maximum Zero Fuel Weight (MZFW), and the Maximum Landing Weight (MLW). It is always easier to work out an example.

Say, an aircraft has weights as follows:

  • MTOW: 100,000 kg
  • MZFW: 90,000 kg
  • MLW: 95,000 kg

Current Zero Fuel Weight (ZFW): 80,000 kg

The fuel is as follows:

Block fuel (total fuel) = 15,000 kg

Trip fuel = 10,000 kg

Taxi fuel = 300 kg

The underload calculation:

  • MTOW: 100,000 – 80,0000 – (15,000 – 300) = 5,300 kg
  • MZFW: 90,000 – 80,000 = 10,000 kg
  • MLW: 95,000 – 80,000 – (15,000 – 10,000 – 300) =10,300 kg

According to the above calculations, the limiting weight is MTOW, and the underload is 5,300 kg. What this means is that if more than 5,300 kg of fuel is uplifted, the MTOW will get exceeded. So, in this case, the pilots can carry 5,300 kg of fuel without busting any weight limitation of the aircraft.

Source: simpleflying.com

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