jet fuel consumption plane

Fill her up please!

Tim Takeoff
12.11.2018
1 picture
5 minutes

Although you’ll often hear this said with light aircraft, it’s unfortunately not that simple with commercial aircraft. A plethora of circumstances and factors need to be taken into account to fill a commercial aircraft up with the optimum amount of fuel.

In the majority of cases, the aim of every flight is to transport the “contents” of the aircraft to a specific destination: this includes the jet fuel itself as well as the passengers and freight. Every kilogramme to be transported naturally generates higher costs, which is why an optimum compromise needs to be found.

Modern “fuel calculations” start on the ground

When you consider the total amount of fuel needed in more detail, a modern “fuel calculation” can be sub-divided into the individual sections of a flight.

Firstly on the ground, the Auxiliary Power Unit, abbreviated to APU, is operated with jet fuel (kerosene): it supplies the aircraft with power, hydraulic systems and air conditioning without the engines running. The APU also starts up the first engine, which does not have a starter motor as there is on a car. The second engine is then “powered” by compressed air from the first engine.

Once the engines have started, “Taxi Fuel” takes the aircraft from its parking position to the runway. The amount is specified by the crew, who can estimate how long they will need to reach the actual runway itself. This naturally needs to be increased if there is a lot of traffic or even aircraft queuing for takeoff. Now the actual flight itself begins …

Aloft – Allowing for the unpredictable

Once off the ground, initially only the flight itself is considered. What would the aircraft need were it to fly the planned route from A to B, with guaranteed no problems en route, and land at its first attempt? At this initial stage, the calculation does not allow for diversions due to weather problems or other circumstances: this is known as “Trip Fuel”.

“Contingency Fuel”, as its name suggests, is then allowed for to factor in the unpredictable. “Conti” might allow for sudden diversions that the aircraft might need to take due to the weather, requests from air traffic control or climbing to a better altitude. This latter fuel amounts to three to five percent of “Trip Fuel”, depending on the scenario.

The “final reserve”

If the aircraft is approaching its destination but cannot land there, either due to extremely poor weather, failed navigation systems at the airport or operational problems on the ground, then an alternative airport needs to be found. The flight from the original destination to the alternative airport is provided for by “Alternate Fuel”. Generally speaking, any aircraft that touches down needs to have a minimum fuel reserve to permit 30 minutes of flight. In technical jargon, this is called “Final Reserve Fuel”.

If you then add Trip, Contingency, Alternate and Final Reserve Fuel together, you arrive at the total “Required Takeoff Fuel”, the amount of fuel the aircraft needs in all cases to undertake the flight (also to comply with the legislation).

Could your carry a little more?

A further two scenarios need to be considered here. First, the aircraft operator can decide whether he wishes to carry additional fuel for financial or operational reasons – “Fuel Tankering” or “Additional Fuel”. The operator might do so if jet fuel is extremely expensive at the destination airport or if the aircraft needs a quick turnaround at its destination.

Statistically occurring delays or recurring events, like queues, also can be taken into account. There are complicated mathematical models in airline planning programs that can factor in these considerations. This amount of fuel is not actually needed but is nonetheless carried for the aforementioned reasons as it has a positive impact on the bottom line.

In spite of this, the crew always needs to bear in mind that any additional weight, in turn, requires additional fuel just to transport it, a consideration that makes the entire calculation all the more complex.

A decision with consequences

The final point on the list of considerations resides solely with the captain and the crew. If they are of the opinion that more jet fuel needs to be carried than is allowed for by the airline’s planning programs, a consideration based on current events or their personal experience, then they can also take into account this fuel. This “Commander Extra” fuel therefore also allows for any final eventualities.

Finally, “Additional Fuel Tankering”, “Commander Extra” and “Taxi Fuel” is added to the “Required Takeoff Fuel”, which ultimately results in the “Final Block Fuel”.

Mass and volume – the effect of temperature

This “Final Block Fuel” is the total amount of fuel needed by the aircraft and is communicated by the crew to the ground team and the fuel attendant. The latter then earths his truck and connects the fuel filler neck to the jet. The fuel attendant needs to adjust the amount depending on the temperature and the associated density of the jet fuel. Essentially the crew requires a mass, without any inaccuracies presented by the jet fuel.

A mass does not change, regardless of how warm or cold it is, whereas the volume does, as liquids have a different density when warm than when cold. By contrast, the weight (mass) remains the same, which explains why the cockpit crew always uses weight rather than litres or gallons when talking about fuel. By way of illustration, you get less fuel for your car at the filling station in summer than you do on a cold day in winter.

As modern long-haul aircraft consume approximately five to ten tonnes of jet fuel per hour, a flight from Hong Kong to Germany requires around 100 tonnes of jet fuel. This may seem a lot at first, but when you break it down to the number of passengers or the tonnes of freight on board, it is more economical (and naturally considerably more expeditious) than were this same volume to be transported by ship or overland.

by Tim Takeoff

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