DALLAS – As it becomes more expensive to fill up our cars, consider the cost of filling up an airliner. Crude oil prices have recently reached new highs, which can be attributed to the conflict in Ukraine.
A strong Dollar has exacerbated the problem for airlines based outside of the US, as the cost in local currency has risen even further.
Refueling an airliner has become a science, with a slew of computers crunching numbers for each flight. The heavier an aircraft is, the more fuel it burns. Carrying too much fuel can cause the engines to burn even more fuel just to carry the extra weight. As a result, carrying more fuel than necessary is not as desirable as it may appear.
How Much Fuel to Carry?
The legalities surrounding how much fuel is required, vary slightly from airline to airline and country to country, but generally follow the following structure. To begin, enough fuel is required from takeoff to touchdown, plus a small amount of fuel to account for unforeseen contingencies.
For example, if the most efficient cruising altitude is unavailable due to airspace congestion, a less optimal altitude will result in a slight increase in fuel burn. This additional contingency has traditionally been a percentage (such as 3% or 5%) or a fixed duration (such as 20 minutes of extra fuel).
A number of airlines now use historical data from previous flights to determine how much extra fuel was used on a specific flight. This allows operators to add an extra amount that bears more resemblance to what is likely to be used.
In the event that a landing at the planned destination is not possible, fuel is routinely carried to allow pilots to fly a diversion to an alternate airport, as well as a final reserve that could be as little as 30 minutes of flying time. While the entire portion of the fuel for diverting can be used, the final reserve should be kept intact. If pilots need to use any of this final reserve fuel, it’s commonplace for a ‘Mayday’ emergency call to be made to Air Traffic Control.
Finally, a small amount of fuel is added for taxiing from the parking gate to the departure runway, as well as a small amount of fuel to allow the Auxiliary Power Unit (APU) to run on the ground prior to take-off.
Zero Fuel Weight
The flight crew will have reviewed their flight plan, which will include the total amount of fuel that must be legally carried. Knowing the aircraft’s weight is essential because a heavier aircraft consumes more fuel. At the heart of these calculations is the Zero Fuel Weight (ZFW), which is the weight of an aircraft plus all payload minus any fuel. The ZFW increases as the number of passengers increases, and vice versa.
An estimated ZFW will be calculated before check-in closes, based on the number of booked passengers and the anticipated weight of cargo. This estimated weight is then used to calculate the fuel for the flight plan that the pilots are provided in advance.
Check-in will most likely close when the pilots arrive at the aircraft and begin their pre-flight preparations, and the pilots will then be given a more accurate estimated ZFW. It is routine to compare this revised ZFW to the original ZFW used in the flight plan, and the fuel required can then be adjusted to reflect any weight changes after check-in has closed.
To put this into context, if an additional ten passengers purchased last-minute tickets on a transatlantic flight on a Boeing 777, the extra weight of fuel required to carry these extra ten passengers would likely be the same as an additional two or three passengers.
The same document that contains the detailed fuel calculations used on the ground will also contain a fuel log that is updated at regular intervals during the flight. This makes use of predicted winds, which are now extremely accurate thanks to modern meteorological forecasting. Forecast wind can vary by as little as a few degrees and knots, which is a far cry from when weather modeling was much simpler.
Nowadays, witnessing the actual fuel burn differ by as much as a few percent on a long-distance flight from one side of the world to the other, would be enough to raise eyebrows on the flight deck.
So, the next time you look out the terminal window at the plane you’re about to board, rest assured that the fuel being loaded into the plane has been meticulously calculated!
Featured image: Lufthansa at MUC. Photo: Photo: Munich Airport