On May 23 2023, a Turkish Airlines Airbus A321neo en-route from Istanbul (IST) to a very busy London Gatwick (LGW) was forced to divert to Birmingham Airport (BHX) after declaring a fuel emergency. The aircraft, registered TC-LSN and operating flight TK1997, had flown a holding pattern prior to attempting a landing which was abandoned due to another aircraft positioned on the runway waiting to depart. The Turkish Airlines crew elected to divert to Birmingham as holding times were anticipated to be up to 30 minutes and the aircraft was nearing its diversion fuel. But how do pilots make such decisions? Here’s our overview of fuel rules in commercial aviation.
Commercial aircraft are required to carry a certain amount of fuel to ensure the safe completion of a flight. Legal requirements define the minimum levels of fuel required and additional fuel is carried to cover contingencies, which makes sure there is sufficient fuel for expected and surprise circumstances. These contingencies can include a variety of factors. Dispatchers study weather, routes, and historical fuel usage to provide a figure to the captain who then makes the final determination on how much fuel to carry on a flight. The amount of additional fuel allocated for these scenarios is determined through careful analysis of the expected route and potential hazards along the way. The mass of the aircraft is also a factor.
How is aircraft fuel categorized?
The fuel loaded onto aircraft is categorised based on its purpose. There are some differences around the world, so we’re going to focus on the rules published by the European Union Aviation Safety Agency (EASA).
| Fuel | Description |
|---|---|
| Taxi fuel | The amount of fuel required to move the aircraft from the gate to takeoff position on the runway. |
| Trip fuel | The total fuel required from the start of the takeoff roll to the completion of landing at the planned destination. Includes takeoff, climb, cruise, descent and approach. |
| Contingency | Fuel to account for chance occurrences or unforeseen events. This could include unplanned deviations from the route, and unexpected changes in the weather. |
| Alternate fuel | The fuel required to fly from missed approach at the destination to the planned destination alternate. This takes the weather into account. |
| Final reserve fuel | This is a quantity of fuel that you must have on board when you land. 30 minutes of fuel is required for a jet, 45 minutes for a turboprop (because jet aircraft can hold at higher altitudes in thinner air). |
| Additional fuel | Fuel that is usually carried if there is no alternate available, or if operating at a lower height than planned would burn too much fuel to reach an alternate airport. |
| Extra fuel | Any surplus fuel carried for other purposes, such as tankering. |
All these combine to form ‘Block Fuel’ – essentially the total amount of usable fuel on board at the ramp before starting the engines.
What does that look like in real life?
Here’s a real world example provided by a pilot at TUI. The following figures are taken from the flight plan of a Boeing 737-800 operating a single sector from Ibiza (IBZ) to Birmingham (BHX).
- Trip fuel – 5616 kg
- Taxi fuel – 155 kg
- Alternate Fuel – 603 kg
- Final reserve fuel – 1,150 kg
- Total fuel on board – 7,800kg
Update – June 2023
EASA modified its fuel policy for commercial air transport operations in late 2022 to improve efficiency and contribute more to environmental objectives. Fuel is now categorized three ways. Here’s a sumary.
- Individual fuel scheme – for commercial operators with fuel monitoring systems that regularly fly to the same destinations. This gives a certain amount of autonomy to the airline to carry an amount of fuel tey typically need (plus contingencies).
- Basic scheme – this simplifies the fuel categories above to simply include 5% contingency fuel.
- Basic scheme with some variations – contingency fuel can be reduced to 3% under certain circumstances (such as advanced weather monitoring being available).
Minimum fuel and fuel emergencies
If an aircraft only has enough fuel remaining to execute the planned route and approach procedure to the airport, but not to account for changes in routing or further delays without breaking the minimum required fuel on board, ‘minimum fuel’ is declared. Whilst this isn’t an emergency, it helps the controller’s situational awareness and can help expedite landing the aircraft. Some operators will require crews to declare a ‘PAN’ at this stage.
Should the situation evolve into a fuel emergency, a Mayday will be declared by the crew. This is a full emergency situation and will be treated as such by air traffic control. According to the FAA, a pilot should “Declare a fuel emergency at the point at which, in your judgment, it is necessary for you to proceed directly to the airport at which you intend to land. Declaration of a fuel emergency is an explicit statement that priority handling by ATC is necessary and expected.”
One of the most famous cases of an accident caused by fuel starvation was the crash of Avianca flight 052 on approach to New York in January 1990. The aircraft made several failed approaches and spent considerable time in the hold before running out of fuel. One of the most significant causes of the accident was agreed to be the crew not clearly communicating to controllers that they were in a fuel emergency.
The strict rules on how much fuel aircraft must carry are one of the many factors that make commercial flying one of the safest ways to travel, and as a result, full scale fuel emergencies are very rare.
Cover image: Conor Clancy, JetPhotos
