In view of negative media coverage concerning Ryanair's three diversions to Valencia on 26th July 2012, I thought I would explain how fuel is calculated for a standard flight, and what discretion a pilot has to carry more fuel.

The official investigation by the IAA was released some time ago and showed that all three Ryanair flights carried additional fuel above that required by law. All three captains of their respective flights acted in accordance with Ryanair's fuel, diversion and emergency policies and these policies have been approved by the relevant safety regulatory bodies (IAA and EASA). Whilst there may be concerns expressed about Ryanair's operations (justified or not) it seems in the case of the Valencia diversions, much of the criticism appears to be unfounded and misinformed.

Fuel Calculations

Most airlines use sophisticated software, together with comprehensive statistical data, to calculate the precise fuel for a particular flight:

Route data -- the track miles of the route planned taking into account the departure and arrival procedures (there are often several routes between two airports, and flight planners choose the most cost effective route for that day). Information on flight levels and altitudes to be flown, and any level restrictions imposed (level capping) are also factored into the equation.

Weather data -- accurate predictions of wind velocity along the route are made. Comprehensive data is used for the entire route at all levels the aircraft is expected to fly at or through. The height of the troposhere is also noted, as is the air temperature and pressure. All these affect the efficiency of the airframe, the drag, and engine performance, and all three influence the fuel burn.

Statistical data -- If an airline has flown this route before (likely for all scheduled airlines) then fuel usage data is stored and used to refine the fuel calculation. Factors such as radar vectoring to avoid crossing traffic (common over the Bay of Biscay, or anywhere in the UK as widebodied aircraft set off across the Atlantic), speed adjustments due to other traffic (very common in France), and stepped climbs (due to congestion above) all affect the fuel usage.

Fuel Requirements

So, all this information is used to calculate surprisingly accurate trip and diversion fuel figures. The fuel requirements are made up of the following:

Trip fuel (from above data)

Alternate fuel (from above data)

Contingency fuel (usually calculated as 5% of trip fuel, or 5 minutes flying at 1500 feet above the destination airfield)

Taxi fuel

Final reserve (thirty minutes flying at holding speeds, 1500 feet above the preflight alternate airfield at estimated arrival weight).

The choice of alternate airfield is often the made based on minimum fuel and might not be the most commercially suitable. For instance, if strong nothernly winds prevail over south east England, then Gatwick might be chosen as the alternate for Luton since the fuel burn could be less than flying to Stanstead, even though the Essex airport is close to Luton. You could probably challenge the validity of that example, but the point is that the alternate is chosen based on fuel consumption rather than track miles or suitability: Gatwick is the busiest single runway airport in the world and would be rather annoyed if Luton's traffic suddenly turned up and added to their workload.

In this case, the captain might nominate a different alternate (further away) and take additional fuel accordingly. There might also be a problem at the planned alternate airfield. Perhaps an approach aid has failed, or the weather is barely better than the destination. Again, a different alternate can be nominated by the captain and extra fuel carried.

Captain's Discretion

When the above breakdown is added together the resulting figure is the minimum legal fuel requirement for the flight. Additional fuel may be automatically factored into the plan if the destination is notoriously busy -- Heathrow and Gatwick, for instance -- but unfactored additional fuel can be carried if the pilot believes unforeseen delays could be encountered.

So what kind of delays are we talking about? Well, busy periods for instance. If I'm flying into Edingburgh at about 0800 hours then I know it's likely to be busy. Yes, statistical data should reveal increased fuel burn due holding or extended radar vectoring on approach, but it might not. My memory does. I'll load probably an extra 500 kg (probably a quarter of my trip fuel from a Southern English airport) just in case, and justify my decision by annotating the flight plan with 'busy period in EDI' or something similar. It has never been queried.

Weather is a factor too. Thunderstorms enroute might require small diversions to avoid and this can quickly use up the contingency fuel and more, so I'm likely to add an extra few hundred kilos just in case. Thunderstorms at the destination can sometimes lead to holding, but this isn't always the case. I would still take extra fuel though (as the Ryanair captains did), but how much fuel I take would depend on the particular destination, the number of runways and the likely volume of traffic arriving at the same time.

Taking Extra Fuel

Why not simply take as much fuel as possible? After all, there's nothing more useless than the fuel left in the bowser. Right? Wrong.

First, it adds weight and extra weight requires more fuel to propel the plane through the air: carrying extra fuel burns more fuel. Fuel is the biggest expense for any airline, and carrying just 200 kg extra (5 minutes flying for a medium sized aircraft -- 737, 320 etc) on each flight would increase fuel costs of a Ryanair sized airline by tens of millions of pounds per year.

Easyjet, for instance, reckons it would save over £1,000,000 per year if pilots taxied out on one engine. Fuel is expensive and airlines are prudent to find ways of avoiding waste.

I fly perhaps around 400 times per year, and in umpteen years of commercial flying I've diverted just a handful of times. It's not a common event. It's far far cheaper for an airline to pay for an occasional diversion than routinely carry lots of extra fuel to avoid that diversion. But the point is, and it's a very big point, the captain has the authority to take extra fuel if he or she believes it would be necessary. And it's a legal matter. If a captain elects to take minimum fuel (plog fuel, as we call it) and the weather clearly indicates that holding might be required, then the captain not the airline, will be held legally accountable should a fuel related incident occur.

There are certainly airlines, or managers within airlines, that pressure pilots to take plog fuel, but within my airline, that pressure is applied only to captains who routinely and unnecessarily take excessive additional fuel -- 'a bit for mum' is an outdated habit from the days when fuel requirements were calculated by multiplying flight times by factory fuel burn figures for your engine/aircraft type and adding a bit for headwind. Times have moved on and it's perfectly safe to launch with plog fuel; and it's perfectly acceptable for me to add a bit if I think I'm going to need it.

Conclusion

The airline uses sophisticated software to calculate accurate fuel requirements for each flight. The captain uses experience and commonsense to assess if that fuel figure is sufficient. Extra fuel is taken if the captain believes it will be needed, but plog fuel is often taken if conditions exist that suggest no delays are reasonably expected. If the unforseen occurs, a diversion is always an option.

The Ryanair captains involved in the Valencia diversions declared emergencies because it is mandatory to do so if you predict the fuel level on arrival will be below final reserve, or if circumstances unfold that suggest this would lead to fuel dropping below this level. From reading the incident report it seems that ATC performance might have had a significant affect on the fuel used prior to and during the diversion, and perhaps played a role in the decision making process of these three captains. But that would be conjecture.