How do we calculate the amount of fuel required?

The word taxi in aviation essentially refers to an aircraft moving around on the ground under its own power and so as pilots we "taxi" an aircraft while on the ground and "fly" it when airborne. As you can probably imagine, getting a 60 tonne aircraft from a stationary position to an approximate taxi speed of 30 miles per hour takes a fair bit of energy which on a jet is known as thrust and on a propeller aircraft, power. Furthermore, the actual engine start process itself uses quite a bit of fuel as does running the air conditioning and powering the aircraft systems should the airport not have means of providing these from connections into the aircraft. This brings us to the total for taxi fuel. 

Therefore, taxi fuel is the total amount of fuel required prior to engine start, for the start of the engines themselves and the eventual taxi to the runway. This figure is seldom fixed and will vary from flight to flight depending on the historic length of taxiing at a given airport. A flight outbound from Bristol will use considerably less fuel for taxiing than a flight out of somewhere like Barcelona which has a ridiculously long taxiing time by comparison. In any case the typical amount considered for taxi fuel on an Airbus A320 is in the region of 100 to 300 kilograms. 

Aviation fuel is always referred to with a mass as all performance calculations depend on the overall mass of the aircraft. Generally speaking to convert the mass of Jet-A! fuel into litres you just deduct 20% and so in this example we'd burn anything from 80 litres to 240 litres just to get to the runway itself. Putting that into perspective, at the upper end that's enough to fill a small family hatchback 5 times.

Trip fuel is the amount of fuel needed from the start of the takeoff run to arriving at the destination. This obviously includes takeoff but also the initial climb to cruising altitude, the cruise itself and of course the associated approach and landing. The exact amount required for a given flight can change from one day to the next. There are several reasons for this. 

One major reason is the weather, be it the more benign such as wind direction or something more severe like thunderstorms, volcanic activity or icing. Airlines have flight planning departments which work out the best route for a specific flight to take and the associated trip fuel comes from that. It's pretty much calculated using your high-school speed, distance, time. By taking the planned cruise speed for the aircraft and how wind affects it (head wind, tail wind) you can work out the speed of the plane over the ground. With that you can calculate how long it'll take to cover the physical distance for the planned route and from that time calculation you can workout the associated fuel burn. Note - your average flight seldom flies in a straight line but it'll be relatively close, airspace permitting.

As an example, a conventional Airbus A320 uses anything between 1000 to 1200kg per engine, per hour. Looking at our example flight from Madrid to Bristol that'd be 2 hours of flight time and around 2400kg of fuel. Of course that's presuming the engines work hard for the entire flight, when in reality during the descent they don't need to provide much thrust and are running at near idle. In addition, if a tailwind is very strong an airline may opt to reduce fuel burn and allow mother-nature to carry the burden. By this I mean instead of a engine-powered speed of 500mph and a 100mph tailwind making it a speed of 600mph... you could slow down the plane to 400mph with the 100mph wind providing "Free" energy to the aircraft. The resultant NET speed is therefore the same, the passengers would arrive at the same time and the amount of fuel burned would be reduced.

I won't delve into it much more than that as hopefully that's enough context to give you a rough idea. 

Just like my driving analogy above, aircraft must also carry contingency fuel. This effectively allows for any changes in the routing of the flight due to either clouds in the way of the planned flight path, unexpected climb or descent and holding above the destination airfield before an approach can be made. The aviation authorities around the world typically dictate the rules behind how much of a contingency airlines registered under their jurisdiction must carry. In Europe the initial recommendation/requirement is the greater of 5% of the planned trip fuel or enough for 5 minutes of holding above the destination airfield. 

There also exists the ability to reduce the unnecessary carriage of contingency fuel by utilising historic statistics over a certain quantity of flights but that's based on the authority and also the airline in question. By the same token, data from historic flights can also be used to gauge a higher amount of contingency fuel if it be deemed 5% may not be enough. Ultimately, the world of fuel planning is very stats based. Data is key in the modern world, it helps us discover trends and mitigate risk. 

There's several more rules on contingency fuel but they're not interesting enough to mention here. There's a reason why Air Law, Flight Planning and Operational Rules & Procedures are the more dull of the topics of the ATPL subjects a trainee pilot must endure. 

At the fuel planning stages it's always important to think about the what if scenarios.

• What if our planned destination airport is closed?
• What if our planned destination airport requires us to hold overhead for too long?
• What if our planned destination airport has bad weather and we can't get in?

Those very scenarios require us to plan alternative airports. We require at least one and at Bristol we tend to use Cardiff. Should the weather at Cardiff also be looking flakey then we might elect to use an airport farther afield such as Birmingham or maybe even London Gatwick. In very rare cases we might even think about using one in the Midlands. Ultimately, even if getting into our destination is 99.99% assured, we still consider a backup plan. 

Alternate fuel is calculated from the point we decide to go-around having first commenced an approach at our original destination. It provides fuel to account for the full-engine spool up of the go-around itself, the subsequent climb, the cruise period towards our alternate and finally the approach and landing at the alternate airport. Using Bristol as the example once again, the route from Bristol's runway to being lined up for a landing at Cardiff takes around 15 minutes, or 700kg in fuel. 

In practice it often becomes obvious we need to divert to our alternate quite early on. For example, in flight we periodically check the weather at our destination and if it makes sense to use Cardiff due to the deteriorating situation at Bristol then we might actually make that decision while we're still over the channel. If we were to then let air traffic control know of our intentions we won't ever attempt an approach to Bristol and as such might find we land at Cardiff without burning any of the alternate fuel at all.

This is the fuel which must be carried in the tanks at ALL times with no exception. You can not plan your flight with the knowledge this fuel will be burned. After every single flight there must always be at least this amount of fuel in the tanks. It is there as an absolute final resort. It is considered that serious an issue to be down to your final reserve fuel that as pilots we must declare a fuel emergency if we're going to get anywhere close to it. It's the final safety blanket available to assure a safe landing. Should said emergency need to be declared then air traffic control would prioritise us over any other inbound traffic. 

To reassure you, in the vast majority of cases an airline will land at it's destination with the alternate fuel still available - because they didn't need to divert to the alternate in the first place - plus the final reserve fuel and then any additional fuel on top (which i've yet to even cover) and as such there's generally plenty of fuel left in the tanks upon landing.

The legal definition of the final reserve fuel is to carry enough fuel to fly around for half an hour above our alternate airport at 1,500ft above the ground. 

Again, further reassurance... to actual get anywhere near your final reserve fuel you must have flown to your destination and attempted an approach which lead to a go-around, made the decision to divert to your alternate where you then also made an approach which led to a go-around, have already burnt all of your contingency fuel by making several weather avoidances or other such changes to the planned flight plan which meant all of your trip fuel was also burned and then have used up all of the captains discretionary fuel too, if any. 

Final reserve on the A320 is about a 1000 to 1200kg.

This isn't often an item that is used at my airline but this additional fuel item tends to considered by airlines that have specific regulatory requirements to take any extra fuel. It may also be used in the event that an aircraft burn more fuel given its current state. As an example of the latter:

An aircraft has faulty landing gear in the sense it won't retract after takeoff and as a result the airline needs to fly it (empty) to another airport for maintenance. However as you may expect, flying an aircraft with its' landing gear extended is incredibly draggy and inefficient. With a draggy aeroplane burning a lot more fuel (about 280% more for the A320 with stuck-down gear) the planning department might account for this additional fuel burn in the additional fuel section of the fuel plan.

This element of the fuel plan permits the pilots to take additional fuel for any reason. It's very common that day-to-day fuel plans do not account for real-world experiences. By that I mean if weather happened to be forecast at an airport a pilots' experience of the subsequent air traffic control delays might mean they expect to be holding for around 20 minutes on arrival. The Captain or indeed his/her First Officer may then request an additional half an hour of fuel as a safety buffer for such an eventuality. It's quite common for us to take extra fuel for one eventuality or the other. Reason might include:

• Weather
• ATC Delays
• Training Flights
• Newly launched route
• Requirement for aircraft-powered Air Conditioning while on the ground
• Anticipation of slot delays and increased on-the-ground time.

This particular item has become the subject of debate recently in the media. Tanking is the process whereby an airline decides to carry fuel with it on the outbound flight in order to operate the flight back in the opposite direction. I can see both sides of the argument. An airline may decide to do this for operational reasons (saving money) or should fuel not be readily available at the destination (strikes / remote islands). 

Here's some examples:

• Flight to Isle of Man. Fuel not readily available as it has to be shipped out to the island especially. You can't just chuck diesel into a plane and the quantity of inbound flights make stocking jet fuel on the island (in large volumes) rather impractical. In such a situation it makes much more sense to take the fuel for the return.
  
  
• Flight to Italy. Striking fuel truck drivers mean there is no fuel service available at any airports. The only way to operate the return flight is to carry all of the fuel required for the return leg with you on our way down there. 
  
  
• Flight to Belfast. At a given moment in time the cost of Jet fuel in Northern Ireland might be dearer than England. It can save the airline substantial money by carrying the return fuel on the outbound trip. 

It tends to be that latter example which has caused issues in the press thanks to the additional CO2 produced from keeping an ultimately heavier aircraft in the air. I personally don't see a flight of England to Northern Ireland being a long-enough one to create a quantifiable amount more CO2, although travelling with tanks full over substantial distance is a different story. However, if we look back to the motorist in their own cars for a moment... How many people only pop enough fuel in their cars for the trip to work and then pop to the petrol station to refill for their trip home? The answer is probably a very small number - unless of course your commute is miles and miles long. Likewise... how many people do you think fill their tanks right up to the brim and carry that around with them in their fuel tanks until the tank is once again empty? A fair few I should say - I know I have from time to time. 

Ultimately, it's something quite political and I won't get involved. I simply wanted to relate the requirement for tankering to our daily lives.

This is now the sum of all of the above components... so its:

Taxi Fuel + Trip Fuel + Contingency Fuel + Alternate Fuel + Final Reserve + Additional Fuel + Extra Fuel + Tankering. 

It's essentially the amount we are planning to leave the gate with.

This is the actual amount we leave the gate with. We always annotate or make note of this for auditing purposes. We might end up with more than we planned for if the refueller doesn't turn off the pumps in time. Conversely, we might even end up with less than planned should we have started burning our Contingency or Extra fuel due to delays on the ground requiring the Air Conditioning for passenger comfort. We always monitor remaining fuel while on the ground to ensure it's still enough for the flight ahead of us. 

Here's example figures for a Madrid to Bristol flight.

As you can see from the above table the flight time itself is planned as 01:55 with an expected taxi time of 00:21 equating to 02:16 total time. However, if you look at the totals row you'll see that when you factor in the final reserve fuel, all of the contingency fuel, the alternate fuel and any extra fuel chosen by the pilots there's enough fuel onboard for a flight time considerably more thank this.