Flight - Lesson Four
-  Fuel Calculation Guidelines  -

Why bother with fuel calculation?

Although sometimes ignored by virtual pilots, fuel calculation is one of the most important steps in the preparing for flight. In real life it's the responsibility of the captain to calculate the amount of fuel that he would take for the flight. In virtual aviation, it's yours. In real commercial aviation the goal is to carry enough fuel to forward the aircraft safely to its' destination, but not to carry any excess fuel that would only make it burn more fuel on the way. In addition, fuel counts towards the payload, so, if you carry more fuel, you can carry less payload. As nobody pays for carrying the excess fuel, airline managers don't like it. But in virtual aviation there are no grim airline managers sitting behind huge desks with calculators in their hands to annoy pilots. So why don't we simply fill our tanks to the brim and leave the fuel calculations to the real pilots? Well, for the following reasons:

• Making a fuel calculation is fun. We, VA pilots, like it "as real as it gets", don't we? We don't just take off somewhere, fly a few circles around the highest building and then land on the first airstrip we happen to spot from above. We fly commercial airliners according to  standard procedures and checklists. Sometimes we even spend hours studying charts and making flight plans, just because we like it. For us, it's not a burden. It's fun. So why would we not make fuel calculations too?
  
• Fuel calculation shows whether you can make it or not. Nobody likes ending his flight in the Ocean or a field "just a few miles off" the nearest airport. But if you don't make a fuel calculation, this can happen. As we don't assign individual aircraft types to routes, it's the pilot's responsibility to chose an aircraft that can make it. Aircraft manuals contain the maximum range of each aircraft, but this only means that the plane can fly this distance under optimal conditions (no wind, optimal altitude & speed, etc.) Only a proper calculation tells whether it can make it under the specific conditions of your flight or not.
  
• When you get rid of excess fuel you get rid of problems. While there are no airline managers in the world of virtual aviation, there are other ways in which excess fuel can cause you problems. Each aircraft has got weight limits (MTOW and MLW) that cannot be ignored without consequences. Our aircraft are all modelled with a certain amount of payload on board. Most of them allow you to depart with full tanks at this payload, but not all! So excess fuel can overweight your aircraft. When you try to take off with an overweight aircraft, you might not be able to lift it off before the end of the runway and then your flight will end before it could have begun...  But in most cases the weight problem will manifest itself only when you try to land. Most aircraft allow you to land with less then half of its full fuel load only! When you attempt to land over the allowed MLW you'll either have to come in faster and risk running off the runway or you'll have to lift the nose much higher, so that you'll be unable to see the runway and the aircraft will be very hard to handle. In case of a longbird like the 757 or the 777 you can even scrape the tail this way...  Only a proper fuel calculation guarantees that by the time you land you'll have used up most of your fuel and then your landing will be smooth.
  

So both too little and too much fuel causes problems and compromises safety. Then how should we calculate the right amount to be carried for a certain flight? Please read on to learn.

The basics of  fuel calculation:

In order to make a fuel calculation we must have our Flight Plan ready and our aircraft's Operations Manual at hand. During the calculation we must take into account the following factors:

• Distance of the destination airport - this is not the distance between the airports measured as the crow flies but the distance that you'll cover when you fly on your planned route. FS Navigator calculates this for you.
  
• Fuel consumption of the aircraft - this is the the fuel consumption rate specific to the aircraft type. You can find it in the Manual, or you can rely on the aircraft performance data in FS Navigator. In the latter case FS Navigator will make a fuel calculation for you but don't use default data! Use the performance data that you can download from HERE. Please note that in case of heavy jets, the FS Navigator data provided use "high altitude cruise" consumption rates for the calculation!
• Cruise altitude and speed (planned) - all aircraft use up different amounts of fuel when they fly at different altitudes and speeds. In case you deviate from your aircraft's normal cruise altitude and speed (see the Manual), you must use a modifier. Please consult the Manual for alternative consumption rates at different cruise speeds and altitudes.
  
• Expected wind direction and speed - if we use Real Weather or fly Online wind becomes a factor. It can hinder us, making our aircraft consume more fuel, or help us, making it consume less. As we don't have access to detailed high altitude wind reports :), we must rely on our knowledge of the prevailing winds. On the Northern Hemisphere the prevailing wind is from North-West. Wind speed at cruise altitude is usually arund 30-50kts. As this is cca. 1/10th of our cruise speed, we use a ±10% modifier depending on our direction.
  
• Distance of the alternate airport - this is the distance of the alternate airport from the destination airport as the crow flies. We need this in order to be able to calculate the emergency reserve. The emergency reserve must enable the aircraft to to hold for 30 minutes at its destination, to make a go-around and a new approach, and to reach the alternate airport and land there. To sum up, this is equivalent with the amount of fuel needed to reach the alternate airport + an additional 45 minutes cruise.
  

As you can see now, the recommended reading through the Operations and Flight Manual before every flight isn't for its literary value. :)

Emerald Air Operation and Flight Manuals are based on data downloaded from the manufacturers' websites and additional information that I obtained from friends. Most data is correct, but at certain points I must have guessed. The numbers in the Flight Characteristics section (ie. takeoff, climb, cruise, and descent speeds, climb and descent rates, power settings, fuel consumption, etc.) have been verified by several test flights and match exactly the values that our flight models produce, so you can rely on them.

Three fuel calculation methods:

Below I'll show you the three methods of fuel calculation that I know, using my recent Dublin-Bodo B752 flight as an example.

1. The Manual formula method: This method is based on the fuel calculating formula that you can find in each Manual.

• According to FS Navigator the route distance is 1100nm
• The trip fuel according to the formula in the Manual is: 5500+22.8×(1100-180)+1040 = 27.500lbs
• My chosen cruise altitude is FL350, speed is 0.80M. According to the Manual his is a "Typical Normal Cruise", so I use no modifier here
• My general direction is North so I expect some headwind. I use a +5% modifier here. I also use a +10% general reserve for unexpected situations, so the corrected trip fuel is: 27.500×1.15 = 31.625lbs
• My alternate airport is Tromso (ENTC) that's 200nm from Bodo. The fuel needed to reach Tromso is 5500+22.8×(200-180)+1040 = 7000lbs
• The mandatory reserve is 45min cruise fuel. The Manual shows that the 757's consumption in normal cruise mode is 10.500pph. So the required reserve fuel is 10.500×0.75 = 7875lbs
• The total fuel needed for this flight according to this method is: 31.625+7000+7875 = 46.500lbs
  

2. The simplified Manual method: This method is based on FS Navigator's ETE calculation and the fuel consumption rates in the Manual.

• According to FS Navigator the estimated time enroute (ETE) is 2h 45'
  
• My chosen cruise altitude is FL350, speed is 0.80M that's a "Typical Normal Cruise". The fuel consumption in this case according to the Manual is: 10.500pph. So the required trip fuel is 10.500×2.75 = 28.875lbs
• My general direction is North so I expect some headwind. I use a +5% modifier here. I also use a +10% general reserve for unexpected situations, so the corrected trip fuel is: 28.875×1.15 = 33.205lbs
• My alternate airport is Tromso (ENTC) that's 200nm from Bodo. The fuel needed to reach Tromso is 5500+22.8×(200-180)+1040 = 7000lbs
• The mandatory reserve is 45min cruise fuel. The Manual shows that the 757's consumption in normal cruise mode is 10.500pph. So the required reserve fuel is 10.500×0.75 = 7875lbs
• The total fuel needed for this flight according to this method is: 33.205+7000+7875 = 48.080lbs (3% difference)
  

3. The FS Navigator method: This method is based on FS Navigator's own fuel calculation based on the EMD aircraft performance data.

• According to FS Navigator the trip fuel needed for this flight is 28.800lbs (as the 757 is a heavy I must have adjusted its' cruise fuel rate within FS Navigator to the "Typical Normal Cruise" consumption rate in the Manual first to get a valid result)
  
• My general direction is North so I expect some headwind. I use a +5% modifier here. I also use a +10% general reserve for unexpected situations, so the corrected trip fuel is: 28.800×1.15 = 33.120lbs
• My alternate airport is Tromso (ENTC) that's 200nm from Bodo. The fuel needed to reach Tromso is 5500+22.8×(200-180)+1040 = 7000lbs
• The mandatory reserve is 45min cruise fuel. The Manual shows that the 757's consumption in normal cruise mode is 10.500pph. So the required reserve fuel is 10.500×0.75 = 7875lbs
• The total fuel needed for this flight according to this method is: 33.120+7000+7875 = 47.995lbs (3% difference)

Fuel management in practice:

• In case of longer flights we usually fill up the tanks with the fuel calculated for the first leg (to the destination airport) only.
• In case of shorter flights, especially to small airports where there's no jet fuel supply (e.g. the Aran islands) we fill up the tanks with the fuel calculated for 2 or more legs (e.g. from Shannon to Inishmore + from Inishmore to Galway .or. from Dublin to Inishmore and back).
  
• If you ever realize in midair that you can hardly make it to your destination with the fuel left in the tanks, you can reduce consumption by climbing higher and flying slower. This way you might reach the destination airport safely.
• If you realize that your remaining fuel is very short of what you need and you'll probably not make it to your destination (due to a fuel miscalculation, an unexpected strong headwind, etc.) then don't even try. Land at any airport on your route that you can reach safely, fill up your tanks, then continue. It's better then ditching your plane to the great amusement of the air traffic controllers. :)
  
• If you ever decide during a flight in progress to continue faster than you've planned (because you want to keep the schedule despite a headwind or for any other reason) always check your fuel flow and make a fast fuel recalculation on the spot for the rest of your flight. High speed means high fuel consumption and it's better to arrive late then not to arrive at all.