You Decide

How to Calculate Fuel Requirements The pilot of the aircraft makes the decision of how much fuel to carry based upon the following information:

• Aircraft operations handbook's specifications for fuel consumption, weight and balance
  
• Payload weight (cargo, baggage and passengers)
  
• Weight of fuel (number of gallons x 6 pounds = total weight of fuel in pounds)
  
• Weather and winds
  
• Total distance of flight
  
• Average flight speed
  
• Number of miles per gallon averaged by the aircraft carrying the weight for this flight
  

Typically there is a legal minimum fuel limit that all aircraft must follow when determining how much fuel to pump into the tanks. The minimum amount of fuel required needs to be able to fuel the following:

• the aircraft from its departure airport to its destination airport including
  
• being able to carry out an approach and go-around and
  
• flying to an alternate airport nearby (in case there is weather impediments at the scheduled destination airport)
  
• plus the ability to fly a 30-minute, holding pattern above the alternate airport
  
• land and taxi to the gate
  

To calculate the amount of fuel needed for a flight, the pilot uses the following equation:

Fuel Flow (gallons per hour) x Time = Fuel Consumed

The pilot uses charts found in the aircraft operation handbook that provides information about the miles per gallon of the aircraft at certain weights. The weight of the fuel is calculated by taking the total number of gallons and multiplying it by 6 pounds. One gallon of fuel weighs 6 pounds. This is usually figured into the charts found in the handbook.

Once the pilot knows the aircraft's fuel consumption rate for the weight being flown and the flight time, the pilot can compute the fuel needed for the flight.

Look at the example below.

First, convert the flight time from hours and minutes to decimal hours:

1 hr 40 min = 1 hr + 40 min/60 min = 1.67 hrs

8.5 gph (fuel consumption rate) x 1.67 (flight time in decimal hours) = 14.2 gallons

The pilot of this aircraft will need to make sure that at least 14.2 gallons of fuel are pumped into the fuel tanks for this flight.

How to Calculate Fuel Consumption
How much and how fast an aircraft uses fuel is known as fuel consumption. The same calculation used for determining a car's fuel consumption is also used for an aircraft with additional consideration for the weight of the payload (passengers and/or cargo). All aircraft have an operator's manual that gives specifications such as fuel consumption at different payload weights and fuel amounts. Remember the weight of the fuel also needs to figure into the equation! For example, let's say a small aircraft has a speed range of 120 - 170 mph and holds up to 40 gallons of fuel. According to the aircraft's specifications at full payload and fuel weight the aircraft uses 10 gallons of fuel per hour. Take the total amount of fuel (40 gallons) and divide that by the number of gallons used per hour (10) and you will find that the aircraft could be flown for 4 hours. Remember that a good pilot will never fly on fumes, and would stop for re-fueling long before the 4 hours are up.

Now let's say that the same aircraft is flying with a full tank of fuel, but only half its full payload weight. According to the fuel consumption specifications for this aircraft it will use 8 gallons of fuel per hour. How long will it be able to fly? Do the math: 40 gallons divided by 8 gallons per hour will provide a little over 5 hours of flight time.