Calculating Fuel and Payload


How much fuel to buy to complete a trip? That’s the question. How you calculate the fuel needed, is explained here. So grab your calculator and do the exercise.

As a sample we take the Cessna Sky-hawk 172SP. In the specifications of this C172 we discover a specific range (this is the maximum range to fly in NM) of 638 NM.
The cruising speed is 124 kts. These 2 figures show us the max. time to fly with full tanks. Divide the specific range by the cruising speed and we get the “Endurance”
:: the Endurance is 638 / 124 = 5,1 hrs; this means that you could fly ~5 hours with max. fuel loaded.
But what is max fuel loaded in terms of  liters/USG or kilo’s…?

In the specifications of this C172 we learn that the max fuel allowed is 318.00 pounds (lbs) or else in gallons: 53 USG.
With this information we can calculate that
:: the Average Fuel Consumption per hour is the (Fuel capacity in USG) divided by the (Endurance) 53 / 5.1 ~ 10 USG/hr

Now the trip:
If you want to fly a trip of say: 248 NM with an average speed of 124 kts, your Flight Duration is:
:: Flight Duration= Distance to fly in NM divided by Average Speed in kts  equals 248 / 124 = 2.00 hrs
Your Fuel Needed = Flight Duration x Average Fuel Consumption per hour and in this case: 10 USG x 2 hrs = 20 USG
In case of emergency there is a 45 minutes rule, saying that you need extra fuel for 0.75 hr; in conclusion we need 2,75 hrs x 10 USG ~approx. 28 USG to buy.

Now launch FSX with the C172. To change the fuel and payload, press Change Fuel in the Aircraft and Fuel and Payload menu and calculate with how many percentage each tank must be filled.  Max. volume was 53 USG, we need in each tank 28/2= 14 USG; as there is a percentage of 14/53 ~ 28% in each tank. Change the values and check the outcome.

OK….where there is fuel weight needed for the flight, you cannot stiff the aircraft with passengers and/or luggage as far as you like…
So we check: the maximum gross weight is 2550 lbs, we have 28 USG on board as fuel, we got 3 passengers added to the payload(170 lbs each) and we see that the baggage weight remains 132 lbs only. And since the passengers have 150 lbs for luggage…Houston…we have a problem…

For your convenience some conversion factors are below:
1 Gallons [US] = 3.7854118 Liters
1 Gallons [US] = 8.3452641093 Pounds
1 Kilograms = 2.20458 5379 Pounds
1 Liters = 0.26417205124 Gallons [US]
1 Pounds = 0.11982844244 Gallons [US]
1 Pounds = 0.4536 Kilograms
How much fuel to buy?
 Specific Range........................ 638
 Cruising Speed........................ 124
 Endurance............................. 5.1
 Fuel Capacity in USG....................53
 Average Fuel Consumption per hour.......10

Flight Time Calculations
 Distance to fly in Nm................. 248
 Average Speed in KTAS................. 124
 Flight Duration...................... 2.00
 
Fuel Need Calculations
 Flight time...........................2.00
 Fuel consumption per hr.................10
 Extra Fuel for 45 minutes............ 0.75
 Buy Total Fuel......................... 28