This How To has been made after a brief discussion at the Reality Expansion Pack Pilots group on Facebook. If you didn’t join it yet, to it now! 🙂

Being a Reality Expansion Pack user means flying X-Plane with one of the most realistic addons around. Have you considered to act as a professional pilot while doing it?

REP is very precise and simulates realistic fuel consumptions throughout the whole flight, thus allowing the simmer to properly calculate the required amount of fuel with no surprises.

Being a professional pilot means two words: Safety First. This article is be about being careful while doing your fuel computations.

How Much Fuel?

The big question. How much fuel should you load in your tanks when at the ramp?

Let’s introduce Block Fuel. That is, the total amount of fuel to have in the tanks when your airplane still sits on the apron.

You calculate your Block Fuel by adding together the following figures.

  • Taxi Fuel
  • Trip Fuel
  • Contingency Fuel
  • Alternate Fuel
  • Final Reserve Fuel
  • Additional Fuel

Let’s analyze each figure alone then make a quick example at the bottom.

Taxi Fuel

This is the amount of fuel required to startup, taxi and hold before takeoff. For a single engine airplane 1USG may be ok. Always consider the airport from which you’re taking off. The Taxi Fuel required in a small backcountry airstrip is much less than that required at JFK!

Beware that Taxi Fuel does not considers taxi after landing.

Trip Fuel

This should include fuel:

  • For the takeoff
  • Climb
  • Cruise
  • Descent
  • Approach and landing

Trip fuel shall be calculated in a realistic way, using the:

  • Performance Tables shipped with the aircraft itself: you find the performance tables in the virtual kneeboard of REP, in the performance section. They give you the fuel consumption for each engine settings at each altitude.
  • Weather forecast: will you have to make a longer route because of weather? What’s the average wind component?
  • Actual masses: a heavier airplane is more a fuel hog!
  • Air Traffic Service restrictions: sometimes flying a DCT from departure to destination is not feasible.
Contingency Fuel

A contingency is a chance occurrence of unforeseen event. That is, contingency fuel is meant to compensate for deviations:

  • Of an individual aircraft from the expected fuel consumption data
  • From the forecast weather
  • From the planned routing/altitudes

Contingency Fuel is the higher of:

  • 5% of planned trip fuel
  • 5 minutes of flight at holding speed at 1500ft

You shall apply contingency allowance only to trip fuel.

Alternate Fuel

This is the fuel you need to fly from the destination airport to the alternate. You shall calculate it using the same rules applied to Trip Fuel.

Final Reserve Fuel

This is the minimum amount of fuel you shall always have onboard after landing. For piston engine aircrafts it’s 45 minutes of flight at endurance speed.

If you find yourself flying with less than the Final Reserve Fuel, you shall declare an emergency (Mayday Fuel).

Additional Fuel

Contingency, Alternate and Final Reserve cover most cases. There are two more cases, however, where you want to load more fuel:

  1. No Alternate: your destination is an isolated aerodrome
  2. No Enroute Alternate and Inability to Hold Height:  you’re long way from home and a malfunction does not allow you to fly at altitude, thus causing higher fuel consumption.
Extra Fuel

Not mentioned in the list above as it’s not mandatory at all. Usually this is some extra fuel that the aircraft commander decides to load because of particular circumstances.

Example Calculation

Let’s do a quick example. Here’s the flight data from A to B, alternate C. Climb and descent fuel flows are negligible (in this case) as the higher fuel flow during climb is averaged by the lower fuel flow during descent.

  • Routing from A to B: 300NM
  • Routing from B to C: 50NM
  • Cruise Speed 130KTAS
  • Cruise Fuel consumption: 15USG/Hr
  • Average Headwind: 10Kts
  • Airport A is small and has a very short taxi route

Let’s calculate the Taxi Fuel. This is quick! 1USG

From the KTAS (Knots True Air Speed) and the wind component, let’s calculate our Ground Speed.
GS = 130 – 10 = 120Knots

Now we can calculate the trip time: 300NM / 120Knots = 2,5 hours (or 2:30, in a more convenient format)

From the trip time and the cruise fuel consumption we get the Trip Fuel: 15USG/Hr * 2,5Hr = 37,5USG.

Now let’s get the Contingency Fuel (5% of the trip): 37,5 * 0,05 = 1,875USG (~ 2USG, be conservative!)

Same thing for the Alternate Fuel: 50NM/120Knots = 0,42Hr => 15USG/Hr * 0,42Hr = 6,3USG

Final Reserve. We don’t know the endurance speed, which provides the lowest fuel consumption. Let’s be conservative and use the cruise fuel flow.

Final Reserve = 0:45Hr * 15 =  11,25USG.

Let’s sum this up!

Taxi 1
Trip 37,5
Contingency 2
Alternate 6,3
Final Reserve 11,25
Total 58

So we start X-Plane, load our favourite REP airplane, put 58 USG of fuel in our tanks and we’re good to go! 🙂

How To: calculate the fuel required for your flight
Tagged on: