Why calculate fuel?

Hello fellow simmers and pilots,

After a few years of simming I want to go deeper into the more serious stuff and really want to learn things as I go.
I love vfr flying and thus I ordered a chart, a plotter and an A6B to start learning the basics of flight planning.

But going through all the hassle of all those calculations, what I really like doing by the way, I’m missing the “why” in it.
Why do pilots need to calculate their fuel rather than just top off those tanks and stay within the action radius?

I hope finding my answer here, that way I can understand the thoughts behind all this planning.

Kind regards


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Well , for example, I can think of a few places that if I tried to take off with full tanks I would never get off the ground. Take off weight really matters and fuel is part of the weight calculation.


Most GA planes can’t take-off with full fuel and full payload while staying within maximum take-off mass.

You then have to optimize the fuel you carry to maximize payload.

This is not a problem if you fly alone, but you can still be mass limited by performance.

  1. No small airplane I know can have all passengers (1, 3, 5) + full fuel. It’ll be overweight. Unless they’re all anorexic.
  2. It might not be possible to top up fuel at all GA fields. At least where I live selling fuel is super complicated, requires special permissions etc. (due to taxes). So often even if there is fuel available it’s only for members of the local flying club / private owners. They’re not allowed to sell it and have to log all their usage.
    3., The more you weigh the worse you fly, so it doesn’t make sense to take too much fuel onboard. In small, underpowered, airplanes it makes a noticeable difference.

Your “radius” can be materially impacted by weather, diverts, hold time depending on where you’re headed. So the “circle” as it is is smaller when factoring in a safety amount for the unexpected. Some aircraft with gravity tanks can’t actually touch a certain amount of fuel in the tanks, and they don’t have a fuel pump to get at that remaining load.


That is true, although most modern airplanes (with Rotax engines, instead of a 1930’s tech Lycoming) can stay in the air longer (with full tanks) than your bladder can handle. I think the main issue is weight.
If you follow the POC and formal limitations and are a slightly larger guy/girl you might even be overweight on your own with full tanks. Especially in ultra lights, where there is a formal artificial MTOW limit. In most cases without grounds in true aerodynamic limitations. The exact same airframe might be sold as a “full” airplane, just 50% more expensive.


And then you find out your buddy brought his full set of golf-clubs, and you’ll have to take other weight out :wink:


Sometimes, tough choices have to be made. The golf clubs are lighter than the buddy though…


take the golf clubs, leave the buddy. Gotcha.


If you really have to take both, then a mandatory fuel stop is in order.

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That’s old school… cool! :slight_smile:

Fun to figure out the influence that winds aloft have on your ground track and speed.

Full tanks limit load, and change maneuvering speed. If you’re flying VFR by yourself with an overnight bag, full tanks may actually be a benefit:

  1. Lower fuel prices at your local FBO (perhaps?)
  2. Increased Va speed enroute (maneuvering speed.)

Have fun with your new Flight Computer!

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And time of day/temperature matters too. As altitude and temperature go up, the engine performance gets worse, there’s less lift at a given airspeed, etc.

As the other posts point out, there is a lot to sort out before takeoff. As much as we do in these aircraft, you never just hop in a real airplane and go fly it without at least some kinds of checks or else you may never climb out, miss obstacles, etc.

There’s a bunch of videos on YouTube of real life pilots who did ignore things like temperature, altitude, and such and can’t clear obstacles or climb, or even take off.

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If I just topped up my tanks each time I flew without calculating I’d be dead :rofl:.

Seriously though fuel calculation is as important as the route itself. You can lose your license for not having the right reserves in your tank when arriving at destinations. It’s great that your doing it manually as flight planning is part of the fun, especially for those who like to be proven right all the time.

Happy planning.

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Another major one for your “radius” is actually wind.

Remember, that an aircraft travels within the airmass it is in, not against the ground.
So the “distance” to the destination can be much more (you fly much longer) then expected. Especially in smaller GA aircraft, the effect can certainly be very pronounced, imagine an 100 kias travel with a 30 knot headwind.


This truly made me laugh out loud! :slight_smile:


The only time my golf clubs are left behind is if I don’t go!


So, why and how we do it in reallife with a C172. If we have pax and baggage we need to calculate the fuel for the trip. Because, as already said, you cant go full fuel amd full payload in a c172 at the same time. So either you take less weight with you (less baggage, or just 1 pax instead of two) or you take less fuel and have to land and refuel once or multiple times on your trip. Also, for example, our runway is very short. And in summer when it gets to around 30c temps, you really have to take the takeoff calculation seriously, as you might not be able to takeoff with full fuel even when alone in the plane. So you can save weight with taking less fuel with you.

When it comes to airliners, weight is also a factor because of the cost. They try to be as light as possible and only take the fuel and reserves they need. Unless the fuel is not available at the destination or just too expensive, then they might go with enough fuel to return to the homebase.

So you see, there a re several factors why fuel is important.


No one wants to expend expensive fuel hauling around expensive fuel unnecessarily.


For easy-to-understand answers to not only your question but also various other similar “why” questions, the following books are very useful. If you give them even just a cursory glance for a few minutes every day, flight planning, calculations etc will no longer seem like a hassle to you as you will understand the importance of it all.

FAA: Airplane Flying Handbook

FAA: Pilot’s Handbook of Aeronautical Knowledge

Rod Machado: Private Pilot’s Handbook

Also watch some Air Crash Investigations TV episodes if possible. Many of them explain how errors and negligence in pre-flight planning and calculations have caused devastating accidents and fatalities in the past.


Thank you so much guys for your answers as they give me much more understanding why calculating fuel is so important.
With this knowledge it really makes more sense to me.

I hope that the performance of the aircraft in the simulator matches reality, so that planning flights in this way will really have an added value.
Let’s find out😄

Thanks alot!!
I know this is old school, but at the same time so important in real life in case of an electrical failure.
You never know😅

Amazing!! Thank you for pointing me to these!!
I sure will have a look.

:joy: :rofl: