# Gross Weight used for Fuel Consumption Calculations

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Hi all,

Just a quick question regarding calculating fuel. For the 737-800, at least, the best fuel flow table per altitude and gross weight I've come across is the 737 series' user manual for Continental (2002). In using it, I'm a bit unsure which gross weight the table uses to list each fuel flow rate; is it the total a/c weight, including an estimate of how much fuel is likely to be required? Or is it just the weight of the aircraft without factoring in the fuel?

Thanks heaps.

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Gross weight by definition means "full weight", which includes fuel, pax, cargo, catering, crew etc.
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Gross weight by definition means "full weight", which includes fuel, pax, cargo, catering, crew etc.

Okay I thought that was the case, thanks. So, if I'm trying to calculate the fuel required in the first place, and don't know it yet, do I estimate the gross weight by including an estimate of fuel that I may require??

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Here's basically all weight limits you should plan by for large transport category aircraft.

Max takeoff weight or MTOW which is the most limiting weight for takeoff.

Max landing weight or MLW.

First you should figure out your MLW to figure out your MTOW. I know it seems backwards but there was is a reason. To figure your MLW you must factor the runway length, field conditions, and max structral landing weight limit.

Here is an example for 737-800:

What's the most limiting of below for MLW?

Structural: 144,000lbs

Field conditions at destination airport: wet, dry, anti-ice on or off during go-around, runway length, etc. (reference performance manual)

Then to figure your MTOW you figure what's the most limiting of below:

Structural: 174,200lbs

Field conditions at departure airport: same as above whether wet, dry, anti-ice on or off during go-around, runway length, etc. (reference performance manual)

Planned fuel burn: MLW+enroute fuel burn to get to destination airport. Does NOT include reserve fuel nor alternate fuel. Just the fuel to go from A to B.

Out of all flights you do, most of the time your MTOW will be the enroute burn limited. However, dependent that you are landing at an airport in low elevation away from any high mountains and field length isn't very short. I.e. 7000' length or less. Most of these numbers are derived from a performance manual or a performance section of the Aircraft Flight Manual (AFM).

Once you know your MTOW, you can now know how much payload you can accept (pax and cargo) because of this formula:

BOW=Basic operating weight means weight of aircraft with oil, crew, crew luggage, full galleys, everything needed for flight. This number can vary from airplane to airplane.

GW=Gross weight

ZFW=zero fuel weight

FOB=fuel on board

BOW+PW = ZFW

ZFW+current FOB = GW

Or

BOW+PW+FOB = GW

Clear as mud?

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Thanks for that thorough explanation, that sure does give me some additional info that I hadn't fully considered yet. But I'm still a bit confused about calculating the FOB. Is it right to use the fuel flow charts from the manual and if so I'm still not sure about using gross weight if I don't know the fuel weight yet?
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You can to get a rough estimate. Problem is you can't figure the fuel needed for climb and descent unless it specifies. Cruise charts are just for that. Cruise.

You can guess how long it would take to get to cruise altitude and then maybe double it (maybe triple) since the fuel flow will be higher the lower you are?

Like for example if the cruise chart says you burn 2000lbs/hr @ FL340 and it takes you 20 minutes to get to FL340.

2000lbs/60 minutes = 34lbs/minute x 20 minutes = 667lbs x 2 = 1334lbs for climb. Probably same for descent.

Note: if it specifies per engine then don't forget to double it!!! LOL!

Climb + cruise + descent = enroute burn

Remember taxi fuel is not included because you can always wait to takeoff to burn fuel however you must be careful because if you have too much, you may end up waiting for a long time to get below MTOW.

To give yourself a guide, you could include taxi out fuel into the enroute burn. To figure that just figure what the fuel flow is at idle thrust. If it's default 737 you may want to get the fuel flow while aircraft is taxiing at steady speed since it takes continuous thrust to taxi.

Same math used here. Example. Fuel rate for taxi about 500lbs/hr. Approx 8 minutes to taxi out. 20 if it's a really busy airport.

500/60 = 9 x 8 = 72lbs or for 20 minutes = 180lbs

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Okay, I'll just estimate it then. And I figured I'd estimate climb fuel by that method, sounds good. Thanks heaps for the help.
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My pleasure.

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Hi Folks,

A couple tips that may make your life a little easier...

FSBuild (payware) (http://fsbuild.com/) - while old - should help and make you life a bit easier as well... It has a ton of default profiles included that will probably get you in the ballpark... Profiles are fairly easy to build if you have the numbers and I've built a bunch - nothing larger than the B1900 though... It's also a fully functional flight planner and loads your flight plan into the FMC on most of the major player's products...

Fltplan.com (http://fltplan.com/) is a real world flight planning website used by pilots - that also happens to free for anyone to use... You probably have to register - but - the beauty of this site is they have tons of real world profiles defined for all kinds of aircraft... Not only will you get real fuel consumption but you get real world routes and such to fly... It's certainly worth a gander as I've used it for years...

Regards,

Scott

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Hmm, thanks for that Scott. I think I'll give fltplan a go, looks good.
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They have an app for Apple tablets. I use their sectionals while flying. Useful for making passenger PAs.
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Hi Folks,

I looked in Fltplan - for 737 they only have the 200, 300, and BBJ defined with a profile. It's more a corporate pilot site I guess - they seem to have business jet and turboprop imaginable...

Regards,

Scott

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They are Scott. That's their main focus is GA/corporate.