# MTOW changes with cruise altitude?

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Hi,Using the nice free B737 fuel planner made by Milan Puta (do a search for his name in the file library) I had a puzzling experience -- the Maximum Take-Off Weight (MTOW) listed in the planner changes depending on the cruise altitude I enter for the flight.Scrutinizing the ini-file provided with the program it is obvious that this is by design. Can anybody explain to me the relationship between cruise altitude and MTOW? According to Wikipedia MTOW may be affected by runway lenght, available thrust and such which makes perfect sense... but the cruise altitude??/ Puzzled regards, Poul

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The weight of the plane will effect how high it can climb and the optimum cruising altitude. Real life Flight Management Systems will compute the optimum and max altitudes for the current weight of the plane. AS fuel burns off those numbers will go higher.

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think of it this way, in easy terms. you are a heavy airplane, your engines have to use more fuel to keep your speed up the higher you go. thats why on short trips with less takeoff weight or towards the end of longer trips when most fuel is burned off do you climb higher.

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Poul,You're right, it sounds odd.The maximum take off weight is the maximum weight (so that's not a variable) the aircraft can take off from the given runway in the given atmospheric conditions. It doesn't matter how much fuel you take, the maximum weight won't change, you just need to check that the actual take off weight is less than the maximum (or you're going gardening).Maybe the software is trying to tell you the *actual* take off weight (does the number go up with decreasing cruise altitude?) rather than the maximum. Maybe it's something else altogether. I'm not familiar with the program so I can't give you much advice other than to email the author and ask him.Hope this helps,Ian

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Hi,Thanks for your comments even though I have yet to find the answer. I have e-mailed the author of the fuel planner, but has not got a response.Ian, I am certain that the weight I see in the fuel planner is the MTOW and not the *actual* take off weight as this is listed separately -- both as Zero Fuel Weight (ZFW) and Takeoff Weight (TOW).Changing the cruise altitude changes the MTOW for the Boeing 737-400 this way:CRZ ALT -- MTOW37000 -- 12000035000 -- 13500033000 -- 140000At lower altitudes the MTOW stays at 140000 lb.So as long as I do not plan to cruise at the 737-400's service ceiling of 37000 ft I should be able to get enough fuel on board to fly the listed max range (2165 nm, fully loaded). The Maximum Zero Fuel Weight (MZFW) is listed as 11700 which only leaves room for 3000 lb fuel if crusing at FL370 -- hardly enough to take me to the runway.I can obviously live with this, but is still puzzles me.... it sure does./ Cheers, Poul

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