Longer flights generally produce better fuel economy in smaller jets, but not always in jumbo jets. Airplanes typically burn the most fuel when they climb, and with short flights, climbing is a bigger percentage of the overall trip. But that rough rule flies out the window with large aircraft because of the heavy fuel load for long trips.
I could not understand the last part of the paragraph.
But that rough rule flies out the window with large aircraft because of the heavy fuel load for long trips.
It would be helpful if you tell us which part you don't understand, but I guess it is rough rule and/or flies out the window.
rough rule means it is not a fixed or defined rule. It is basically the rule but there could be exceptions.
flies out the window This is a set phrase that means that something can be disregarded or ignored. (The image might be throwing something you don't want out of a window.)
In the example, it means the rough rule (the previous two sentences) is basically meaningless or does not apply to the situation mentioned in the this sentence.
I am not a native speaker or a master of English, but I guess that the meaning of the part is that the rule that earlier mentioned cannot be applied to the large aircraft since they should carry more fuel than smaller ones.
