I’ve been pondering this for the longest time, but have been unable to come to a conclusion.
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1$\begingroup$ If I understand your question correctly, then at an airport where the expected taxi-out fuel burn is larger than the difference between maximum ramp and maximum takeoff mass, the mass at start of takeoff would always be restricted by the maximum ramp mass (if not by other factors like performance). $\endgroup$– Cpt ReynoldsCommented Feb 15, 2018 at 13:19
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$\begingroup$ Assuming you are referring to Max Ramp Wt (MRW) vs. Max Takeoff Wt (MTOW). If the MRW is 241,000 lbs and MTOW is 240,000 lbs, then 1000 lbs of fuel would have to be burned during taxi so as not to start the takeoff roll beyond the maximum weight limit. $\endgroup$– user22445Commented Feb 15, 2018 at 13:50
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$\begingroup$ I don't really understand your question, but does this one answer you? $\endgroup$– PondlifeCommented Feb 15, 2018 at 13:54
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$\begingroup$ @Pondlife: I guess the OP is asking if the limit on the MRM could, in particular conditions, or for a particular aircraft, be so low that it would prevent the MTOM to be reached. But some clarification in the post would be good. $\endgroup$– minsCommented Feb 15, 2018 at 18:34
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2 Answers
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@vasin is correct. I'll put some actual numbers in a scenario.
A 747-481F has a max ramp weight of 913000lb, a MTOW of 910000. That 3000lb difference between max ramp and MTOW has been the same for 747s since the -100 airplanes.
The 3000lb is usually sufficient to cover the taxi fuel, but not always. Think having to taxi out at JFK when departures are heavy and the runway in use is the one farthest away from your ramp. Or, same thing at Honolulu and because of your weight you need the reef runway.
The dispatcher/load planner for the flight believes in following the rules, and he believes the taxi out will consume 4000lb. Thus, he has to use 909,000 as the max takeoff weight. Thus the max ramp mass has limited the MTOW
This kind of thing happens most on freighter flights because they're loaded as often as possible right up to their max zero fuel weight, that's where the money is. So let's say it's a freighter flight, JFK to Tokyo Narita. With the airplane near MZFW, you can't load enough fuel to get to Tokyo, but if you stretch a string on a globe between JFK and Narita, it goes almost right over Anchorage, and Anchorage is pretty much at the halfway point. Indeed, Anchorage has been used as the refueling stop for freighters for years and for pax aircraft as well, but especially for freighters.
Anyway, the load planner will put as much cargo aboard as possible to put the actual takeoff weight right up against the MTOW allowed.
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**assume the allowed takeoff mass means MTOW.
MTOW means the maximum weight at which the pilot is allowed to attempt to take off, due to structural or other limits.
This includes all mass involves in the flight including MZFW, fuel and consumables. Both maximum ramp mass and MTOW are a fixed number regardless of temperature or airport altitude. since maximum ramp mass and MTOW are fixed number, if fuel burn during taxi out is greater than the difference between maximum ramp mass and MTOW, maximum ramp mass will limit maximum weight an aircraft can takeoff with.
answered Feb 15, 2018 at 13:31