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What is fuel rate?

Large aircraft fuel burn rate as it lightens?

  • Does an aircraft require less fuel to maintain height and speed as it becomes lighter?. ie. Suppose a 747 takes off with a full fuel load and climbs to its cruising altitude of 37,000 feet. After 6 hours of cruising, it'll have used up a fair amount of fuel. Will the engines be using as much fuel to maintain the height and speed as they did 6 hours earlier when the aircraft was heavier? (assuming nil head/tail wind). This also begs the question - would a 747 with very little fuel on board be able to go faster than one with a full fuel load?

  • Answer:

    Yes, you got the right idea - As the fuel gets burned in cruise the airplanes step climb, and as a result burn less - However there is nothing much in changing cruise Mach speeds - And being full gross weight at 37,000 feet does not exist for a 747 - The way we did in the 747-200 (sorry, I never flew the 747-400) was this way - Suppose we takeoff at the maximum weight - which is 377,000 kilos - We do a flight Buenos Aires to Madrid - that is a 11.5 hrs long flight - At that weight - the highest we can climb to is FL 280 - Our initial cruise Mach is .86 - After 1 hour, we request climb to FL 290 - Or maybe FL 300 if the temperature is cold enough - Brasilian ATC ask us when we can be at FL 330 for the oceanic crossing - We tell them the time ETO Recife - (look where Recife is on a map) - We can climb some 1,000 feet higher for every hour of cruise - For the oceanic clearance, they give us FL330, at Mach .85 - So you see, you do not accelerate because lighter weight - You reduce power, to stay at you Mach number (for traffic separation) - Reducing power = reducing fuel flow - Maybe we have another plane 15 minutes ahead of us at Mach .85 - During the cruise, we talk to ATC and get further step climbs - FL 340, then 350, 360 finally 370 - Our crossing is completed over Las Palmas (Canaries) so we can slow down - We finish the cruise at Mach .84 (economy cruise) and climb to 380 or 390 - Two hours later it is our arrival in Madrid... all routine - check the weather - See if we have enough fuel for the alternates, Barcelona (or Malaga) - If you know the technique, we do a re-release (re-file) over La Palmas - We initially depart for Las Palmas and use it as destination with reserves - Then we come close to Las Palmas, we talk to operations if we got enough fuel - And get re-cleared by dispatchers to continue to Madrid, our real destination - Hope you now understand that is not a question on "how fast" - It is a matter of not having a slow plane ahead of us, like Airbus at Mach .82 - Or then given the weight, see if we can go 1,000 ft higher and fly faster - With the above, we generally landed with 12,000 kilos of fuel remaining in Madid - Enough to go to Barcelona if need be - .

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Yes, burning fuel substantially reduces the weight of an aircraft, and so it also reduces the fuel burn rate. The more fuel you burn, the less fuel you need to burn to stay aloft. Large airliners fully loaded with fuel on long-haul flights will often "step climb," meaning that they will start at a relatively low cruising altitude and climb to higher cruising altitudes in steps during the course of the flight. This allows them to save fuel as they burn fuel. For aircraft, it's not so much a matter of changing maximum airspeed with weight as it is a matter of changing fuel burn with weight.

Techwing

you are correct. P.S. the other guy wants to sit there and write a dang novel about this, but the answer is STILL yes. thumbs down or NO thumbs down, THE ANSWER IS YES! just as a lighter CAR can go faster, a lighter A/C (at maximum throttle) can go faster. it's not DONE of course......

been there, done that.

The answer to your first question is YES. Weight of fuel contributes to the total weight of the plane. The lighter the weight, the less power is needed at the same speed or Mach number, at the same altitude. In fact, the technique for long range cruise is to climb later on during the flight to save even more fuel, depending on the weather, of course. The second question involves aerodynamics and can get complicated. But assuming a lighter weight aircraft vs. a heavy is the same as a car or truck performance. The main difference is that the lighter plane can accelerate faster while taking off or even in cruise up to a certain point. And that is when aerodynamics gets into play. In fact, a lighter plane, at calculated weight, will not go faster but WILL use more fuel to maintain that speed. So, for long range cruising, for the altitude and weight combination, to achieve max range, as fuel decreases it is best to slow down the plane. Excellent question. Best of luck.

Ramon C

Yes, as the aircraft burns fuel it becomes lighter and that reduces drag and fuel consumption. It also allows the aircraft to climb higher and further reduce fuel consumption. On long flights the aircraft climbs in stages. Concorde was the best example of this, she was cleared for the block from 45,000 to 65,000 and was set at constant throttle and Mach number and she climbed all the way, the SR71 does the same thing. No. A pretty heavy 747 can reach Mmo so losing weight does not allow them to go faster. Scheduling and fuel consumption define actual cruise speeds. Hah, wasted my time, Skipper747 already wrote the book.

Chris H

Skipper 7 has, as always, provided an authoritative answer. There is a slight confusion in other answers. The drag of an aircraft is an inherent feature of its design and airspeed and is not affected by its fuel load.

GC

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