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In many cases, the maximum landing weight of an aircraft is lower than the takeoff weight due to the amount of fuel it carries.

While some aircraft have fuel dumping systems, others like the A320 family and Boeing 737 do not, and so have to cycle nearby until they have burned enough fuel. (And in case they have to land immediately, they will simply to, but need further inspection due to the high stress on brakes and landing gears)

My question is: If an aircraft has to cycle to burn fuel, are there measures to burn fuel faster? Lower altitude requires more thrust and so needs more fuel, but are there more ways? Like a slight air brake setting?

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    $\begingroup$ It'd (also) be interesting to compare the fuel dump rate and the normal fuel burn rate to see how much useful the dump can be (as indirectly suggested by @aeroalias' answer). $\endgroup$ – mins Dec 27 '15 at 15:21
  • $\begingroup$ Fly faster, if possible, to run the engines higher. Do it at a lower, less aerodynamic altitude. Put out landing gear, other draggy items. $\endgroup$ – CrossRoads Aug 13 '18 at 15:20
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This question (sorry to say) is sort of moot because:

  • if you have an urgency to land (e.g. cargo fire), you don't care if you are overweight and you'd land anyway
  • if you have time to dump fuel, the situation isn't as desperate, and you can easily wait just a bit longer.

The large jets (e.g. Airbus A380, Boeing 777) have a fuel dumping system because, with full fuel, these planes can travel up to 12 hours or more! That is to say, if you don't dump fuel, you might have to fly around all day to get down to your normal landing weight.

Airbus A320 and Boeing 737 are different: they usually fly 1~3 hours route, and may stretch to 6 hours on a full tank. So, to begin with, you need less time to burn fuel. Plus, these (relatively) small planes can land on shorter runways, which means your airport choice will not be as restricted as with a large jet.


Now, as to actually answering your question......

Flaps and gears have speed restrictions. You cannot fly full throttle with these deployed. That being said, if the pilots put the airplane in landing configuration (full flaps + gear down), drag will increase significantly and they will need more thrust to maintain altitude and airspeed.

You can technically achieve that with spoilers, but it's dangerous. Spoilers destroy lift, so what you're really doing is reducing the effectiveness of your wings and compensate that by flying faster and/or increasing your angle of attack.

Then again, if you're desperate, just land right away......

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    $\begingroup$ "these planes can travel up to 12 hours or more!" xkcd.com/870 $\endgroup$ – Robus Dec 28 '15 at 9:05
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    $\begingroup$ Side comment on your first sentence: you may have an onboard emergency (medica, for example) requiring to land ASAP, but without the risk of landing overweight and damaging the plane. $\endgroup$ – kebs Dec 29 '15 at 16:06
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Fuel flow is proportional to air density, so it is advisable to stay low. Since the propulsive power requirement scales with the cube of airspeed, flying faster is also advised. However, when flying below 10.000 ft all civilian traffic has to obey a speed limit of 250 KIAS.

My recommendation is to

  • stay low and fly in approach configuration (gear, flaps and slats out) as close to 250 KIAS as the configuration allows. A Hapag-Lloyd A310 demonstrated in 2000 how much an extended gear will increase fuel consumption.
  • climb to an altitude just above FL100 and fly as fast as possible. Depending on the aircraft, spoilers or speed brakes might still be used to increase drag, but the gear and high lift devices have to stay in.
  • fly a sawtooth pattern: Climb and descent in rapid succession, and use all drag-increasing means during descent, including sideslip.

Which version is best depends on the particular airframe. My personal favorite is number three: Few maneuvers beat a proper sideslip for sheer fun.

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    $\begingroup$ I'm going to go out on a limb and say that fun is not high on the priority list when looking for ways to burn fuel due to an in-flight emergency. However, fun can be an added benefit to highly effective. ;) $\endgroup$ – FreeMan Dec 29 '15 at 15:25
  • $\begingroup$ @FreeMan If it's an emergency haven't you already landed? $\endgroup$ – David Richerby Nov 24 '16 at 9:55
  • $\begingroup$ @DavidRicherby as per kebs comment on Kevin’s answer, in a medical emergency you don’t want to risk the lives of everyone else by landing over weight. $\endgroup$ – Notts90 Aug 13 '18 at 12:49
  • $\begingroup$ Speed limit does not apply in an emergency. $\endgroup$ – mongo Dec 30 '18 at 19:50
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For increasing fuel burn, the pilot can simply increase drag by increasing the speed. This results in requirement of more thrust, which requires more fuel. He can do it in lower altitudes, but the holding altitude has to be cleared by ATC, I guess.

Note that even jettisoning of fuel takes quite some time.

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    $\begingroup$ Can you do something the analog of pressing the gas pedal with the parking brake on?e.g. Add thrust with spoiler + flaps + speed brakes ON + gear down + RAT deployed or some allowable combination of all those that does not exceed allowable speeds? $\endgroup$ – curious_cat Dec 26 '15 at 16:37
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    $\begingroup$ Flaps and gears have speed restrictions; flying faster than that can damage them (or even tear them off the aircraft!). RAT won't increase drag significantly. $\endgroup$ – kevin Dec 26 '15 at 16:43
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Another method - one employed by F-16 aircrews as the jet doesn't have a fuel dump vent - is 'burner and boards', that is selecting afterburning with the speedbrakes deployed.

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Many times burning fuel off is not necessary. However there are times when it is. One pilot who flew for a company I worked for was ferrying a large Cessna from Kansas to Africa, with essentially a waterbed full of avgas. He had a gear problem at takeoff. The decision was to empty the bladder inside the cabin, prior to a landing with one gear down and one main someplace else. Probably a good decision, but this type of situation is rather rare, thankfully.

One human factors consideration is that an extended burn or dump time can increase crew fatigue and stress.That can increase risk.

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