Fuel dumping question

[Garfield1966]

I have heard several times lately that when an airplane was in trouble, they had to circle for several hours to burn fuel before they could land. I know this is to lighten the a/c so as to not damage the landing gear. Isn’t there an easier and quicker way to do this? I thought I remember hearing about a way for the cockpit to open a valve and vent the fuel. My understanding is that if done at a high enough altitude, the fuel vaporizes before it hits the ground. If there is a problem, isn’t it best to get on the ground as quick as possible instead of circling for hours?

 

[FWAAA]

Some aircraft have valves that can be opened, like the 747. Their use doesn't cause an immediate "dump" of fuel (imagine the disaster that might cause if such an imaginary trap door ever opened uncommanded inflight) but instead cause a spray from the wingtips. Jet fuel is volatile enough to vaporize even at relatively low altitude - the fuel never reaches the ground. The Air India 747 flew circles while dumping fuel for about a half hour the other night.

The TV newscasters went on and on the other night about circling "OVER THE OCEAN TO DUMP FUEL" as if it mattered where the airplane flew. Given the crazy litigious world we live in, if the airplane circled over Malibu and Sherman Oaks, kooks would probably claim that they were harmed by the jet fuel, despite the impossibility.

Other aircraft (like the A320) do not have the capability to dump fuel, and thus must fly to burn it off. The JetBlue emergency landing (nose gear) at LAX a couple months ago flew at low altitude (of course with its gear down) to lighten things prior to landing.

Dunno why some airplanes can't dump fuel. Maybe it's more important for very heavy airplanes like the 747 which can have much higher takeoff weights than the max safe landing weight.

 

[scorpion]

Depending on the situation less weight might be a valid reason for dumping fuel but common sense would dictate less fire hazard. If you were on an aircraft that had to make an emergency landing would you want the fuel tanks full or empty?

 

[Bob Owens]

Weight is the issue. Aircraft can take off heavier than they are designed to land. If they do an "overweight landing" an inspection must be accomplished, the depth of the inspection depends on what the pilot reports his sink rate as and how softly he landed. The decision to fly around to burn off fuel is usually optional, if need be they could turn around and land immediately but then an overweight inspection must be accomplished. Overweight landings are pretty routine, if someone has a heart attack or other medical emergency do you think they are going to fly around for an hour to burn off fuel before they land?

Most older aircraft like the 747, 727, etc have fuel jettison systems so its not the size of the aircraft that determines whether they have the systems or not.While the fuel that is jettisoned does atomize due to being pumped out into a 200mph airstream the system is designed to get the fuel off the aircraft not to vaporize the fuel so it doesnt reach the ground. Atomizing the fuel creates a huge safety hazzard because in its atomized state jet fuel is easily ignitable. The fuel thats jettisoned does eventually reach the ground but its so disbursed that its not likely to be detectable by people. If it was done low enough it might be because the fuel is simply pumped out(at a high volume rate) of a 2" pipe at the wingtips so it is possible that some of the fuel could reach the ground at enough of a concentration to be detectable by people.

 

[oldbscaler]

Generally speaking only wide body long range airplanes are the only ones that can dump fuel. The only narrow body airplane I can think of is the good old 727. Newer 767-300 can dump fuel but the older 767's cannot dump. The 747 and 777 can all dump. The reason the larger planes can dump is the tremendous fuel loads the can carry in auxiliary fuel tanks. Generally only the aux tanks dump fuel to get down to the max landing weight of the aircraft.
As far as burning fuel so as not to stress the landing gear, lighter planes have slower approach speeds, which of course is safer if there is a problem of any kind.

 

[kirkpatrick]

Reducing landing weight has more to do with getting the aircraft stopped than reducing stress on the landing gear. Bringing three quarters of a million pounds to a stop means dissipating a lot of energy either by burning up braked pads or reverse thrust, and reducing that weight can mean a lot in landing distance.

I've had three fuel dumps in my over thirty year career as a flight attendant. Once in a 707 when a hydraulic problem made a landing at MCI necessary and twice on the 747, once out of Athens and once out of Rome when we lost engines right after takeoff.

I remember the ATH incident quite well because the engine blew before we had cleared the end of the runway. To say the climb rate was unspectacular would be an understatement. I swear we blew people's laundry off the line as we climbed out to the northwest. We went out over the Aegean and dumped 120,000 pounds of fuel. That would take my car around the world over twenty times. The dump valves on the 747 are near the wingtips, and by the time the fuel stream reaches the number five doors it's ten or fifteen feet in diameter. Quite a sight!

MK

 

[Garfield1966]

Thanks for the info everyone.

 

[Wretched Wrench]

Reducing landing weight has more to do with getting the aircraft stopped than reducing stress on the landing gear.

Overweight landing inspections are taken very seriously, and an overweight landing is avoided whenever possible. Not only for safety reasons, but because of the time involved in the inspection.

But there is no doubt that a heavier airplane needs more runway to stop.

As a side issue, a full tank is much less explosive than a tank with air space containing fuel vapor. But a full tank can spill more fuel if ruptured. And a full tank is more likely to rupture due to the weight and stresses involved. Sort of a damned-if-you-do and damned-if-you-don't, as far as fire or explosion danger.

 

[BoeingBoy]

For the exact answer to Garfield's question, FAR part 25 section 1001(a) says:

"(a) A fuel jettisoning system must be installed on each airplane unless it is shown that the airplane meets the climb requirements of §§25.119 and 25.121(d) at maximum takeoff weight, less the actual or computed weight of fuel necessary for a 15-minute flight comprised of a takeoff, go-around, and landing at the airport of departure with the airplane configuration, speed, power, and thrust the same as that used in meeting the applicable takeoff, approach, and landing climb performance requirements of this part."

Part 25.119 & 25.121(d) refer to climb performance in the landing configuration, with the most restrictive (25.121d) being with one engine inop but the normal all-engines operating landing configuration.

In other words, aircraft climb performance at max gross weight (less 15 minutes of fuel burn) with one engine inoperative and normal landing configuration determines whether a fuel dumping system is required or not. If the plane can meet the performance criteria, no fuel dump system required. If it can't, it's gotta have a fuel dump system.

As for the fuel dump system itself, it has to be able to dump enough fuel in 15 minutes for the aircraft to meet the above requirements. Obviously, by dumping longer one can get the aircraft weight down even further for landing.

Additionally, it must be shown that any fuel dumping system meets these requirements:

1) The fuel jettisoning system and its operation are free from fire hazard;

(2) The fuel discharges clear of any part of the airplane;

(3) Fuel or fumes do not enter any parts of the airplane; and

(4) The jettisoning operation does not adversely affect the controllability of the airplane.

The 727 is the only U.S. manufactured aircraft that I know of that has a fuel dump system and rear mounted engines. Notably, the fuel dump outlets on the outboard trailing edge of the wings was well outboard of the engines, and on the -100 model was actually slightly behind the engine intakes. All aircraft with wing-mounted engines have the dump point well outboard and behind the wings since they're also on the outboard trailing edges.

Jim

 

[Ifly2]

BEAUTIFUL....TRUE Message Board mentality...

someone posts the real actual factual answer to an honest question...[/i]and no on even notices...all the attention is on the speculation posted by those who don't have a clue....and the name calling...


ssdd
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