Why does a plane have to 'dump' fuel before landing?

https://www.yahoo.com/news/more-details-delta-jet-fuel-123829765.html

Is the concern about a fireball if it crashes or because the landing gear can't support the extra fuel weight?

 

They dump fuel for a softer, safer and easier landing. Less fuel = less mass, which means they can go slower with the flaps extended without stalling, and also slow down faster with less stress on the spoilers when touched down.

 

Also less demand on the engines.

 

Structural damage... Most aircraft have a maximum fuel landing weight. Fuel dumping usually occurs when an aircraft must return shortly after take-off when its at its maximum fuel weight. The max fuel landing weight is unaffected by passenger and cargo weight.

 

Just curious how many gallons they'd dump ? 100 ? 1000 ? I have no idea. What's the cost / gal ?

MDS

 

At 6.5 lbs per gallon, the dump amount could be thousands of gallons to get below the maximum fuel landing weight. Airlines negotiate long term contract prices for their fuel. I’ve heard as low as < $1.00/ gallon compared to retail of up to $7.00 /gallon.

https://www.boeing.com/commercial/aeromagazine/articles/qtr_3_07/AERO_Q307_article3.pdf

 

747-8's Max takeoff weight is 987,000 lbs. However its max landing weight is only 688,000 lbs. So they could theoretically have to dump close to 300,000lbs (46,000 gallons) of fuel. This would normally be done above 10,000ft, with exceptions of 7,000ft in the winter and 4,000ft in the summer, down to as low as 2,000ft in an emergency. At the normal altitude the fuel will all evaporate before it hits the ground under normal conditions.

Overweight landings are allowed as well, however they may reduce the airframe life or require additional inspections.

 

That's incredible when you think about it

MDS

 

Interesting....thanks for the info!

 

I think it is just Max Landing Weight (MLW), and is irrespective of fuel, passengers, cargo.

 

Took off last year from Hong Kong in a 747-8 and couldn’t retract the landing gear. Had to dump almost 100,000kgs to get down to MLW. Kind of a strange feeling to open the valves and dump all that out into the atmosphere, but what’re you gonna do?

 

Also they are not supposed to dump fuel below 8,000? ft, so it evaporates before reaching the ground. They dumped at only 2,000 ft !!!
Tower asked if they wanted to dump fuel, pilot said NO, then dumped anyways. I'm sure tower would given them different path/instructions.

 

I thought I read the 777 was 186,000 lbs over max landing weight, jet fuel weighs 7lbs a gallon so about 26,000 gallons had to go.

 

Depends on the aircraft. Some airplanes can have no passengers, be well below the maximum gross weight, and still have too much fuel to land without causing possible structural damage.

 

That would be a violation of FAA rules which is being investigated now.

 

I think that is what I said.

You originally wrote "maximum fuel landing weight". I said it is actually "maximum landing weight". Amount of fuel has nothing to do with it other than that is the only thing that can be expelled to remove weight on commercial passenger aircraft. Military aircraft have same thing, i.e. maximum landing weight, but they can also off-load passengers, cargo, and ordinance to reduce weight in addition to fuel.

 

It'll be interesting, I can't remember a case like this but last I checked when you declare an emergency you can do anything it takes to save your plane, crew and passengers.

I read this morning a lawsuit was filed against Delta by the school victims and their parents.

 

Nope. Maximum fuel in the wings can cause structural damage to the airframe on landing regardless if the plane is at gross or not.

 

Nothing stopping a pilot from landing above maximum landing weight for reasons he believes are urgent. His call. An example would be a medical emergency where time matters. The aircraft would then be taken out of service, inspected and depending on what damage repaired. The inspection alone would be $$$.

 

Looking right now at a W&B manual for a B777.

Under Certified Weight Limits the following are listed
Maximum Taxi Weight (MTW)
Maximum Takeoff Weight (MTOW)
Maximum Landing Weight (MLW)
Maximum Zero Fuel Weight (MZFW)
Minimum Flight Weight (MFW)

That is all.

If you can find a reference to "maximum fuel landing weight" please provide a link.

 

I said “most” not all aircraft. Do you just like to argue or are you an expert?

 

yes

 

Yea, I get that, an overweight landing needs an inspection, most take 30 minutes but at 186,000 lbs over max landing weight, I'd think the gear would break if it wasn't an absolutely perfect 1 engine landing.

 

or the tail

note: this was flight test and part of certification, and far exceeds what anyone here as likely experienced. That sink rate far exceeds what would be expected with a landing at excess weight.

 

You do not want to land on a single engine at max landing weight unless you really have to.