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Trimeresurus
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How long does it take a 777 to dump fuel to reach maximum landing weight?

Wed Jan 20, 2021 11:01 pm

Currently, there's an ANA 77W squawking 7700 and flying circles over lake Michigan, presumibly dumping fuel before it can land. But seemingly it has been doing that for the past hour or so, and if it's a medical emergency on board, I'm afraid it might already be too late for that. Does it really take an hour of fuel dumping to reach the landing weight from an 12 hour fuel load?

And what's the consequences of a landing above that weight? Surely some damage to landing gear suspension and brakes wouldn't be worth more than human life?
 
shamrock137
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Wed Jan 20, 2021 11:46 pm

Trimeresurus wrote:
Currently, there's an ANA 77W squawking 7700 and flying circles over lake Michigan, presumibly dumping fuel before it can land. But seemingly it has been doing that for the past hour or so, and if it's a medical emergency on board, I'm afraid it might already be too late for that. Does it really take an hour of fuel dumping to reach the landing weight from an 12 hour fuel load?

And what's the consequences of a landing above that weight? Surely some damage to landing gear suspension and brakes wouldn't be worth more than human life?


How fast fuel is dumped depends on the aircraft, what they were fueled to, and flight conditions. Most fuel jettison systems that I know of are gravity fed, meaning the flow rate isn't constant. An overweight landing can be completed, usually requiring a maintenance inspection. They don't usually result in damage. These days, very few aircraft can actually dump fuel, and its an optional system on most that can. For example, its optional on the A330, and I believe the 350, Starlionblue would know...

In the event of a critical incident, be it a mechanical issue, or a severe medical issue, an overweight landing can be accomplished. However, landing ASAP is not always the best choice depending on the situation. The crew may need to contact company maintenance to accomplish troubleshooting procedures, run checklists, find a more suitable airport to land or simply come up with a plan to deal with the situation.

A good example of slowing things down to make the best decision is JetBlue 292, where the nosegear became stuck down and twisted 90 degrees. The crew took the time to do all the troubleshooting they could, including a visual inspection by ATC, worked with maintenance and their company to come up with a plan, diverted to a more appropriate airport, and circled for a few hours to burn fuel since the A320 is not equipped to jettison fuel. In the end they had the best outcome they could, by slowing things down, and taking their time. Emergency's don't always mean land ASAP.
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Starlionblue
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 12:34 am

While certainly you want that medical emergency on the ground ASAP, this should be weighed against the additional threats of an overweight landing. Normally there isn't any damage but it isn't really about the risk of damaging the aircraft. Threats would be high brake temperatures, which can lead to a fire, the potential of runway overrun, and poor go around performance. If you do end up with a brake fire or overrunning, you may have to evacuate, and during an evacuation injuries are commonplace.

In most cases, you would not consider an overweight landing for a medical emergency. The FCTM states that overweight landings "can be performed in exceptional conditions" (emphasis mine). Look at it this way: They could have been over the North Pacific, two hours from a diversion port when the person became ill.

Adding to this, long haul aircraft have a fairly comprehensive stock of emergency medical equipment. Cabin crew are medically trained, and have access to doctors via satphone.

Fuel jettison system is optional both on the A330 and on the A350. However, I don't know if any operator has ordered the A350 without it.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
DTVG
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 11:50 am

Any reason why fuel dumps (here and in general) seem to be conducted over uninhabited areas (or is that they just go to less busy air spaces?).

I would have said the fuel evaporates or the wind blows it far away before reaching ground in some form...
 
T54A
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 12:38 pm

Not B777 info, but just as an example. An A340-600 on a 16.5hr flight (JNB-JFK for example) will need to dump about 110 000kg of fuel for a turn back. It dumps at 1600kg/min, so about 68min less engine fuel burn. The A340-300 dumps at 1000kg/min, so that can also take a while.
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shamrock137
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 1:30 pm

Starlionblue wrote:
While certainly you want that medical emergency on the ground ASAP, this should be weighed against the additional threats of an overweight landing. Normally there isn't any damage but it isn't really about the risk of damaging the aircraft. Threats would be high brake temperatures, which can lead to a fire, the potential of runway overrun, and poor go around performance. If you do end up with a brake fire or overrunning, you may have to evacuate, and during an evacuation injuries are commonplace.

In most cases, you would not consider an overweight landing for a medical emergency. The FCTM states that overweight landings "can be performed in exceptional conditions" (emphasis mine). Look at it this way: They could have been over the North Pacific, two hours from a diversion port when the person became ill.

Adding to this, long haul aircraft have a fairly comprehensive stock of emergency medical equipment. Cabin crew are medically trained, and have access to doctors via satphone.

Fuel jettison system is optional both on the A330 and on the A350. However, I don't know if any operator has ordered the A350 without it.


Didn't think about hot brakes, that's a really good point. Even with a severe medical no point in getting it on the ground, blowing the fuse plugs and being stuck with flat tires... Thanks as well for the info on the 330/350.
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Horstroad
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 4:16 pm

Example for the 777-200LR:

MTOW: 347,500 kg
MLW: 223,168 kg
-> Maximum amount Fuel to dump: 124,332 kg

Maximum jettison rate with both nozzle valves open and all jettison pumps operating: 2,449 kg/min
Jettison rate with only one nozzle valve open and only wing tank jettison pumps operating: 1,224 kg/min.

So dumping the maximum amount of fuel could take between 51 and 102 minutes.

The jettison rate with only one nozzle valve open and all jettison pumps operating is probably the lowest possible flow rate in a scenario where you want to dump the maximum amount of fuel, as the jettison/override pumps of the center tank need to be operational in order to be able to dump that amount of fuel.
This gives a minimum jettison rate of 1,406 kg/min and a maximum time of 89 minutes.
 
889091
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 11:08 pm

Horstroad wrote:
Example for the 777-200LR:

MTOW: 347,500 kg
MLW: 223,168 kg
-> Maximum amount Fuel to dump: 124,332 kg

Maximum jettison rate with both nozzle valves open and all jettison pumps operating: 2,449 kg/min
Jettison rate with only one nozzle valve open and only wing tank jettison pumps operating: 1,224 kg/min.

So dumping the maximum amount of fuel could take between 51 and 102 minutes.

The jettison rate with only one nozzle valve open and all jettison pumps operating is probably the lowest possible flow rate in a scenario where you want to dump the maximum amount of fuel, as the jettison/override pumps of the center tank need to be operational in order to be able to dump that amount of fuel.
This gives a minimum jettison rate of 1,406 kg/min and a maximum time of 89 minutes.


Don't you have to account for the fuel that is burned during that 51 or 102 minutes? I don't think it'll be a linear correlation.
 
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Horstroad
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 11:27 pm

889091 wrote:
Don't you have to account for the fuel that is burned during that 51 or 102 minutes? I don't think it'll be a linear correlation.

Of course there are other factors to consider. After take off you will already have burnt off some of that fuel, so you would never have to dump 124 t anyways. I just wanted to give some numbers.
 
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Starlionblue
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Thu Jan 21, 2021 11:45 pm

889091 wrote:
Horstroad wrote:
Example for the 777-200LR:

MTOW: 347,500 kg
MLW: 223,168 kg
-> Maximum amount Fuel to dump: 124,332 kg

Maximum jettison rate with both nozzle valves open and all jettison pumps operating: 2,449 kg/min
Jettison rate with only one nozzle valve open and only wing tank jettison pumps operating: 1,224 kg/min.

So dumping the maximum amount of fuel could take between 51 and 102 minutes.

The jettison rate with only one nozzle valve open and all jettison pumps operating is probably the lowest possible flow rate in a scenario where you want to dump the maximum amount of fuel, as the jettison/override pumps of the center tank need to be operational in order to be able to dump that amount of fuel.
This gives a minimum jettison rate of 1,406 kg/min and a maximum time of 89 minutes.


Don't you have to account for the fuel that is burned during that 51 or 102 minutes? I don't think it'll be a linear correlation.


I think a heavy 777-300ER burns around 8 tonnes/hour. So yes, it makes a difference in this context, but not a big one.
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BoeingGuy
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Fri Jan 22, 2021 4:13 am

Starlionblue wrote:
While certainly you want that medical emergency on the ground ASAP, this should be weighed against the additional threats of an overweight landing. Normally there isn't any damage but it isn't really about the risk of damaging the aircraft. Threats would be high brake temperatures, which can lead to a fire, the potential of runway overrun, and poor go around performance. If you do end up with a brake fire or overrunning, you may have to evacuate, and during an evacuation injuries are commonplace.

In most cases, you would not consider an overweight landing for a medical emergency. The FCTM states that overweight landings "can be performed in exceptional conditions" (emphasis mine). Look at it this way: They could have been over the North Pacific, two hours from a diversion port when the person became ill.

Adding to this, long haul aircraft have a fairly comprehensive stock of emergency medical equipment. Cabin crew are medically trained, and have access to doctors via satphone.

Fuel jettison system is optional both on the A330 and on the A350. However, I don't know if any operator has ordered the A350 without it.


The one time I can think of that Boeing recommend landing ASAP even if overweight is in the case of an unidentified or uncontrollable smoke or fire.
 
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Starlionblue
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Re: How long does it take a 777 to dump fuel to reach maximum landing weight?

Fri Jan 22, 2021 5:29 am

BoeingGuy wrote:
Starlionblue wrote:
While certainly you want that medical emergency on the ground ASAP, this should be weighed against the additional threats of an overweight landing. Normally there isn't any damage but it isn't really about the risk of damaging the aircraft. Threats would be high brake temperatures, which can lead to a fire, the potential of runway overrun, and poor go around performance. If you do end up with a brake fire or overrunning, you may have to evacuate, and during an evacuation injuries are commonplace.

In most cases, you would not consider an overweight landing for a medical emergency. The FCTM states that overweight landings "can be performed in exceptional conditions" (emphasis mine). Look at it this way: They could have been over the North Pacific, two hours from a diversion port when the person became ill.

Adding to this, long haul aircraft have a fairly comprehensive stock of emergency medical equipment. Cabin crew are medically trained, and have access to doctors via satphone.

Fuel jettison system is optional both on the A330 and on the A350. However, I don't know if any operator has ordered the A350 without it.


The one time I can think of that Boeing recommend landing ASAP even if overweight is in the case of an unidentified or uncontrollable smoke or fire.


That's the one that comes to mind.

Maybe if you're down to a single hydraulic system and it is leaking, but that seem pretty far fetched.
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