Fuel jettison time

[petic537 2]

I have found that "fuel jettison time" is the same if I use 1 or both nozzles. Is that correct?

2 nozzles:

https://ibb.co/cZVf85
 

1 nozzle:

https://ibb.co/e5Ydvk

Same jettison time 50 min.

 

[downscc 1,327]

Same pumps... same hydraulic head, only difference is one open pipe instead of two and in any case I doubt there is anything near a maximum flow in the pipe.  So yeah, I'm the wrong kind of engineer but it makes sense.

[Qavion2 39]

Petr, did you wait 90 seconds for the jettison system to compute the jettison rate? When the system is first activated, the jettison system uses standard fuel jettison rates to compute jettison time, but, after 90 seconds, it will compute the rate at which fuel quantity is changing to produce a more accurate value. This second value will also consider the rate at which the engines are using fuel. Carry out your tests with the engines burning fuel at a constant rate.

http://www.pprune.org/tech-log/81634-b747-400-fuel-dump-rate.html

Does the value change if you start turning off pumps?

 

[petic537 2]

Hello John, I have tried to open both nozzles and then I waited 2 minutes. After that I closed the left nozzle and waited for next 2 minutes - fuel jettson time was the same for both scenarios.

If turned off CTR PUMP or OVRD PUMP then fuel jettson time was increased.

 

[Flamin_Squirrel 71]

16 hours ago, downscc said:

Same pumps... same hydraulic head, only difference is one open pipe instead of two and in any case I doubt there is anything near a maximum flow in the pipe.  So yeah, I'm the wrong kind of engineer but it makes sense.

Not true. There are two aspects to the hydraulic head here: what the pump generates, and the back pressure resisting it (see red and blue lines on the graph below). These figures match during steady flow.

By opening a second dump nozzle, it will cause the ""system head loss" to reduce, which will cause the blue line to flatten. The curves will intersect further to the right, indicating the pump will be generating less pressure but higher flow. Hope that helps.

[downscc 1,327]

I'm not sure the total dynamic head is going to vary significantly between one or two dump nozzles.  Interesting topic..., any mechanical engineers around here with a basic knowledge of aircraft fuel systems?

[Qavion2 39]

 

3 hours ago, downscc said:

any mechanical engineers around here with a basic knowledge of aircraft fuel systems?

I think you'd want someone with a very good knowledge of the 744 system, Dan. There are too many variables. Some pumps have the assistance of gravity (HST). You would probably need to know specific pump outputs and manifold pipe diameters. e.g. an HST pump has a nominal flow rate of 17,000kg/hour (althought this may be on a manufacturers work bench)

Maybe you could compare refuelling rates with jettison rates. I'd guess each jettison nozzle would have half the cross section of a typical refuelling pipe. Normally two refuelling pipes are used on the left wing. However, I don't know truck pumping pressure.

[Captain Kevin 586]

On 4/8/2017 at 8:54 AM, Flamin_Squirrel said:

(see red and blue lines on the graph below)

[downscc 1,327]

With all respect Kevin, your graph is a generic pump curve probably used to teach basic hydraulics to High Schoolers.... looking for something a little more concise here. Thanks.

Nice giraffe,  is that April and her new baby?  I don't see the relevance to the topic however.

[berts 97]

Boeing built a lot of safety features and redundancy into the 747's systems so that if any one item failed there would normally be at least one backup.  I don't know why you would want to dump with one nozzle closed, but assuming one fails to open then the fuel flow rate out of the operational nozzle will be much greater.  The average dump rate (approx 2,000kgs/min) should therefore remain more or less constant with the same fuel load and pumps working.

Occassionally the FUEL OVRD 2, 3 message might be displayed during jettison due to reduced back pressure from the open nozzle valves (low pressure condition).  Extending or retracting the flaps between 1 and 5 was not recommended because fuel could come into contact with the wing whilst the LE flaps are in transit.

Bertie Goddard

[Qavion2 39]

It's interesting to note that there is no MEL specifically for the nozzle valves. However, this may be more a case of probability of failure of the remaining one rather than of fuel jettison flow.

[Captain Kevin 586]

8 hours ago, downscc said:

Nice giraffe,  is that April and her new baby?  I don't see the relevance to the topic however.

That picture was taken in 2008 at the St. Louis Zoo, so highly doubt it. It was basically just a play on words since both sound similar.

[downscc 1,327]

16 hours ago, Captain Kevin said:

That picture was taken in 2008 at the St. Louis Zoo

I was at that zoo several times during my tour at Scott AFB..., very nice zoo. I'm sure much has been added since the late 80s.