Fuel spraying from engine? - 777 from Narita

[BobbyDTripod]

I was flying as a passenger on JL401 out of Narita on 30th Oct. The plane turned back to Narita after 2 hours flying with engine vibration according to the pilot.
I looked out of my window and watched a trail directly from the exhaust of engine the which looked like a thick vapour which I believe to be fuel spraying out of the engine for the 2 hours up until landing and then continuing to leak out once the plane had landed, leaving a liquid trail behind us on the ground and then gathering in a pool below the engine upon stopping. Groundstaff looked agitated and a fire crew were on the scene right away. The engine was possibly making more sound than I would normally expect once we were on the ground also.
Please see the response below which I have had from the JAL people. I'm not sure if they are being entirely honest with me - maybe you can shed some extra light on this for me? I've never heard of dumping fuel through the engine? (although they did do a fuel dump from the end of the wings before descent).
Any thoughts on the matter would be most welcome as I am very interested in what may have been going on?

"According to the department which is in charge of flight safety,
the reason why the aircraft you boarded on returned to Narita was
because the vibration of the engine seemed to be larger than usual.
Therefore we decided to check the safety of the aircraft by flying
back to Narita.
We deeply regret to worry you by the sight that the fuel was
spraying out of the engine. However, please let us inform you that
the part of the aircraft which fuel was spraying out was the part
of which was built by intention that the fuel will spray out from
there. Also, the amount of fuel sprayed out was not the amount
which would affect the safety. Also, the thrust of the engine was
enough to return back to Narita.
Furthermore, please let us explain to you why we decided to return
back to Narita instead of flying to the nearest airport. The reason
why the aircraft flew back to Narita was because there was no urgency
for this case to make emergency landing at the nearest airport."

[barit1]

However, please let us inform you that
the part of the aircraft which fuel was spraying out was the part
of which was built by intention that the fuel will spray out from
there.

Possibly a faulty or missing seal (O-ring?) on some fuel plumbing. I had this happen on a flight test engine, with incomplete documentation, so it took a phone call or two to sort it out. :O

[EEngr]

However, please let us inform you that the part of the aircraft which fuel was spraying out was the part of which was built by intention that the fuel will spray out from there.Possible explanation: From your position, fuel being dumped in preparation for returning to Narita appeared to be emanating from the engine. Dumping fuel is a normal procedure in the event an aircraft is rerouted and would otherwise have to land with a heavy load.

[TightSlot]

B777 Fuel Dumping

http://www.youtube.com/watch?v=RlaUOEa2UGY

When an aircraft takes off for a long flight, the aircraft weight will exceed the maximum weight at which it can be safely landed. The aircraft weight can be reduced to the safe landing weight by both burning fuel in the normal course of flying, and dumping fuel as shown above.

[PAXboy]

One can see that, from a window further forward, the exact point of the fuel leaving the wing would not be seen. It may well be the case that it appears to come directly from the engine, rather than a nozzle several meters outboard of the engine.

[Hartington]

I've re read the original post. The poster refers to fuel being dumped "from the end of the wing" which suggests a separate vapour stream to the one from the engine. He also refers to a pool on the ground after they stopped - fuel dumping is completed before the landing approach - it certainly does not continue on the ground.

Just some thoughts.

[Bealzebub]

It is entirely possible that the aircraft had a fuel leak. It can sometimes take up to thirty minutes to establish that a leak actually exists. Although fuel is normally stored in the wings and wing box fuselage area, it is delivered to the engines, as such there is a lot of pipework around the engine and engine strut area. Although quite rare, the engines are obviously a prime location for leaks to occur if and when they do. Once a leak is confirmed, it can often be stopped or reduced by shutting down the relevant engine. That might still result in spray being observed, or some pooling after the aircraft has stopped on the ground.

Engine vibration might be causal to, caused by, or entirely independent of whatever could also cause a fuel leak. Vibration is often dealt with by operating the engine at a reduced thrust setting, or even shutting the engine down. Troubleshooting often takes more time than you might imagine, and any decisions taken regarding the continuence of the flight or a diversion, will often only occur after that process is complete.

For a heavy aircraft on a long scheduled flight, it is very likely that the maximum regulated landing weight would be exceeded for a considerable part of the initial journey. If a decision to divert is made, then the weight reduction is achieved by opening "dump" de-fuelling valves, that allow a limited amount of the fuel to vapourize at a controlled rate from pipes at the back of the wing. This process can also take a considerable time, and this time management will also be instrumental in deciding on the most suitable diversion airport.

The explanation from the Captain, and subsequently from the airline are both likely to be broadly correct, but neither would be likely to involve very much elaborate detail simply because it rarely achieves the required levels of accuracy and technical comprehension suitable for the general recipients. What might be entirely routine and understandable to a pilot or engineeer, might be truly terrifying to a nervous flyer or the majority of passengers with less specific technical knowledge. You would always strive to be honest, but also ensure information is reassuring (as indeed it should be) and open to the absolute minimum amount of potential misunderstanding.

Similarly, Public relations departments will also have a limited amount of specific technical detail for general information, but would usually attempt to be as honest as possible with the tools at their disposal.

Specific technical details are (depending on the seriousness of an event) often available from public data sources when an investigation has been carried out in accordance with mandatory reporting events.

[BobbyDTripod]

Thanks for all you replies... I understand all the points made here.
The spray was definitely coming from the engine as I was sitting in the best seat possible right behind it to see it emanating directly from the exhaust. And indeed yes, there was also an seperate orthodox fuel dump from the nozzles down the wing as one would expect. So from this I can surmise there was a leak from the engine somewhere. My question is how dangerous is that?

[NutLoose]

Some engines have a collector tank to prevent and fuel oil etc on shutdown dumping onto the apron, when the engine is ran at power this then is sucked via a pipe through the exhaust and vapourised, I wonder if they have a leak say from a case drain that is feeding into the tank it could be that you are seeing clearing.......