DV,
Sorry, but the QinetiQ report sets the risk as being tolerable (see para 1 of conclusions).
You must be reading a different document to the one I've seen. Putting aside the document, and whomever wrote it (I recognise the authors, incidentally) for the moment, can you explain to me, how a risk is tolerable when it doesn't exist? The reason I say it doesn't exist is this (my apologies to the grannies for the egg sucking lesson):
Lets say that a pipe containing a hydrocarbon liquid, anywhere, at any time, leaks; maybe even sprays out of the pipe. That, in itself, is not a fire hazard. A health hazard, maybe. Therefore, with no fire hazard, there is no risk.
However, it will become a fire hazard if there is any potential for it to ignite.
Oxygen abounds in the atmosphere, so the liquid needs only some form of ignition in its environment. Fuel is ignited only by heat (heat accelerates or provides the combustion/oxidisation process, otherwise known as fire). A spark contains heat, which is the only reason that a spark will ignite fuel. Some hydrocarbons are very volatile and vulnerable to oxidisation and require only a little amount of heat (or a small spark); gasoline is a good example. Clearly, an aircraft, unlike a car cannot stop if there is a fire on board, so jet-engine fuel has to be resistant to accidental/easy ignition. Kerosine fits the bill. Kerosene, unless atomized, is very difficult to ignite. Atomization of kerosene occurs under very high pressures only. Kerosine vapour will spontaneously ignite (flash point) at approx 45C. Kerosine liquid will ignite at different temperatures dependant on the time that it is subject to that heat, bearing in mind that it will vapourise under heat and, due to any air local currents, move away from the heat source (convection). Therefore, in order for us to deliberately ignite kerosine, we need either a huge spark (with sufficient heat) or an existing heat source. In a jet engine the latter is achieved by constant combustion, whereby we simply add more fuel to the existing hot fire. However, we still need to start that combustion process and to do that we have to generate lethal voltages to produce very powerful sparks in the igniters in the engine combustion chambers. Furthermore, the kerosine has to be atomised before it gets to the spark. Once it has lit, we have constant combustion and the ignitors can be switched off. If you believe we are overdoing it with the atomising process, we experience engine running problems with carbonised burners which do not atomise the fuel properly. The 7% lower flight idle RPM on the No 4 engine during the descent on XV230 was a classic example of carbonised burners (Mick Bell, my old friend: a fuel leak, not. A 1000 gall/hr fuel leak at flight idle would result in a flame out because the flight idle fuel bleed would be starved; I think you forgot that bit)
Conclusion1: Kerosine cannot be accidentally ignited by a low voltage spark. Therefore, in this aspect: no risk
It can only be accidentally ignited by a very hot surface or a very powerful spark. Outside of the engine bay ignition system, in all aircraft , incl Nimrod, there is no possibility of a spark of that magnitude occuring unless it is caused by a massive electrical failure. It would have be a failure so significant that the circuit protection devices will switch off that circuit before it reaches the magnitude required to ignite kersosine. We have been discussing fuel safety here, but electrical safety requirements in aircraft have always insisted that circuits are protected by high quality insulation, protected against arcing and sparking by fast acting cut-outs and good maintenance practice.
Conclusion 2: Although kerosine can be ignited by HE energy sparks, there has to be a triple failure in an aircraft for this to occur: the fuel must have leaked (1st failure), a nearby HE electrical system (200 volts?) must have failed in its isolation (2nd failure) and the associated circuit protection has not acted in time to stop the fuel igniting. Single point failure risk due to a HE spark in a Nimrod: none, because we do not have any 200V power supplies anywhere near the fuel pipes. The designer wasn't that stupid.
We've discussed hot pipes ad nauseum. But, to add to the growing knowledge in this forum, please note that, with the exception of the crossfeed pipe, all HP air pipes are inside fire zones. The crossfeed valves are inside the hotter (fire) zones of the outboard engines. Therefore, approx one third of the crossfeed pipe (ie the pipe between the valves) is inside a fire zone with the appropriate protection. We do not, now, open that pipe, except to start the engines. While that pipe is open we limit the engine speed such that the pipe's external temperature does not exceed the degree required to ignite fuel or hydraulic liquid that might come into contact with it. The IPT has sponsored tests to ensure that we are below that ignition temperature. We are not guessing and using our "gut feelings", here. We might be under-resourced, but we are professional and we will do the right thing and we will not assume we are right.
Conclusion 3: the kerosine on a Nimrod cannot now possibly ignite due to coming into contact with a hot surface. Fire risk: none.
This isn't a tolerable risk. Its a non existant risk.
(in the case of single point failures)
The copy of the QQ report, that I have read, fully supports my assertion. It states in the Summary:
"
Principally, the key factor in the Very High and High risk assessments has been the proximity of a hot surface relative to the fuel system.....It is considered that these areas of risk are addressed as part of ongoing inspections or aircraft limitations, specifically, limited use of bomb bay heating, HP Bleed Air cross feed and the SCP"
These are the facts that I referred to in my previous post.
DV, I'm keen to know more about the document you refer to. The one I have refers to the only analysis that QQ has done in recent years.
I'm also prepared to discuss HP air pipes and any perceived associated risks.
Ed
ps. I had a pprune name faff over the weekend, so I've changed my name (slightly). Do not write privately to edset100; I will not receive it; edsett100 works.