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Date: Tue, 20 Dec 2005 12:38:01 -0500
From: Chad Robinson <crj@lucubration.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fuel filter selection
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al p Wick wrote:
> Debris killing pump:
> Let's guestimate how often this happens when you have no filtering of
> fuel inlet. How about one pump shut down every 500 hours of operation? So
> on scale of 1 to 10, pump shut down is a 6. But hold it, we have two
> pumps, two independent failure odds. So that risk is 1 in 250000 hours.
> Because we only loose fuel if BOTH pumps die. So now that risk is a 1 on
> scale of 1 to 10. One of our lowest risk items on the plane. 
> To keep this analysis simple, I'll ignore odds of noticing before flight,
> and effect on flight components. 

Al, I like the analysis, but I'd disagree with the 1 in 250,000 factor for two 
pumps. That assumes that the debris that killed the pump was completely 
independant between the tanks. For many this may be true, BUT:

1. Those with a sump feeding the pumps will stay at 1 in 500 because the 
contamination will probably be in the sump. Call it 1 in 1000 because maybe 
you get lucky and the dead pump sucked up ALL the debris, and will hold it 
when you switch to the other.

2. Maybe the source of the debris was put into both tanks. That is, maybe the 
debris is from a deliberate contamination to both tanks (gasp), something in 
the fuel rail or similar that's releasing debris (metal shavings from bad 
machining?) and will release that debris to either tank, regardless of which 
you select, etc.

So, it's probably still rare, but maybe not as rare as 1 in 250,000 just 
because there are two pumps. I think the "most likely" root causes of such 
contamination contain enough cross-over risk to keep this number lower.

Regards,
Chad