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To: flyrotary@lancaironline.net
Date: Tue, 20 Dec 2005 12:26:39 -0800
Subject: Re: [FlyRotary] Re: Fuel filter selection
Message-ID: <20051220.122644.2572.0.alwick@juno.com>
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From: al p wick <alwick@juno.com>

Good points Chad. Rephrasing your statement, we need to make sure we take
advantage of independent failure modes. So that means, pay attention to
how far the inlets are from each other, make sure both pumps aren't on
the same fuse, stuff like that. If the pump inlets are 1" apart from each
other, it's not too difficult for one object to clog both inlets. If
inlets are 4" apart, risk drops dramatically.

I have one inlet with that big self cleaning filter sock on it. Same sock
used by most OEM car mfg. The other inlet has no filtering. I think it's
reasonable to use two socks and treat pump failures as independent
probabilities.

In hind sight, I think I was foolish to have external fuel pumps. With my
new engine conversion in process, I am tempted to convert fuel sys to wet
pumps. Huge risk reduction.


-al wick
Artificial intelligence in cockpit, Cozy IV powered by stock Subaru 2.5
N9032U 200+ hours on engine/airframe from Portland, Oregon
Prop construct, Subaru install, Risk assessment, Glass panel design info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html


On Tue, 20 Dec 2005 12:38:01 -0500 Chad Robinson <crj@lucubration.com>
writes:
> 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
> 
> --
> Homepage:  http://www.flyrotary.com/
> Archive and UnSub:   http://mail.lancaironline.net/lists/flyrotary/
> 
> 


-al wick
Artificial intelligence in cockpit, Cozy IV powered by stock Subaru 2.5
N9032U 200+ hours on engine/airframe from Portland, Oregon
Prop construct, Subaru install, Risk assessment, Glass panel design info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html