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Date: Tue, 07 Jun 2005 11:05:05 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EC2 problems - solved / rotary risks
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David Staten wrote:

> In Al's defense, my interpretation is that the CAS is a critical 
> "single point of failure". While none of us have developed a failure 
> at that point, a failure there is a show stopper. He was taking a 
> "theoretical" failure risk and doing the math with it. Going from a 
> 1:1000 chance of failure to a 1:1,000,000 chance of failure is a 
> significant result.
>  


And I think what most others have been saying is that we don't know that 
it isn't already a 1:1,000,000 chance.  Adding the complexity to have 
two sensors and the necessary interaoperability drops the probability of 
each one down to 1:1000.  So now you've added some weight, fabrication 
and maintenance headaches to get back to square one.  The CAS hasn't 
demonstrated any problems, so why go chasing through the bushes after 
rabbits. 

Most of the people taking exception with Al are saying that the method 
can't be applied because it requires a historical dataset and there's no 
history to base it on.  Without the history, it's all meaningless 
voodoo.  The CAS issue has caught a lot of attention, because Al used it 
as an example for how the EC2 is dangerously unreliable, and most of us 
(being new to FMEA) now view the technique with a jaundiced eye.

> To take the theoretical into practical, the Shuttle guys THOUGHT they 
> had a 1:200 flight risk of catastrophic failure. 2 destroyed orbiters 
> later, that risk is actually playing out to 1:50. It's about risk 
> management, and the CAS is just one (of MANY) single points of failure 
> that can be identified. That being said, there are many more risks out 
> there that take precedence.
>
> Dave
>

And there could still be a 1:200 flight risk of catastrophic failure.  
The 50 flights so far were just unlucky.  That's the problem with 
statistics. 

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