[Info-vax] OT: Aircraft pitot tubes and clustering.

[Tom Linden]

On Fri, 18 Dec 2009 10:31:20 -0800, glen herrmannsfeldt  
<gah at ugcs.caltech.edu> wrote:

> David Mathog <mathog at caltech.edu> wrote:
>> JF Mezei wrote:
>
>>> BUT, they found cases where 2 probes failed at the same time and by the
>>> same magnitude.
> (snip)
>
>> This could happen if the most common tube failure mode was to take the
>> output voltage to ground or supply voltage.  Then if two failed in the
>> way they are most likely to they will read exactly the same thing.
>> Presumably the software should reject readings in these failure mode
>> voltage ranges out of hand, but it might not.
>
> Well, you might hope that they check for that one.
>
> But say two filled with ice at about the same rate, while the
> other didn't.  (I think I remembered ice being part of the problem.)
> It is a problem of statisical independence.  If there are things
> that statistically could happen to both at the same time, then the
> test doesn't work.

A spurious false vote can occur under several different scenarios as has  
been
pointed out.  It would seem that an independent and differnet test ought to
be included as a sanity check, e.g., speed can be determined from satelite  
data
and the approximate wind speed is known from meteorological data, but I  
don't
know if that has adequate accuracy to disbelieve the two false positive  
votes.

BTW, the IBM360/65 had triply redundant cpus and employed a similar  
strategy, IIRC.
>
> -- glen



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