I have been struck while flying twice, once it melted the lens on the iv-p
and blew open the wing extension, the second time in a 737 landing in dfw,
Once in a million? I guess I am unlucky

Sent from my iPad

On Jan 20, 2012, at 7:06 AM, Gary Casey <casey.gary@yahoo.com> wrote:

Just a quick comment on the redundancy issue.  I read the comment that a
vacuum system is not a good candidate for a back-up because of the known
poor durability of most vacuum pumps.  But that doesn't by itself reduce
reliability.  What happens if one has two electronic systems and both get
disabled by a lightning strike?  Say there is a one in a million chance of
getting struck by lightning and there is a 1 in 2 chance that both systems
will be disabled.  That says that there is a 1 in 2 million chance that you
will have a really bad rest-of-the-day.  Now put in a vacuum system that
will absolutely not fail as a result of a lightning strike.  There is, say,
a MBTF of 500 hours for the vacuum pump(I think it is more like 1,000
hours).  It will take maybe 30 minutes to get on the ground after the
lightning strike takes out all the electronics.  What is the odds of the
vacuum system failing in that 30 minutes?  Presumably 1 in 1,000.  So the
odds of the electronic system failing AND the vacuum system failing in the
next 30 minutes is 1 in a million times 1 in a thousand, or 1 in a billion.
 You only need the vacuum system to keep working for the time it takes you
to get to the ground.

So the all-electronic system will have a 1 in 2 million chances of killing
you, while the combination of an electronic primary and the not-as-reliable
vacuum system has a 1 in a billion chance of killing you.  500 times better
than the all-electronic system.

That's why I have an engine-driven vacuum pump and a vacuum AI in mine,
along with the electronic EFIS.
Gary Casey