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Date: Fri, 20 Jan 2012 09:42:42 -0500
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From: Andres Katz <bu131@swbell.net>
Subject: Re: [LML] Re: Steam Gauge Backup


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I have been struck while flying twice, once it melted the lens on the iv-p a=
nd blew open the wing extension, the second time in a 737 landing in dfw,=20=

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 v=
acuum 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 reliabi=
lity.  What happens if one has two electronic systems and both get disabled b=
y a lightning strike?  Say there is a one in a million chance of getting str=
uck by lightning and there is a 1 in 2 chance that both systems will be disa=
bled.  That says that there is a 1 in 2 million chance that you will have a r=
eally bad rest-of-the-day.  Now put in a vacuum system that will absolutely n=
ot fail as a result of a lightning strike.  There is, say, a MBTF of 500 hou=
rs for the vacuum pump(I think it is more like 1,000 hours).  It will take m=
aybe 30 minutes to get on the ground after the lightning strike takes out al=
l 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 fail=
ing AND the vacuum system failing in the next 30 minutes is 1 in a million t=
imes 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.
>=20
> 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.
>=20
> That's why I have an engine-driven vacuum pump and a vacuum AI in mine, al=
ong with the electronic EFIS.
> Gary Casey

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<html><head></head><body bgcolor=3D"#FFFFFF"><div>I have been struck while f=
lying twice, once it melted the lens on the iv-p and blew open the wing exte=
nsion, the second time in a 737 landing in dfw,&nbsp;</div><div>Once in a mi=
llion? I guess I am unlucky<br><br>Sent from my iPad</div><div><br>On Jan 20=
, 2012, at 7:06 AM, Gary Casey &lt;<a href=3D"mailto:casey.gary@yahoo.com">c=
asey.gary@yahoo.com</a>&gt; wrote:<br><br></div><div></div><blockquote type=3D=
"cite"><div><div style=3D"color:#000; background-color:#fff; font-family:boo=
kman old style, new york, times, serif;font-size:12pt"><div>Just a quick com=
ment on the redundancy issue. &nbsp;I read the comment that a vacuum system i=
s not a good candidate for a back-up because of the known poor durability of=
 most vacuum pumps. &nbsp;But that doesn't by itself reduce reliability. &nb=
sp;What happens if one has two electronic systems and both get disabled by a=
 lightning strike? &nbsp;Say there is a one in a million chance of getting s=
truck by lightning and there is a 1 in 2 chance that both systems will be di=
sabled. &nbsp;That says that there is a 1 in 2 million chance that you will h=
ave a really bad rest-of-the-day. &nbsp;Now put in a vacuum system that will=
 absolutely not fail as a result of a lightning strike. &nbsp;There is, say,=
 a MBTF of 500 hours for the vacuum pump(I think it is more like 1,000 hours=
). &nbsp;It will take maybe 30 minutes to get on the
 ground after the lightning strike takes out all the electronics. &nbsp;What=
 is the odds of the vacuum system failing in that 30 minutes? &nbsp;Presumab=
ly 1 in 1,000. &nbsp;So the odds of the electronic system failing AND the va=
cuum system failing in the next 30 minutes is 1 in a million times 1 in a th=
ousand, or 1 in a billion. &nbsp;You only need the vacuum system to keep wor=
king for the time it takes you to get to the ground.</div><div><br></div><di=
v>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. &nbsp;500 times b=
etter than the all-electronic system.</div><div><br></div><div>That's why I h=
ave an engine-driven vacuum pump and a vacuum AI in mine, along with the ele=
ctronic EFIS.</div><div>Gary Casey</div>  </div></div></blockquote></body></=
html>=

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