From: utzoo!decvax!harpo!eagle!mhuxt!mhuxj!mhuxi!mhuxv!burl!sb1!ll1!ihldt!ihnp4!ihuxa!cfiaime
Newsgroups: net.aviation
Title: Re: IFR insurance low cost
Article-I.D.: ihuxa.182
Posted: Wed Jan 26 08:17:00 1983
Received: Fri Jan 28 21:01:30 1983
Reply-To: cfiaime@ihuxa.UUCP (Jeff Williams)
References: ihuxr.295 avsdT.271 ihuxa.180 alice.1418

Question:  Has anyone out there in "Netland" ever had an altimeter 
failure with a properly maintained unit?  (The one in my Funk was
useless until it was rebuilt, but that was not PROPERLY maintained.)
The point is, a skydiver altimeter, an auto altimeter are not meant
for use in aircraft at all.  Do you wish to vent this unit to the cabin?
Or do you wish to install it with a seperate static source?
If you really wish a backup altimeter, many instrument shops have good
used units for under $100 (see Trade-A-Plane).

Lets review the FARs for a moment concerning the altimeter.  First, it
is required for VFR (and IFR) flight as per FAR 91 something.  But,
while flying VFR, if the altimeter breaks (and they dont break catastrophicaly)
the pilot still has the ground in sight and can still fly without reference
to the altimeter.  Hence, in VFR, and altimeter failure is not an
emergency.  For IFR flight, the altimeter is inspected every two years.
The only way for an altimeter to fail is:
     1)  the diaphram ruptures
     2)  various gears and screws internally loosen
     3)  the case ruptures
     4)  the static system malfunctions.

In the first two cases, the biennial check of the unit will show if there
is a developing problem.  In the third case, you have more problems than
just an altimeter problem.  In the last case, an alternate static source
keeps the altimeter working.

Some encoding altimeters WILL fail if the electrical system malfunctions.
The Cessna 300 series encoder is like this.  If you notice, the aircraft
with the Cessna 300 series encoding altimetter installed also have a 
normal barometric altimeter installed in the panel.  (This is a certification
item for the use of the Cessna 300 encoder, by the way.)

So, for my money, the altimeter will be backed up by having an alternate
static source only, not a skydiver or auto altimeter.  However, this is
only my opinion, so don't take it as the Gospel.

Jeff Williams
BTL - Indian Hill