From: brent@greatcircle.com (Brent Chapman)
Subject: Re: Rescue Questions (ELT SNAFU)
Originator: brent@napa
Organization: Great Circle Associates; Mountain View, CA
Date: Fri, 16 Oct 1992 00:31:37 GMT

>Also, small aircraft (with some exceptions) are required to have an Emergency
>Locator Transmitter which is activated by sudden g-forces, eg a crash or real
>bad landing.

Which go off all the time by mistake (something like 97% false positive
rate), and don't work worth a damn when you want them to (something like
50% false negative rate).  I don't have a definite source for these numbers,
but these are the figures you usally hear quoted within CAP; I believe the
fales positive rate (if anything, it's actually higher), but I'm not so sure
about the false negative rate (on the other hand, even if the ELT _does_
go off, the antenna has often been damaged in the crash, or the aircraft
has come to rest upside down, or any of a number of other things keep the
signal from getting out).

>They operate on 121.5MHz and the CAP is good at homing in on them.

Yup, on airports (or in the pattern; some guys from my squadron spent a
few hours chasing a wandering ELT signal all over the airport before
the finally tracked it to a plane that had just landed and shut down,
after spending a couple of hours shooting touch and goes), in boat
harbors, in sporting good stores, in moving boxes (I spent a full day
in August chasing down an ELT that turned out to belong to a family
which had just moved, and went off when the movers dropped the box it
was in; they had to open quite a few boxes before they found the one
with the ELT in it), in word processors (CAP units in Los Angeles
recently chased down _TWO_ Smith Corona Word Processors, about a week
apart, that were transmitting quite a strong signal on 121.5), in
active and reservist flight gear, in trucks rolling down the Interstate
(I've heard a story that Texas Wing once chased down a truck driving
through the state that had something like 40 ELTs going off in the
back; seems somebody new at the factory had armed them before shipping
them...) and, oh yeah, very occasionally in crashed airplanes.

The ELT system is one of my favorite examples of the worst possible
case of "design by committee": "Design by Congress".  I don't have all
the details at hand, but here's the story as I remember it (maybe some
of our historians can correct the errors and fill in the blanks).
Sometime in the 50's or 60's, a Congress Critter was on an airliner
that crashed and didn't get found for a while (after all the survivors
had starved).  Congress took exception to this, and passed a law that
required an ELT system to be developed and deployed within some
ridiculously short period of time, like 1 year.  The design was very
rushed, and really stinks.  Funny thing is, large passenger aircraft
(i.e., airliners, which is what originally supposedly led to this whole
mess) are NOT required to carry ELT's; it's everybody ELSE who's stuck
with the damn things.

Gang, I (and lots of other CAP folks) spend a significant amount of time
chasing down bogus ELTs.  We chase them so agressively, despite the fact
that it's almost always a false alarm, because all the false alarms keep
us from hearing the _real_ ELT trips, rare as those are.  Please, help 
cut down the false alarm rate by paying a little more attention to your
ELT.  Take a second to listen to 121.5 after shutdown (especially if you
happen to make an "arrival" rather than a "landing" at some point).  If
you remove an ELT from your aircraft for service, disarm the damn thing,
preferably by removing the battery! 

You'd be amazed how often we chase these things to someone's garage or
living room, where the ELT is sitting on a shelf somewhere; often, kids
accidentally knock it over while playing, it goes off, the kids put it
back on the shelf, and the parents have no clue anything happened until
CAP knocks at the front door at 4 a.m. with the cops in tow.  Around
here at least, we never go knocking on doors without support from the
local law enforcement agencies.  Their presence usually makes our life
much easier, and is safer for us.  The cops aren't there to ticket
these folks (there's no law against ELT false alarms, much as some of
us might wish for one sometimes), they're there for the safety of the
CAP folks.

Enough ranting...


-Brent
--
Brent Chapman                                   Great Circle Associates
Brent@GreatCircle.COM                           1057 West Dana Street
+1 415 962 0841                                 Mountain View, CA  94041