Message-ID: <000c01c00fa0$c8a03360$cc6b1b3f@oemcomputer>
Reply-To: "dfs" <dfs@gateway.net>
Reply-To: lancair.list@olsusa.com
From: "dfs" <dfs@gateway.net>
To: "Lancair List" <lancair.list@olsusa.com>
Subject: Re: Circuit Breakers for Hydraulic Pump Relays and run-away trim
Date: Sat, 26 Aug 2000 14:00:46 -0700
Mime-Version: 1.0

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A simple method of creating a run-away-proof trim system is to use what the
Human Factors wonks at Rockwell referred to as a (excuse me) "Dead Man
Switch" (DMS).

This is a spring-loaded switch that in it's normal un-actuated position,
removes ALL power to a critical circuit or control system. This is in
addition to any other control actuation, like a trim switch. In the case of
a trim circuit, the DMS could be a small push-button or trigger switch on
the stick grip that needed to be squeezed to allow the trim control to
function. This wouldn't impose very much additional work-load on the pilot
(probably become second nature in no time at all) and would allow almost
instant reaction time to kill the trim system by merely releasing the
switch. Further, a potential run-away condition could be recognized as soon
as the DMS was actuated, since the trim would start running before the trim
toggle switch was touched. If a run-away condition was detected, just
keeping your finger off the DMS until you pulled the breaker makes it a
non-event.

If ALL power to the trim circuit is removed by the DMS (similar to pulling
the breaker) it can't continue to run away. Also, during normal operations,
inadvertantly depressing the DMS without diddling the trim switch would be
of no consequence unless a run-away failure condition existed.

This concept was used in the rear cockpit of the B-1 where a built-in,
computer controlled aircraft test system could actually turn various systems
under test on and off, and which could include the movment of control
surfaces. To run these tests, (ground tests only, active tests were locked
out in flight) a test person had to manually hold the DMS actuated while
potentially dangerous (to other personnel) tests were running. To kill the
system if necessary, the test guy just released the switch.

Simple, neat and effective.

Dan Schaefer

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