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As a result of another local case of gear retraction on the ground, I am
going to install a safety system to make it harder for me to switch from
those who will to those who have.

There have been several schemes using pitot pressure actuated switches to
keep the up circuit disabled until airborne.  These seem heavy, expensive
and require precise calibration to get full protection.  It is also not easy
to verify the calibration.  For full protection it seems to me that the
system would have to reliably actuate at something over lift off speed and
below Vx.  On my 235 that is only about a 5 MPH span.  That is a lot to
expect from even a good pressure switch let alone a washing machine part as
has been used in some systems. I suspect most of those flying won't do it.

Why not just a throttle position switch set somewhere above half throttle?
It seems to me that it would be much simpler and would protect from all
cases of carelessness except starting a takeoff run with the gear switch in
the up position and inadvertent retraction on an airplane parked with the
throttle open.  The take off case can be made adequately safe by adding a
gear switch position check to the run up check list and the airplane parked
with the throttle open case seems to me to be an acceptable risk.

Anybody doing it with a throttle position switch?  Are there shortcomings I
haven't thought of?

Ross W. Colebrook N7828

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