On 06/27/2011 09:38 PM, Jos Okhuijsen wrote:
> It's a pity Mark, that you have not checked the max DC specification of
> those microswitches.
The max DC specification of the switches is of little value. Max DC is
mainly limited by the opening arc of the switches under load. In normal
situations, the limit switches are never opened as the controller cuts
the power long before the limit is reached. These switches are just
sitting there all their life closed, waiting for a controller failure
which is unlikely to happen anyway.
In a previous statement you claimed that the run away was caused by
failed micro switches. This is similar as claiming that you have had a
trim servo run away because of a failing end stop in the trim servo.
Such a claim would ignore the fact that the trim servo can only run
away, with or without end stop, if something keeps feeding power to it
after it reached the desired position. It would require a faulty cockpit
switch or faulty pilot in addition. Similary: Your situation, even with
shorted micro switches, could only occur if the controller or some other
circuitry was failing at the same time: something was delivering power
to the prop while it was already over the target RPM. Even then, this
would never let the CB pop. There is only one controller and it can not
give power to two opposite directions at the same time whatever failure
mode you can devise.
Someone asked whether you had modified the circuitry and you denied
that. However in the archive (before the accident) I found a message
where you wrote that you had the reverse circuitry modified to simplify
the process of entering reverse (which is cumbersome in the original
setup but maybe there was a reason for that), and if I recall it
correctly a relay was used to switch the controller and feed power to
the reverse ring of the prop at the same time.
If this modified circuitry failed, it would explain fully what happened:
While feeding unwanted power to the reverse ring of the prop the prop
could reach 0 or negative angle because the reverse slip ring bypasses
the limit switches (for logical reasons) AND the controller at the same
time, and the CB would pop if the controller tried to reverse the engine
polarity.
The whole story of short circuited limit switches does not explain the
problems in any way. I would be confident to operate the airplane with
shorted limit switches AND shorted or busted diodes, as these items are
never used while the controller is working properly.
The function of the diodes is to allow the motor to run in the other
direction once one of the limit switches opened (otherwise you could
never recover from reaching the prop limit as the limit switch cuts ALL
power to the prop). In normal operation the diodes are never used.
Neither shorting them or opening them would affect the normal operation
of the prop. Again, whether these diodes are fully up to specs wouldn't
matter at all in normal situations as it is unlikely that any current
will ever flow through them.
Your situation further differs from standard configurations by using the
scimitar blades, which are not pitch bias neutral under load but have
strong aerodynamic resistance in one pitch direction. It could well be
possible that changing the pitch in one way caused excessive current,
while in the other direction the aerodynamic loads would try to changing
the pitch even further, and maybe drive the blades over the limit
causing the limit switches to open. And in such a situation it might be
possible that the limit switches finally give up the ghost (but
logically they would burn through and open up, making it impossible to
change the pitch from there). Again, even in this scenario it would not
lead to pitch run aways and popping CB's, but just to a prop which
doesn't respond anymore to pitch change commands.
Frans
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