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1. 04:09 AM - Re: Re: Servo Failure (Steven)
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Subject: | Re: Servo Failure |
John,
I've got a pretty good article on the Bendix servo. Monday I can scan the
thing and post it to our company's website.
If the fuel boost pump pressure and the engine fuel pump pressure were about
the same, I can't see that the servo should care which one is feeding it.
(For the single speed IO-540 pumps).
>From what I understand (and it's limited) the servo is set up on the bench
for normal aircraft fuel pressure.
All things being equal, if you change the ship's fuel pressure, you change
the fuel flow. For example at W.O.T, setting the engine driven fuel pump
pressure up and down will have a direct relationship to the fuel flow. The
last 'rebuild' on our servo came back twice with a wide open throttle fuel
flow lower than what we hoped to see. The second time was better, but we
compensated somewhat by running slightly higher fuel pressure on the right
side.
With this last rebuild it's finally right, but with us running the right
engine fuel pressure toward the high end of the green arc, the WOT fuel flow
was now too high, and the ship's fuel pressure was dropped back to the
middle of standard.
On the IO-540's the 'fuel flow' isn't really a measure of flow, but another
measure of pressure, this time at the fuel distribution spider. Plug an
injector and it would appear you've got more flow, when all you've got is
really more pressure.
Also keep in mind with these servos that with differential pressures being
used, some of the failure modes are fuel crossing to places it shouldn't
ought to be.
I think everything I said is true.
Steve
Also Robert, keep in mind that you're flying a Continental-powered
Commander, so you have low & high boost pump settings. Us Lyc-powered guys
only have "on" and "off" :-).
And Steve, the theories sound good. I wonder what effect the failure of an
engine-driven pump would have on the fuel pressure? Is the servo
self-regulating or is the EDP always the first link in the fuel pressure
management process? Being just the guy who pushes the levers around in the
cockpit, I'm probably light on the more esoteric points of the IO-540's
systems. Perhaps it's time to rectify that.
I'm going to dig around and see if I can find any articles or other
documentation on boost-pump-related engine floodings on approach. I've seen
ONE story along those lines related to Commanders, but I don't remember
which model the author was flying. I want to assume a 500A or 685 because of
all the Bonanza-specific issues I've heard about with the hi/lo boost pump
settings & engine floodings, but I don't know that the Lycs are immune to
similar occurence.
/J
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