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Posted for "Zavatson, Christopher J (US SSA)" <Christopher.Zavatson@baesystems.com>:
Matt,
You have three different issues here:
1. Performance characteristics of diaphragm pumps naturally produce
higher pressure at low flows rates and decreasing pressure at higher
flow rates. You should be able to see this during take off and initial
climb at low altitude. Pressure will drop from 6 to possibly 1.5-2 psi.
At idle, fuel pressure should be at its highest, which you observed
during landing.
2. Fuel tank venting. Some positive pressure is good, but be careful.
You could theoretically pressurize the wing with full ram pressure
~1psi. At this pressure the wing skins will bulge. You will be able to
see all the rib locations when the light is right. I observed this on a
flight after an unrelated change that lowered the vent farther into the
slipstream. I measured the tank pressure and found almost full ram
pressure. Some filing on the vent tube brought the pressure down.
Vents can only contribute +/- 1 psi so that isn't the whole story
either.
3. You are not alone in your observation. After observing the same
thing primarily on long, high altitude flights, I discussed this with an
A&P here at work. He used to work for Piper and they saw the same
thing. He tells me they spent lots of time instrumenting the system
trying to figure out the root cause of this occasionally pressure drop,
but they came up empty. As you saw, the boost pump does not eliminate
this odd behavior. This points to something inside the carburetor.
Does your pressure come back up after a time in constant cruise?
Chris Zavatson
N91CZ
360std
www.N91CZ.com
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