Bill Boyd

Bill Boyd

Well Known Member
Just took a post-maintenance test flight today and was tooling around at 3,500' LOP at about 60% power and 9.5 GPH (IO-540) when I decided to drop lower and see if a friend was in his farm hangar. Rolled into a descending turn and felt a roughness in the engine as I dropped down to about 2,000 feet - quick glance at the fuel flow showed 2.8 GPM. Mixture was advanced slowly with little response, then shoved all the way in - right back to smooth operation at above 10 GPH. Rest of flight (reverting to LOP in a few minutes) was nominal.

I've had "creep" before when trying to GAMI-balance my injectors where I would lean a bit and the flow reading would step down and stabilize, only to begin a slow drop to an even lower reading that took maybe a half inch of quadrant lever movement to bring back to steady-state. Don Rivera said they know about this behavior in some installs and was not concerned at all. My control cabling all seems squared away per the latest annual inspection last week.

I've seen the fuel flow creep before but never to this extent. It was attention-getting. This was on the 100LL tank, so I don't think vapor lock was in play despite it being a hot day. And it doesn't seem like that modest descent would cause over-leaning with a fixed mixture control setting, but maybe...

Anyone else seen this?
 
Bill, that behavior is way out of norm.
And "No concern" just doesn't seem like something Don would say. If you're sure the problem was communicated fully, at least contact Precision before flying it again.
 
How many years has it been operating since new or overhaul?
Precision recommends 12 years calendar. Even if it isn’t the issue, not a bad idea to get it to Don for service if it has timed out.
 
System components were rebuilt by manufacturer in 2020 prior to first flight. What I was discussing with Don was a sort of hysteresis in the control response where the fuel flow would move not at all or very slightly in the opposite direction of mixture lever movement when trying to dial in a specific FF for charting GAMI spread while balancing of injectors was being done, or the newly selected FF value that appeared stable for 10 seconds or so would change by perhaps two or three tenths of a GPH after I logged the FF and EGT data and looked back at the EFIS. He said he had seen this before most commonly in quadrant installations and did not think it a cause for concern.

I'll dig deeper and inspect the spline engagement, etc and report back. It's a bit annoying and has now caught my attention as something not to just live with.
 
Do you have a quadrant? I have to hold one or the other, mixture or throttle, as they will slightly move each other depending on how the friction lock is set. Makes sense.
 
That makes more sense. Sure, lots of installs show wobbly fuel flow indications...nothing unusual about a few tenths. The "9.5 became 2.8" is cause for worry.
 
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