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Date: Mon, 07 Dec 2009 11:51:38 -0500
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Subject: Re: fluctuations in static system
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I experienced something similar to this in my 235 years ago.  The static 
system did not include a "loop" over the top of the cabin to force any 
liquid in the line to drain back out the port, and there was a small 
static leak causing the tube to draw air into the cabin (in flight the 
cabin tends to have lower pressure than the static port position).  The 
result was that when flying through rain water would be "sucked into the 
line," and would settle in the lowest spot causing "burping" (large 
momentary transients).  Alternate static always immediately corrected 
the problem.

This scenario was more extreme than you report, but is an example of how 
ANY problem in the static system can cause dangerous errors.

Regards,

Bill R

Keith Smith wrote:
> Hello again,
>
> Watching the videos from previous flights, I notice the VSI/ASI and
> altimeter 'bouncing' around (in the case of the altimeter +/- 10-15ft)
> after power is applied during the takeoff run.
>
> I haven't yet established more a pattern than that (ie. any trends
> related to engine power or speed once airborne), but the problem is
> more or less consistent. I haven't tried engaging the alt static port
> in flight, but will do that on tomorrow's test flight before the plane
> goes to the shop.
>
> Has anyone encountered anything like this before? I have seen cockpit
> videos of other 360's and haven't seen the issue, so it's certainly
> not a problem native to the design of the aircraft.
>
> I'm not sure if there is a burble around the static port due to some
> strange airflow around the ports, or if there's a small blockage
> within the static system.
>
> Any thoughts on likely causes? I'll report back after the alt static
> test tomorrow, and then, ultimately on the diagnosis from the shop
> once it's done.
>
> Thanks,
> Keith
>