Jay Honeck wrote:
snip
Immediately after this was done, I noticed our suction was lower than
before. It would top out in the 4.8 region -- at the very low end of the
acceptable scale.
At the same time we also noticed that our newly overhauled DG was precessing
excessively. It would precess 10 degrees every 15 minutes or so. Sometimes
less.
Jay,
Generally speaking, 4" of vacuum is sufficient to drive modern gyros,
and a properly functioning DG shouldn't precess any more than 5 degrees
every 15 minutes (which is a high rate of precession, but that's
apparently the number manufacturers use to determine warranty coverage).
My experience with two (properly functioning) Sigma Tek DG's is far
better -- around 3 degrees every 30 minutes, assuming we're not doing
pylon 8's.
Before we pulled and returned the DG, I wanted to make sure that the lower
suction wasn't part of the problem, so today I had my A&P adjust the vacuum
regulator. It's now reading 5 at 2000 rpm, and almost 5.2 (the high end of
acceptable) at full throttle in a ground run. We left it at the high
setting to see where it settles out after a flight or two.
I have noticed that even with ALL NEW parts in our vacuum system, before
the first flight of the day vacuum starts out at the top of the green,
particularly if it's cold outside ( 50 degrees). It then routinely
settles into the middle of the green for the remainder of the flight.
Don't ask me why that happens, but it does, and since everything is new
and otherwise works fine, I'd consider that normal.
I would suggest you make sure that your regulator is functioning
properly. Look very closely at the vacuum gauge while you increase RPM
slowly from idle / 1000 RPM. Vacuum should rise relatively quickly
until 1500-1800 RPM and then STOP, as if it hit a "peg".
If it rises further with RPM, even a few tenths, the regulator is worn
out and needs to be replaced. PMA'd examples are about $250. Don't go
back to Piper...they likely stock the OEM Airborne, and that lists for
about $950.
Question: Will this increase in vacuum help the DG's precession problem, or
is the DG toast?
It's likely toast. It does happen...and it's not necessarily the
manufacturer's fault. If the DG is dropped in shipment or (worse)
dropped on the bench (even from a few inches), it can display the
symptoms you're reporting. IMO, gyro shipping boxes should display a
warning label like "Radioactive material in glass shipping
container...Handle with care". Maybe then they'd get the respect they
deserve.
One other item...have you swung the compass lately? Before you pull the
DG, do that (and not at the airpot rose...have your avionics shop use
their calibrated compass). That will reveal whether some or all of your
"precession" is actually "caused" by an inaccurate compass. I say this
because that's the first thing Sigma Tek asked my avionics tech when we
requested warranty service. It's apparently a common problem.
For the benefit of those who haven't seen it, check out the two-part
article I wrote about the fun we had with our 172's vacuum system.
Click through Aviation-Articles-Maintenance-50 Hour Inspection
-Doug
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Doug Vetter, CFIMEIA
http://www.dvcfi.com
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