Folks, My 1975 Sierra went into its annual last month
and one of the items that needed to be looked at was
the sudden fluctuation of the RPM in flight. I have a
Horizon 1000 and as you all know that gauge is
accurate down to 1 RPM. The gauge indicated that the
engine "lost" up to 25 REVS, initially a few times in
a flight, but it started to get more frequent in the
last months before the annual was due. This Rev drop
also manifested inself with a slight "shudder" ie a
vibration that was never there before. So at the same
time that the Revs dropped, the aircraft ran with a
slight vibration.
Into the annual, a few weeks later and a few thousand
dollars lighter we went flying again and the issue was
still there. It was suggested that maybe the mags (
close to overhaul at 500 hrs) were the culprit, or
maybe the prop was slightly out ( I have a 3 blade) of
balance. So I got the prop balanced and they told me
that the balance was extremely good, they made it
"perfect" and I saw the charts, almost zero vibration
induced by the prop. The balancing activity also shows
other potential issues such as bad mags, bad spark
plugs etc because they effect the "harmonics" of the
engine. I got told that the engine was in SUPER
condition ( which also was confirmed by the annual
with compressions in the mid 70' for all 4 jugs. So
what next? I went flying again after the balancing and
the aircraft still did the same thing, and now it was
losing up to 50 Revs several times in a row, sometimes
one event every 3-5 minutes and it shook some more.
After some serious discussions with the mechanics we
felt that, because the engine seemed not to "hold" a
constant speed, there could well be something wrong
with the governor.
So, off the governor came and it was overhauled. Seems
the counter weights were worn out, one of the shafts
needed to be opdated ( replaced with a later part
number). I got the governor installed again yesterday
and I went flying today.
Well, good news, the Rev drops are gone, the
engine/aircraft runs as smooth as it can be and it
runs exactely up to 2700 RMP redline.
Note. The governor had never been overhauled. My
aircraft has 1950 TT hours and about 350 Hrs SMOH.
Just something to keep in mind when you have this
issue or when the engine is due of an overhaul. When
something changes in the "feeling" or behavior" of an
aircraft, find out what is happening... generally
there is something happening that you may want to know
Now, I have only one thing left ( for the time being),
my oil temp gauge reads 2 needle widths higher than
before all of this started to happening. First things
first , of course the Vernatherm will come out and
will be checked for acurate temp sensing. The only
thing that changed was the fact that , during the
annual, the oil cooler was flushed and that should
have a good influence on the oil temps ( if there was
no vernatherm to regulate the temp ) ie better
More to come.

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