Here is a head's up for owners of older Lancair IVs.
My original kit is circa 1992 and came with the now-obsolete
MAC 6A servo for the elevator trim tab. Returning from a
big military-civil air show in Melbourne, Australia, we experienced
an interesting incident that taught several lessons. The guy in
the right seat was flying the airplane while we descended below an
overcast, manuvering between rain showers toward our destination airport
in the late morning. We were indicating about 180 KIAS with a few
bumps when we got severely hammered with a violent down
draft.
Reconstructing the event it appears the following
occurred:
1) The co-pilot's arm was raised off the arm rest. He hung onto the
stick, but his hand slid up far enough to knock off the round pad at the
top containing all the trim tab buttons.
2) Immediately following was a savage up push, probably arising
when we exited the down draft. The co-pilot's arm was pulled
down, but was moved off the arm rest and so continued downward while he
was still holding he stick. Result: a big pull up.
3) Instinctive response when the nose went way up: he pushed down
hard and we went negative G and everything hit the roof.
4) He handed the airplane over to me and I locked the stick in
position to re-establish more or less horizontal flight and stop what
was becoming a pilot induced oscillation.
5) Event over. Elapsed time about two to three seconds after
which the guy in the back seat complained about something behind his
back. Meanwhile holding horizontal flight required a large
stick force nose down. The elevator trim tab was
non-responsive.
I slowed the airplane way down, put out gear and flaps, and landed
in about 10 minutes holding the stick forward the entire time.
The trim tab is somewhat larger than stock for the earlier models
because of reports of insufficient trim in the pattern. The larger
trim tab imposes a larger load on the servo.
Result:
1) The hammer used to drive in the tie-downs and a fire
extinguisher in the baggage compartment ended up in front of
the rear seat back and behind the back of the guy sitting there.
They had to migrate up and past several luggage bags to arrive at that
location.
2) We found the trim tab could be moved up and down a bit
when wiggled by hand. Clearly something in the servo was
wrong.
3) We pulled the servo and found that the force on the trim
tab during the upset had been enough to apply a compression
load that broke the back side (thrust bearing side) of the trim servo
out by breaking its containment grooves out as shown in the attached
photo. This allowed the final driven gear on the push shaft
to disengage from the gear train making it inoperative.
We were still four hours from home, and so jury rigged a fix with
cable ties and wire that worked for a while but ultimately slipped
allowing the servo to fail again during the next descent (when further
trimmed nose down). We finally locked the trim tab in place with
some carved wood behind the servo and a lot of tape on the trim tab
itself, made a couple of flights until we got it about right, and
flew it home with a gentle tug on the stick while we trucked along
staying below 170-180 KIAS.
I wrote to Gary Menzimer at Ray Allen Company and found I was not
the first to experience a servo failure in an older LIV during a severe
upset. Another pilot had the same thing happen when a side window
blew out the door on a P model, and probably pulled the pilot's arm out
(and back on the stick) during the decompression.
The fix:
There is a direct replacement Ray Allen 7A servo that has
been substantially modified to increase strength. Additionally, a
sister company, Bristol S. G. Inc. offers a newer, much stronger and
upgraded servo that is a near drop-in for the 6A-7A models dubbed
the B6 series. See
www.bristolsg.com One
can also purchase new clevis rod ends which are also significantly
stronger than the earlier units. The jack screw exits the back of
the box which may necessitate drilling a small hole in the elevator spar
web, but otherwise it is a near drop - in installation.
The Lessons:
1) The forecast included thunderstorms forming in the
afternoon. We descended below the overcast after seeing some
building cumulus but no thunderstorms during the late morning. We
thought we had beat the strong convective activity. Wrong.
Always be suspicious even when it is relatively smooth. An
isolated down burst can ruin your whole day with no warning.
2) Keep your arm solidly on the arm rest to avoid unintentional
stick movements.
3) Keep the baggage net spread over the baggage (non-P
models). We got complacent during the long trip (seven hours
going, seven and a half returning) and did not bother to pull it over
everything. Result: some potentially dangerous stuff migrated
forward of the rear seat back making for some uncomfortable seating back
there.
4) If you have an old MAC servo in your elevator, consider an
upgrade. The servos have moved forward two generations since the
early 1990's and are much stronger which is appropriate for these fast
airplanes.
Fred