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.

 

 

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