[hharney]

Does the trim tab play cause boom bounce? Some have indicated that it does. I have experienced this phenomena in the years past and really wasn't sure I was really feeling the boom bounce back then. I remember at least 5 or 6 years ago that I had these symptoms and could actually look back and see the boom moving. Then I saw others talk and post messages about it. So it's a real thing? We talked about it in Dayton and several other members said that they had experienced it. Loose cable tension, elevator play, trim tab play and other theories were circulated around the room. Some thought that the adjustments they did worked and others were still skeptical. Here is my dilemma.

I had not felt or seen boom bounce for 6 or so years. Then just 2 weeks ago it was rearing it's ugly head on a flight at 9,000 ft, cloudy, wet day in the air and relativity smooth air except in the clouds. I found that it was more pronounced than I remember. I could look back at the tail and see the booms moving up and down. I know what you are thinking, yes I have that DANGEROUSLY MODIFIED AIRCRAFT. Well, this boom bounce was around before I made the last modifications that caused the Cessna Company to write the SID's, or some think that this is the case. I could actually make it stop by moving the control yoke fore and aft. I could also change the RPM on the rear engine and cause it to change or stop.

Anyway, back to the point, we looked at the trim tab and it does indeed have some play. Maybe around 1/4 to 3/8 inch. Too much in my estimation. But the other attention getter was the play in the elevator. It was again like 1/4 to 3/8 inch of play when one would move the elevator without allowing the cables to move. We found the play in the rod end fork form the cable that attached to the bearing bracket on the elevator. The bolt was slightly worn and semi loose on the bearing. The book says to torque 10 to 40 which leaves a lot of area for question. We tightened the bolt on the bearing to the higher book value and the play was gone. All of it, no play at all. The next two flights, one 8 hours in the air and the other 5.5 hours in the air resulted in no boom bounce or elevator flutter. But this has happened before, as I said above it was a time of like 6 years between a noticeable incident. Well the trim tab still has the play and I thought it would be good to fix one area first to see if that stopped the problem or not but I don't really want to wait 6 years before I fix the trim tab. So maybe a few more flights with a good sense of detection and then fix the trim tab.

I thought I would just do some investigating on the position of the trim tab while in flight. I marked the trim wheel so that I know where the tab is perfect in line with the elevator. Interestingly enough the tab does not stay even with the elevator. If you already know this sorry to bore you with my findings. The photos below indicate the position of the trim tab when the aircraft is in perfect level flight with one and two aboard in the front seats. Some luggage in the rear and in the middle seats but no big load. I just find it interesting because I had never done this before and thought you might like to see this. It would be good to have others try this too and see if it is different with different models. I know that Cessna changed the size of the tab later in the G and H models.