I thought I had chased down everything that could be coming in contact with the cowling, but I still had a low frequency vibration that was in the airframe and not in the stick.

 

In a discussion with Tom Giddings, he asked, "When did you last check the engine mount to firewall bolt torques?"  "Ah, probably two years ago," I answered.  Then I used some lazy man's excuse like "Well, I probably would have checked them next month during the annual condition inspection." I added, "The lower cowl is such a nuisance to remove."

 

Well, I did pull the lower cowl and used 5 drive extensions to get a torque wrench socket thru the foot well to the aft-side firewall nuts only to find that 5 of the 6 required 2-4 faces turned before the proper torque was restored (all three on the left side, the middle and bottom on the right). 

 

After re-assembly, I went for a test flight.  I could tell that things had improved just while taxiing out.  One trip around the pattern demonstrated that the low frequency frame vibration was gone (almost reduced to nothing).

 

Remember that epoxy/glass/carbon can creep a bit under pressure and heat, and it is wise to check the torque of these bolts periodically.  Uh, especially if vibration is beginning to felt where operations were once smooth.  This is relevant with any Lancair that has a composite firewall (wood/glass, all glass, all carbon).

 

My 320 mount uses AN bolts that are 7/16" x 24 with a torque range of 450 to 500 inch pounds (37.5 to 42 foot pounds).  I used 40 foot pounds.

 

Thanks again Tom,

 

Scott Krueger AKA Grayhawk