Friday, October 21, 2016

Service Bulletin SB 16-08-24 : Loose engine mount bracket rivets

Each one of the two upper engine mount brackets has 22 LP4-3 rivets attaching it to both the panel base and the upper firewall.  On two high-time RV-12s the aft-most six rivets on each bracket have been observed to be "smoking."  Two solid surfaces which are mechanically held in contact with each other and which are subjected to vibration can experience fretting corrosion.  Fretting corrosion
occurs when the two contacting surfaces experience relative motion, often of microscopic magnitude.  In the case of aluminum, the small-scale rubbing between mating surfaces produces fine aluminum power which quickly oxidizes, causing a smokey appearance around the rivet head.

Van's fix is to replace the offending LP4-3s with much stronger Cherry rivets.  This, of course, required drilling out the 12 LP4s in the engine mounts.

Some time back, Van's switched from the Garmin SL-40 com radio with separate intercom to the Garmin GTR-200 com radio with built in intercom.  Naturally, I had just installed the radio tray towers (the grey objects shown in the pictures) for the old radio when the announcement came out about the replacement, so those rivets had to come out as well (6 LP-4s total), so I did all the rivet removed at once.  The manufactured heads on the rivets were on the bottom of the of the panel base, so I originally started the process by awkwardly lying on my back in the fuselage drilling upward.  The drilling process in rivet removal is quite delicate, since enlargement of the #30 hole is to be avoided.  A clear view of the rivet head and a steady hand are required, both difficult to achieve in that position.  It occurred to me, somewhat belatedly, that this process would be much easier with the fuselage on its side, allowing me to sit upright to do the drilling.  Once again, I'm reminded of what a great decision it was to put off installing the tail cone as long as possible.  Without the tail cone, I have great access to the area between the aft bulkhead and the seats.  There's still lots of stuff to do there.

As much as I'd like to install the landing gear, I'm resisting that urge as well.  With the gear on, even without the tail cone, it'll seem much more like a real airplane.  I would no doubt be unable to resist the urge to sit in the cockpit and make airplane noises.  The downside, of course, is a lack of options for positioning the fuselage on its side while installing the many systems still to go in the fuselage.

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