Main wing spar Issue 3 - probable overtightening of the AN3 bolts

Removing the spar caps involves removing AN3 bolts. Out of curiosity I used the torque wrench to assess how tightly they were done up in the first place. I successively raised the wrench setting until the nuts visibly shifted tighter. The in-lbs figures were very high, ranging from 80-120 in-lbs, well above the specified maximum of 40 in-lbs. I contacted TAF about this, they stated this method does not give a valid estimate of the actual torque applied at manufacture. This could be correct as other mechanisms could come into play, such a cold-welding, which could create an impression of greater torque when trying to crack tighter after a number of years.

TAF recommends torqueing of these bolts to 37in-lb, including the nylock torque.

Despite these torque values - taken at face value - being up at the levels where AN3 bolts (threads in particular) might fail, I noticed no evidence whatever of thread stripping or distortion.

TAF have stated these bolts are employed primarily in tension.

In any case I decided to retire all these AN3-12A bolts. I noted that the existing bolts have no washer under the head - not essential but possibly desirable given the tension role. I also noticed that the grip of the bolt does not reach fully though the spar layer stack, falling short by about 1mm at the last layer. Given the tension role, TAF have stated this is not an issue. However both issues can be solved by replacing with a slightly longer AN3-13A bolt, combining with one 1/16" AN washer under the head and two 1/16" AN washers under the nut. This setup is consistent with AC43.13-2B, which limits the number of stacked washers to two (it is worded a bit vaguely). This configuration will result in the nut having exactly the same position with respect to the thread as before.

I noticed some very slight lateral "slop" in the AN3 bolts. The holes appear to be chuck reamed to #12 (4.80mm = 0.189"). The grip range for AN3 is 0.186"-0.189". Maybe some of the bolts have a few extra thou of clearance due to bolt tolerances alone. I contacted TAF recently and they have stated these holes (in the spar caps at least) are fabricated to 4.9mm, even less tight. And given they work in tension a very tight fit is not so important. I considered upgrading to AN173 close tolerance bolts, but for the above reasons have decided against it.