Re: Landing gear bolt broke
Posted: Sat Mar 04, 2023 2:01 am
We have had some experience with broken upper gear leg bolts up here as well. When this happened the first times, we stuck to the SB and replaced the stainless bolts with AN types. In addition, we safetied the bolts in such a way that the lower part of a broken bolt could not fall out (for some reason the bolts always seem to break in the middle). This would bar the titanium leg from rotating and cause a calamity.
However, the problem persisted as discovered during periodic maintenance. It appeared that the suggested 1/4 inch bolts just could not handle the torque loads imposed on the gear legs by the wheel geometry. What we then did was to ream out the upper holes in gear leg and engine mount to 5/16 inch (8mm), and replace the AN4 with Unbrako 12.9 class bolts and long nuts. The whole assembly was torqued up to a considerable pre load on the bolts (12.9 class bolts are extrmely strong!)
After this, the issue went away, and we have been flying happily the last years. Remains the question why the original bolts would break in the first place. We can only speculate that for some reason we could not get the wheel geometry aligned exactly to plans, and that his imposed excessive loads on the bolts.
However, the problem persisted as discovered during periodic maintenance. It appeared that the suggested 1/4 inch bolts just could not handle the torque loads imposed on the gear legs by the wheel geometry. What we then did was to ream out the upper holes in gear leg and engine mount to 5/16 inch (8mm), and replace the AN4 with Unbrako 12.9 class bolts and long nuts. The whole assembly was torqued up to a considerable pre load on the bolts (12.9 class bolts are extrmely strong!)
After this, the issue went away, and we have been flying happily the last years. Remains the question why the original bolts would break in the first place. We can only speculate that for some reason we could not get the wheel geometry aligned exactly to plans, and that his imposed excessive loads on the bolts.