lml@lancaironline.net Mailing List Archive

All the comments below are true, but I believe the root cause is design-related and there has been a lot of discussion about this on the the ES forum. The mount absolutely begs for a tube to connect the two front mounts. The force on these tends to spread the mounts roughly in line with the axis of the mount bolts. Besides adding a connection, a diagonal brace is also a very good thing to add. The other shortcoming is the stiffness (or lack thereof) in the nose gear mount area. Imagine holding both arms to the front and then twist both clockwise and then counterclockwise - this is the the loading on the nosegear mount plates that causes the problem. This is the probably the motion that cracked the tube in the photos. The solution is to weld a transverse beam (a flat plate will do) between the two plates. Lancair has recently modified the mount by adding such a flat plate - a good thing. But they have also added gussets where Michael's welder suggested - a completely inadequate cure, in my opinion. And, as many owners have found, it doesn't take a nose gear shimmy event to cause the failure.
Gary Casey

Michael,

I make a living making aerospace parts….Boeing is my largest customer….so I think my advice is sound.

4000 series steels like 4130, 4330 and 4340 are susceptible to cracking after weld if they are not preheated and post heated during the weld process. I do not know what material your engine mount is made of but the 4000 series steels (chrome moly) are quite popular for this kind of application. I would be very leery of welding or re-welding steel tube and not knowing what kind of steel it is. Additionally, the quality of weld on your engine mount is not very good. I would suggest replacing the mount unless you can confirm what material the mount was made from. If you choose to repair the mount, Lancair should have specified the material and the fab house should have material certs from the original build. Whether or not that information is still available….I don’t know. Finding a good welder at an airport may be challenging. Another thought…losing the nose gear due to mount failure would be very expensive.

Craig Berland

IV-P N7VG

<<The two attached images show a crack in the engine mount on my Lancair IV-P. #4637 shows context and #4641 details. The opposite side cracked in a similar fashion two years ago. (I have not had nose wheel shimmy.)

I would like advice on appropriate repair. My mechanic is suggesting some form of gusset which could improve the overall strength and prevent a future occurrence.

I wonder if this might not be asking for trouble.