spar tip repair

[Dan Rhinehart]

Hi all,

I have a spar tip in need of a new section of the spar cap replaced (about 18"). I looked up the procedure in the FAA AC 43.13, and it only seemed to cover splicing a section of a built up spar, not the procedure of replacing a section of spar cap.

Any ideas???

[Paul Rule]

Dan,

Use the Culver "General Repair Instructions" from the parts book.  These instructions can be considered approved data.

Par. #5 says: "... if the (damage requires) greater then aproximately 20-24 inches it is advisable to replace a section of the spar cap..."  This clearly allows a scarf repair of a cap section anywhere on the spar.

Although this paragraph is addressing inboard damage it is conservative to apply it to outboard sections as the loading decreases as you go outboard.   The essential thing (in my opinion) is that you do a 20:1 scarf as specified. 

The spar repairs in 43.13 allow a 10:1  scarf with reinforcing plates because the (CAA at that time) believed that a 20:1 was too difficult for the average mechanic to do in the field.  The Culver repair data pre-dates the 43.13. 

Both Piper and Aeronca were approved to make 20:1 scarf splices ANYWHERE in their wood spars and did so on a routine basis.  Aeronca also glued smaller boards together both horizinally and vertically the length of the spar (effectively a laminated spar) to build up spars.  I have seen several of these and recently replaced an Aeronca Chief spar that was made up of 7 pieces of wood.

From this it would appear  20:1 scarf without reinforcing apparently generates 100% (or more?) of the required strength.