Kitfox service advisory 12 - prop bolts.

KITFOX SERVICE LETTER # 12

SUBJECT:    1/4” PROPELLER BOLTS

TO:   ALL KITFOX OWNERS

FROM:   DENNEY AEROCRAFT COMPANY

3 1/4” X 1/4” PROPELLER BOLTS: Because of a recent failure of the 1/4” prop bolts, Denney Aerocraft strongly recommends replacing the 6 center 1/4” prop bolts with 8 mm x 3 1/2” bolts. On a recent flight of a KITFOX, all 6 of these 1/4” bolts sheared off in flight and the prop departed the aircraft. The pilot was able to land with out incidence on a football field in L.A. and the prop was recovered with damage only to the spinner. Exact reason why the 1/4” bolts failed is undetermined. However, one possible reason is due to the extreme humidity changes in that particular area. The wood may have swelled and shrunk again leaving the bolts in need of re-torquing. This is the only reported incidence of failure we have received. After over 1000 hours on our prototypes, we have experienced absolutely no problems with the 1/4” bolts. The engine importer claims no other cases of bolt failure with the 1/4” bolts, either.

We now have 8 mm x 3 1/2” bolts to replace the six 3 1/4” x 1/4” bolts. These bolts will be long enough to accept elastic stop nuts behind the prop flange so safety wiring the heads should not be necessary. The prop flange on the gear box is already drilled and tapped for 8 mm but you will have to drill out the 1/4” prop holes using a letter 0 drill bit (a 5/16” drill bit is a little small). The bolts should be torqued to 15 to 18 foot pounds.

The cost of these bolts with nuts and shipping will be $12.00 per set.

In the event that you do not replace the 1/4” prop mounting bolts, re-torque the bolts to 10 foot pounds every 10 hours or oftener if your area has significant humidity changes.