By Gary Wolf
President, RAA Canada
Photos by Gary Walsh

In late June of 2004, RAA (Canada’s Recreational Air-craft Association) member Gary Walsh experienced a thrown prop blade and the near departure of his engine from the mount of his 912S-powered Kitfox amphibian. The prop was a 3 blade NSI CAP 140. CAP stands for “Cockpit Adjustable Propeller”. The prop had only 200 hours on it since new, and one of the blades chose this time to leave the hub. Gary remembers flicking off the ignition, but the vibration was so intense that both carbs came out of their spigots and may have been the real reason the engine shut down so quickly. Fortunately all of the hoses and wires and a few engine mount tubes retained the engine within the cowling, so the C of G was not affected when the engine mount failed.  

Gary had previously owned a Cessna Aerobat and had frequently practiced dead stick landings, so he managed to get the Kitfox safely on the ground. After landing he got out to survey the damage, and found that the departed blade was embedded in the top of his float, taking out the nose gear retract linkage in the process. The blade was intact except for the lower part of its retaining cuff, which was still secured to the hub.

This photo shows the machined undercut groove in the corner of the sleeve. (Refer to the arrow). The fracture appears to have originated in this groove.
We inspected the broken blade and saw that there was a beach mark, typical of a fracture that had been waiting to let go. Gary contacted Lance Wheeler, owner of NSI, to

Looking closely at this photo, you’ll see one of the three blades missing from the NSI hub and firmly planted into the top of the Kitfox’s port float.

find out if this sort of event had happened before. Lance said than this was the first failure where there was no previous damage history, such as a prop strike. Even though Lance felt the cause was torsional vibration caused by the higher compression of the 912S engine, Lance promised that he would take care of Gary’s expenses in terms of the prop; Lance has never promised to address any other damage.

A bit of history - Gary had bought his Kitfox with this CAP 140 propeller two years ago and it had not come with a prop manual so he ordered one. A few months before this prop threw a blade, Gary had noticed some play at the tips of the prop blade and called NSI for advice. The prop hub was also throwing grease. Lance said that he should not idle the engine below 2500 rpm and that the play was normal. Centrifugal force would eliminate this if the prop were not idled below 2500 engine rpm. 

Back to July - Gary removed the engine and took it to Tri-City Aero for an inspection of the gearbox. He also made photographs of the hub and blades and then took the damaged parts to Cambridge Materials Testing, a local test lab that spends its days determining why bridges fall down and auto engine blocks crack. This lab inspected the parts and found that the crack had been growing for quite awhile. They issued a report that pointed at the sharp inside corner radius of the cuff as the culprit. At Lance’s request Gary shipped the entire propeller with hub to NSI in Arlington, WA, USA. Gary also met Lance at Oshkosh, and delivered a copy of the lab report. Gary then began the wait.  

At first it was Oshkosh that prevented Lance from responding. Next, Lance had a medical condition that had to be taken care of. Gary meanwhile retrieved his 912S and bought a new motor mount. He found someone to talk to about the eventual repair of the damaged fiberglass float. There were many calls to NSI but none were