During the month of October there were two cog belt breaks with the AP cog belt system. The first one occured in the State of Main on a ship with quite a few hours on the belt. This system was purchased directly from AP and installed and inspected by perhaps the most experienced installer of that system, so it was installed correctly. The second belt break occured one week later and was purchased from the RW factory as an upgrade to the Pro-Drive system that was on the ship. This belt was also installed by a highly experienced builders helper prior to the ships first flight.

The owner/pilot told us that one week prior to the break that occurred while Orv Neisingh was instructing, his own cog belt broke while in hover flight. The landing gear was slightly damaged, and the tail rotor contacted the gound during the unexpected set down.

We now have two documented AP design cog belt system breaks in one month. The belt manufacturer stated that the belts broke from the cords being bent beyond their minimum allowed radius between the cogs. This can happen if the belt is run too loose which allows the cogs to ride up onto the driving cog sprocket causing a hyper extention of the structural fibers leading to their failure. This may happen when the engine is being started. The tensioner has no oil pressure so it does not provide tension while the engine is hammering the drive train during the starting procedure.

The first cog belt break was in Main and was a Kevlar cord (black mold release) belt, the second one occurred in Missouri and was a Carbon fiber cord (blue mold release) belt. We have spoken with both pilots with the understanding that their names be kept confidential. The manufacturer of the system is aware of both breakages.

VP is now in the process of researching and testing a new and much stronger cog belt along with a new belt tension system to eliminate the belt experiencing high loads during engine start before the engine oil pressure builds to where it is able to properly apply the tension forces to the tension wheel. It appears that the existing system may have a defect in the design and if it does, we need to determine how to eleiminate that defect.

The Photo below is of one of the belts that broke in flight. Stay tuned as we are working on a solution to this issue.