Prop bold torque

July 1993

Not again, you say? We are all well aware that there have been many admonishments in the CP over the years regarding the critical importance of correctly torquing your prop bolts, and of doing this often, and at regular intervals. Recently a friend borrowed a wood prop from an EZ builder in a state where the moisture is much higher than it is here in Mojave. This prop was installed, the bolts, were torqued to the recommended value, the bolt heads were safety wired. After only about 30 hours of flight here in the dry desert air of Mojave, this friend discovered that the safety wire had broken and that all six prop bolts were loose enough to be able to turn the washers under the bolt heads with his fingers! What happened? This prop lived for a couple of years in a damp climate.

The wood absorbed some of this moisture and swelled a little. After a few weeks in the dry climate of Mojave, the wood lost most of this excess moisture and shrank. The bolts were no longer squeezing the prop between the crush plate and the prop extension flange. The prop began to move just a little, causing the face of the prop to char slightly. The bolts began to unscrew themselves and it literally would not have flown more than a few more minutes before this prop would have come off the airplane. Wood props, used correctly and prop erly maintained, are very safe and have an excellent safety record over many years. However, the torque on the prop bolts must be checked regularly. If you have a new prop from a wetter climate than where you live, check the torque every 10 hours for the first 100 hours. Once the prop settles down, you can extend these checks to every 25 hours. Do not omit this simple safety check. It could be extremely costly if you do.

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