MT Prop Nut Caution!

[Andy Young]

As those of you with MT props likely know, the low-pitch blade angle on these is set by adjusting a large (1-1/4" wrench size) nut at the front of the prop, under the spinner. The only thing that keeps this nut from moving on it's own is a jam nut of the same size that it tightened up against it. Once these are tight to each other, they are safety-wired, but only to each other; these critical nuts are NOT in any way safetied to anything else. This makes it extremely important that these nuts are adequately jammed against each other. Complicating this is the fact that the nuts are a bit thinner than a typical wrench of this size, making it hard to get good torque of one against the other. A friend of mine recently experienced the prop nuts on his coming loose and spinning off completely (they ended up in the spinner). This allowed the prop to go to an extremely flat pitch. There is a service bulletin that says these nuts should be torqued against each other at 74 ft. lbs. Getting that much torque on it is most easily done by grinding down an open-end wrench so it will grab the inner nut without binding on the outer nut, then using a socket and torque wrench on the outer nut.

If you or someone else has been playing around with the low-pitch stop adjustment on your MT, you might want to do whatever you need to to be sure yours is torqued properly. This might be as simple as asking your mechanic how he did it, or it might require pulling the spinner and re-checking it.

Just a heads-up.

[bobguhr]

Thanks for the heads up Andy, I let loose with some AMU's and was told to expect an 84" MT on my door step begining of June, I'll make certain the jam nuts are set properly.