Why my new prop spinner cracked.

[GJ Humphrey]

I'm looking for a rear prop spinner, Cessna part number 1457306-5. New or excellent condition. Please respond to gjhumphrey@comcast.net.

Here's why my prop spinner, installed new five months ago, cracked.

Pardon the elementary explanation, but I'm not an A&P.

As can be seen from the exterior, the spinner is attached to the bulkhead by a multitude of screws.

When the spinner is attached, it is supported internally at about the mid-point between the base and the apex by a plastic disk, about 6.5 inches in outside diameter, and shaped something like a man's dress hat. In the parts manual is called "Support - spinner."

The open end of the hat snugly fits on the cyllindrical dome of the propeller. The "brim" of the hat, about .75" inch in thickness, with a flanged edge matching the taper of the spinner, snugly contacts the inner surface of the spinner, as said, about halfway between the base and apex..

As a means of ensuring a good fit, Cessna sells plastic spacers in the form of disks about 1/32" in thickness and precisely the diameter of the open end of the "hat." The disks are installed on the inside of the "hat", in other words they are located between the top of the prop dome and the inner surface of the "hat." Up to four disks may be used.

When the mechanic removed the cracked dome today, we found first of all that none of the screw holes was enlarged or distorted. Revealingly, we found that of the four spacers one was non-standard and undersize. The three were identical in diameter and thickness and fit precisely in the "hat." The fourth was about .5" smaller in outside diameter than the other three and almost paper-thin. Because it was smaller in diameter it was free to move around, indeed, we found a generous amount of white plastic powder indicating this disk was moving around, chafing and growing thinner and thinner.

What all this means is the spinner was not adequately braced by the spinner support and the inner surface of the spinner and the plastic spinner support were themselves chaffing, proof being a lot of black, oxidized aluminum dust between the support and the apex of the spinner. Further proof: the black powder had collected on about about 150 degrees of circumference. The remaining 210 degrees of surface was clean aluminum.

It's clear the mechanic who installed the new spinner only months ago, used the same non-standard spacer that had caused the previous cracking problem: inadequate internal bracing of the spinner.

Spinners are expensive. Further, I wonder what would happen if a rear spinner flew apart in flight. I'd rather not find out first hand.

Checking the proper bracing of a spinner, it turns out, is pretty simple and inexpensive in terms of a mechanic's time. Inadequate bracing means some degree of wobbling, perhaps focussing stress at vulnerable point. On my spinner the crack appeared on the radius cut-out that accommodates the prop root, just outboard (aft, in this case) of the bulkhead, right where the bulkhead no longer braces the base of the spinner.

One more lesson: I'm getting my props dynamically-balanced.

Hope this info helps someone save money and aggravation.

GJ Humphrey
Concord, NH
1973 T337, P-model, SN P3370063, N3CU