Desert Center Emergency Landing - Part 3

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Cause:

The proximate cause of this incident was probably one of two things.

Bill Seibold thinks that there might have been a problem with the prop bolt torque.  Although I had checked them at the Condition Inspection in September, that's certainly a possibility.  The aft face of the prop extension and the extended portions of the bolt bushings show some fretting on their surface.  Whether that was due to long term rubbing from loose bolts, or short term rubbing as the prop was departing, I don't think I'll ever know.  The propeller is in a section of Joshua Tree National Park that's designated wilderness area and has no roads.  Although I could probably localize it to within a 1 square mile area, I'd be surprised if it's found within the next 100 years by hikers.  I won't get a chance to examine it.

Another possibility is a blade loss.  Although I've never heard of a Catto 3-blade prop failure, anything's possible.  Starting about 1/2 hour before the prop departure, we had been noticing an intermittent vibration.  It would come and go for a few seconds at a time every 5 minutes or so, and even Deanie noticed it.  All engine gauges read nominal, so I was thinking that one of the wheelpants was a bit loose and vibrating at certain airspeeds.  I played with the throttle and mixture when the vibration appeared and was always able to get it to go away.

A few minutes before the "BANG" (which I now think was the prop hitting the lower winglet, not the prop departing the engine), the vibration had returned, just as we passed over Twentynine Palms airport.  In retrospect, obviously the right thing to do would have been to land there and investigate.  We even discussed it, and since I had been able to get the vibration to go away by changing RPM (and therefore the forcing function) before, I told Deanie that we'd land in Blythe, about 15-20 minutes ahead.  Stupid decision making on my part - no question.  About 10 seconds before the "BANG", the vibration returned with a vengeance.  Not enough to shake the crap out of the plane or anything, but enough to know that something was really wrong.  I retarded the throttle to 1800 - 2000 RPM, played with the mixture, and advanced the throttle a bit, but couldn't affect the vibration. Then, "BANG".  Quiet and smooth as glass.

My theory is that there was a crack in a blade which was growing, bringing the resonant frequency of the prop in and out of the RPM operating range as it got bigger and the vibration mode frequencies changed.  One blade leaves, then 10 seconds later the hub gives up the ghost.

I THINK that I can actually trace the vibration back four weeks, to my flight to Marysville, CA for the altitude chamber class.  I noticed a slight vibration on the flight up there which lasted for a minute or so, but then went away.  I flew a couple of flights to Thermal, CA to help repair the Hoffmann's Long-EZ, a long flight to Bisbee, AZ for thanksgiving, to Watsonville, CA for a concert in Santa Cruz, and did not have any vibration on any of those flights.  On the short flights to John Wayne airport (KSNA) for a CATIA class last week, I did notice a couple of VERY short periods of vibration, but they went away quickly on their own. Or maybe Bill's right, and these symptoms could have been created by undertorqued bolts and fretting between the prop and the prop extension.

My big error in judgment was in not landing at Twentynine Palms, where there's an FBO, a town, and which we were right over when the problem manifested itself.  The decision to continue on and land at Blythe was stupid, and could have cost us far more than a lost day and an airplane parked in the middle of nowhere.  Deanie has tried to make me feel better about it by saying that we might have landed, found nothing, and then had the prop disappear as we were taking off, which would have been worse.  Maybe so, but far more likely is that by that time the problem was large enough so that I WOULD find something and ground the plane in civilization, with an intact wing and winglet (and prop extension and spinner).

But I'll never know.

To be continued in Part 4:


Cause - Addendum:

While my original theory was that a blade broke, I'm now 99.99% convinced that it was, as Bill Seibold surmised, a prop bolt torque issue. The vibration we felt was nowhere near enough to be that caused by a departed blade - even a small part of a departed blade.  Also, an examination of the prop extension shows severe fretting on the propeller contact surface - a sure sign of not enough pressure to maintain the friction force.

The fact that all six prop bolts had broken clean at the root of the thread also indicates fatigue and high bending stress levels. That, plus my poor technique at bolt torque checking, and my infrequent checks, lead me to be almost completely sure that the Catto prop is NOT to blame, and that _I_ am, for not having my propeller bolts torqued correctly.
 


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e-mail: marc_zeitlin@alum.mit.edu

Last updated: February 21, 2007