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First Tail Rotor Failure


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#1 UNDCFII

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Posted 06 January 2003 - 12:32

Well, like most things in helicopter flying, the outcome depends alot on timing.  Story follows: Hopped in one of our H300C's after a nice long X-mas break.... I'm refreshed but it has been two weeks since I've touched the controls.  Next to me is a high time fixed-wing instructor we are training in to add to our helicopter instructor staff.  This is somewhere around his sixth to seventh lesson.  During run-up, about the time we are doing a mag check, I notice a very small, hardly noticeable intermitent high-frequency vibration in the airframe, but almost imperceivable over the normal vibrations of the machines we love so dearly.  Additionally this ship has one of those dream T&B's on it right now, where it is running super smooth....that is probably the only reason I felt anything.  It sets off one of those voices in the back of the head..."hmmmm, I wonder where that is coming from." We are on the ramp, which is covered by half cleared snow and ice that has been compacted.  It seems that we are just stting on some of it funny and I mention that to the student.  My instincts tell me to keep an eye on it though.  So we are ready to go, and thankfully he is in no hurry to pick it up. I am in my "instructor defensive position", especially since he is a low time hoverer and hasn't quite found the hover button yet.  We had maybe 15" MP in and just barely light on the skids when I noticed the student was starting to input alot of pedal, about then the aircraft does a right face about 90 degrees in the time it takes me to take control, push the collective back down maybe three inches, and roll off the throttle.  "Huh.....that's wierd".  So I look back and the tail rotor does about two revolutions and stops, but the engine and main rotor are happily spinning at idle.  Well, that's no good.....I don't think we can MEL that :) So A quick shutdown to keep from messing anything up further.

Some things to note, both of us (A&P) preflighted the main rotor and tail rotor, and noted no problems.  Before I ever let a student pick it up we always clear the area and I look back at the tail rotor to see if it is clear, spinning, and to check for any oscilations in the tailboom.  The little prop was spinning just fine.....then!!  Anyway, I'm kicking myself for not trusting my instincts on that tiny, intermitent vibration, but was happy with my instinctive corrective action.  Also, this emphasized the importance of picking the aircraft up smoothly.  I'd much rather find out about a control problem while I'm on the ground not to meniton dynamic rollover.  Last note, the reason I say timing is important....that aircraft had flown 3 times that day, and the previous flight had just got back 20 minutes before.  I can't think of a better time for a tail rotor to fail, than on the ground during pick-up.  For those wondering, one of the end caps on the T/R driveshaft had a hole about 1/2" in diameter drilled into it by the T/R gearbox spline retaining nut.  This allowed the driveshaft splines to disengage from the gearbox.  We are not sure why yet, but it looks like improper installation.
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#2 Sonic04GT

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Posted 12 February 2013 - 15:02

Interesting story and nice reaction!

#3 apiaguy

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Posted 12 February 2013 - 17:54

trying to understand the op... Was it an older style T/R driveshaft with grease fittings and one of the grease fittings came out? 1/2 in. hole in the end of the D/S?? How exactly did that allow it to disengage? The driveshaft is squished between the driving spline on the input pinion and the driven spline of the T/R gearbox... so unless one of the splines stripped (aka service bulletin B-299.1) or the safety wire and driveshaft fore or aft nut backed off I'm having a hard time understanding exactly what happened.

#4 SBuzzkill

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Posted 12 February 2013 - 18:52

Sounds like the splines stripped.

We had some slight high freq vibes on the OH-58 not too long ago. Found some spread disc packs but that was it.

Great writeup thanks for sharing.

#5 Wally

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Posted 13 February 2013 - 10:20

"Also, this emphasized the importance of picking the aircraft up smoothly."
Most excellent.

Just a pilot (retired, so I have a LOT of time)...


#6 Gomer Pylot

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Posted 14 February 2013 - 12:38

Yeah, get in the habit of jerking the aircraft off the ground, and you will regret it, sooner or later.
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Best Regards,

Gomer




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