The analysis - so far!
The analysis - so far!
We were waiting & waiting - all late afternoon & evening for the winds to abate ..... they were supposed to be ~6kt @ 7pm .... still 9-10 gusts @ 14 & a tad cross I did several runs .... solid balancing runs.
The windsock was finally drooping enough - well under 10kts & I pre-rotated with the plan to do one of the low full powered passes - just as the nosewheel lifted I heard SNAP CRACK BANG & instantly pulled throttle & leveled the rotor ....... sat there stunned disbelief while the rotor spooled down... thinking WTBH did I leave laying around back there -/ what could possibly have come loose???? Desmon was waiting down midfield by the windsock & called to ask what was up ... I said I was taking it in to post flight - something had come loose!
When we looked it over - first prop blade 2 big gouges - edge split - a chip off the second & scuff marks on a third - of course - the one with the old chip wasn't hit!!!
Desmon spotted a deep bolt-head sized ding & gouged powder-coating on the rear keel tube ..... so we were looking for some big bolt that had departed the aircraft! I finally spotted a hole up on the rotorhead where there should have been a bolthead in the slider pivot block with a couple of big washers!
Then with the sun setting we go out on the taxiway to search for the missing pieces we found the broken bolt head,2 washers & one of two steel bushings - the 2 rubber washers were gone!
The aircraft spruce source AN6-40 bolt had sheared at the 1/3 mark from head -end ..... thankfully the other 2/3 stayed put or things could have got really ugly & I'd need new rotor & who knows what else!!
The central 1/3 of the bolt showed fresh metal fracture .... it is probable by the darker coloring around the outside that a partial shearing had happened earlier!
Now the questions????
possible causes ... flawed bolt??? undersized bolt for the job??? stresses from harmonic vibration in pre-rotator pulleys -other????
Future action .... grease & inspect or just replace every 20 hours (I have 38 hours - & LOTS of pre-rotation cycles with all the B-O-M work on the machine! ) Look into using a bigger bolt???? have to remachine the pivot block & torque tube!!! Close look at stressors & fix the high vibration PR system!
Before I went to bed I emailed the alert to Larry & every Monarch/Aurora owner I have emails for!
Hoping Larry will contact the others.
Took close up photos for Jeff T & he had lots of good ??'s I answered & we are sharing the analysis with Vance .... agreeing with Larry's idea it seems there was insufficient torque on the bolt ( even tho castle nutted & pinned) - as well as unique stressors associated with the aggressive MLS PR system - the Aurora uses ..... there are problem vibration areas ( belt & shaft pulleys) we need to machine & balance to run smoother to reduce the stresses on the system!