COURTESY ATSB
input had broken. The assembly uses a series of flexible plates at either end to accommodate relative movement between the engine and transmission in flight and includes a fail-safe feature that allows up to 20 minutes of continued operation after the fracture of a single plate. In this case, though, “the flex plates from the drive- shaft had fractured into multiple segments.” One of the bolt holes in the outer flex plate at the gearbox end had broken due to what materials examination showed to be a fatigue crack that had propagated through about 90% of the flex plate’s width (see photos above). Scoring on the section of the rear transmission-mount support assembly under the driveshaft matched discoloration on the failed outer flex plate.
This pilot dumped lift at 10 ft. and was able
to walk away. A free fall from 30 ft. wouldn’t have been nearly as benign. A shrewd decision made without delay was the
difference between life-threatening injuries and an exciting hangar story.
The investigators also found that five washers were missing from four different fasteners connecting the flex plates, allowing the flex plates to move relative to one another. In each case, witness marks indicated that the washers had apparently been present at one time, with the ATSB unable to explain their absence. The driveshaft assembly isn’t field serviceable; its
maker, Kamatics Corp., stresses in its maintenance and repair manual that “evidence of turning fasteners by wrench or other means” renders the entire assembly unairworthy. Any loose or missing hardware, including evidence of slippage on the torque stripes, is cause for immediate replacement “with a serviceable unit.” Both in- and out-of-aircraft inspections are purely visual, looking for evidence of fretting, corrosion, or abra- sion from contact with other assemblies as well as loose, broken, or slipping fasteners. Moreover, the main- tenance shop that had removed and reinstalled the
82 ROTOR MARCH 2021
driveshaft while performing an engine change 74.5 flight hours earlier provided investigators with quality-control photographs showing that, at that time, all fasteners visi- ble in the photographs were assembled correctly. Kamatics advised the
The outer flex plate at the gearbox end fractured through the bolt hole (left), leaving a small section remaining (right).
ATSB that the failed driveshaft had originally been sold to the US Army in 1980. At that time, the UH-1H drive- shaft wasn’t subject to any service life or time-before-overhaul (TBO) limitations; replacement was done
strictly on condition. This was not the case with any of the company’s driveshafts for other models. In 2018, Kamatics sent the FAA recommendations that included replacing UH-1H driveshafts after 5,000 hours of operation or, “if the time cannot be determined from historical records,” bringing those recommenda- tions in line with those for Kamatics’s other helicopter driveshafts. That recommendation had no legal authority at the time of the accident, however, and the time in ser- vice of VH-OXI’s driveshaft had not been tracked because doing so hadn’t been required.
The Takeaway
In-flight emergencies requiring immediate response aren’t unique to rotorcraft. The NTSB’s analysis of a hor- rific 2014 King Air accident found that just 20 seconds elapsed between the first indications of a loss of thrust in the left engine and the airplane’s inverted crash into the roof of FlightSafety International’s training facility at Wichita (Kansas) Mid-Continent Airport, now known as Wichita Dwight D. Eisenhower National Airport (KICT). The pilot likely had less than 10 seconds in which to react—a time frame that might seem downright luxuri- ous to a helicopter pilot recognizing signs of impending mechanical failure while operating at low altitude and air- speed over obstructed terrain. How quickly to put the ship down may depend in part
on how rapidly those symptoms are intensifying, but it’s rare that sooner isn’t better than later. And if things do fall apart, the altitude at which that happens matters. This pilot dumped lift at 10 ft. and was able to walk away. A free fall from 30 ft. wouldn’t have been nearly as benign; from 50 or 100, probably unsurvivable. A shrewd deci- sion made without delay was the difference between life-threatening injuries and an exciting hangar story that offers words to the wise.
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