Recommendation R20030211

Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: R20030211
Date issued: 06 November 2003
Safety action status:

History of the flight

The Robinson Helicopter Company model 22 helicopter (R22) registered VH-UXF was engaged in aerial mustering operations with another R22 helicopter registered VH-AOP. The pilot of UXF had returned from a refuelling stop and had recommenced operations in the mustering area. When no radio transmissions were heard from UXF for approximately 15 minutes, the pilot of AOP commenced a search for UXF. After a short search, the wreckage of UXF was located at the edge of a claypan. The helicopter had impacted the ground heavily in an upright attitude and both the occupants survived the impact with what appeared to be severe injuries. After making them comfortable the pilot of AOP departed to seek medical assistance. He returned approximately 80 minutes later with a doctor. The doctor reported that in the intervening period of time, both occupants of UXF had died as a result of their injuries.

Wreckage findings

On-site examination of the helicopter revealed that it had impacted heavily with little forward speed. Both main rotor blades exhibited evidence of low speed rotation at the time of impact and also exhibited minor damage from small tree strikes. The tail rotor exhibited evidence of high speed rotation at the time of impact and was destroyed by ground contact. The helicopter remained upright and onsite examination revealed a failure in the clutch shaft which on initial inspection appeared to be from impact damage. The helicopter was recovered to an engineering facility for a more detailed examination.

The examination at the engineering facility revealed what appeared to be a pre-impact failure of the clutch shaft. The shaft was taken to the ATSB laboratory in Canberra for further examination. The remainder of the examination of the recovered helicopter revealed that the engine was operating at high power when the helicopter impacted the ground. In addition, no pre-impact damage to other helicopter systems was found that would have contributed to the accident.

Failed Part examination

The clutch shaft had been installed in the helicopter on 30 October 2002 and had 886.2 hours time-in-service since new.

The clutch shaft fitted to UXF (P/N A166-1, S/N 5570) had fractured at the point of connection to the main rotor gearbox flex-plate yoke (P/N A907). The fracture displayed clear evidence of torsional fatigue cracking, following a spiral path from within the yoke connection that extended around the shaft for approximately 340 degrees over an axial length of around 25 millimetres. The area of final overload failure represented approximately thirty percent of the overall shaft cross-section and showed mostly ductile shear features.

Disassembly of the shaft-yoke connection found low levels of break-out torque required for both assembly bolts. The inner and outer bolts loosened at torque values of 7 and 4 foot-pounds respectively; markedly below the 20 foot pounds (240 in-lb's) tightening torque that the aircraft maintenance manual specified for the connection. Removal of the bolts and bearing blocks subsequently revealed that the blocks had been installed directly over the painted yoke surface. Inspections found no evidence of any attempt to remove the paint from the block seating locations prior to assembly.

Both connection bolt shanks showed fretting and other prominent evidence of loaded contact against the yoke and shaft hole sides. The orientation of the bolt damage indicated that the looseness of the connection had allowed the transmission of shaft torque loads via shear loading of the bolts. Extensive fretting and corrosion of the mating shaft and yoke surfaces, found after removing the fractured stub provided further evidence of looseness and movement within the connection. Both mating surfaces showed no evidence of the primer coating specified by the maintenance manual. An oily residue remained in some areas, as did an accumulation of a soft yellow coloured material, typical of a jointing or similar non-drying compound.

Initiation of the torsional fatigue cracking had occurred from the bore of the inner shaft bolt hole, at an area of fretting damage similar to the damage on the bolt shank surfaces. The fretting damage was confined to one side of the hole, and similar damage was also found within the other shaft bolt holes and the aligning yoke holes.

Metallurgically, the clutch shaft material and heat-treatment were sound and complied with the manufacturer's specification for the component. No evidence was found suggesting any contributory deficiencies within the design or manufacture of the clutch shaft.

Examination of Serviceable Assembly

A comparable clutch shaft & yoke unit was obtained from the same aircraft operator and disassembled at the ATSB laboratory. While the yoke surfaces in that case were unpainted and hence the bolt tensions remained higher (evidenced by break-out torques of 12 and 16 foot-pounds), the connection had still been unstable and showed appreciable fretting of the mating surfaces and on the bolt shanks. The similar, non-hardening jointing compound had been used within the connection in lieu of the prescribed primer coating. Cracking was not found within any of the shaft bolt holes.

Previous Failure

In 1992, a R22 helicopter sustained a torsional fatigue failure of the A166 clutch shaft, with the aspects of the failure appearing very similar to the failed component described above. A review of the technical report provided by the then Civil Aviation Authority concerning that failure confirmed the prior instability and looseness of the shaft-yoke connection as being a factor contributing to the failure. Similar fretting was found over the mating surfaces of the shaft and within the bolt holes. The torsional fatigue cracking had initiated from the same location and propagated to a comparable extent before failure.

A subsequent inspection of the shaft that had been retained for exemplar purposes also found no evidence of the use of a primer or similar coating over the shaft connection.

Use of non-approved primer

The R22 maintenance manual states that a zinc chromate primer or epoxy resin be sprayed onto the mating portions of the A907 yoke and the end of the A166-1 shaft. These two components are then "wet" assembled (joined together while the primer or epoxy is still wet) and the bolts torqued to the pre-determined value prescribed in the manual. The company maintaining the helicopter had substituted a compound called "Mastinox" at some point early in their operation and maintenance of the type. The chief engineer stated that the use of Mastinox was instigated well before he had joined the company and that it had become an accepted maintenance practice. The use of "Mastinox" was not in accordance with the R22 maintenance manual, and communication with the manufacturer has confirmed that the use of Mastinox is "not approved".

R44 Clutch shaft-to-yoke assembly

The R44 helicopter has an identical assembly of the clutch shaft to yoke, only the part sizes are larger than those of the R22. There have been no documented cases of failure of the R44 clutch shaft in Australia.


The A166-1 clutch shaft fitted to UXF had failed as a result of torsional fatigue cracking, originating from the bore of the inner bolt hole used for securing the adjoining drive yoke. The initiation of fatigue cracking was directly attributed to the looseness of the shaft-yoke connection, which allowed the transmission of rotational loads through the connection by shear forces acting on the bolt shanks and transmitted via the bolt holes. The point loading about the holes produced by that behaviour produced a significant stress-raising effect on the localised material structure. In the presence of the associated fretting and corrosion damage that was sufficient to initiate fatigue cracking under normal shaft loads.

Under normal intended security of the connection, rotational forces are transmitted uniformly via the friction between the shaft and yoke surfaces. Security of the connection is established by adequate bolt tension and the assembly of the connection with a curing or drying primer. Corrosion protection is also assured with the use of the manufacturer approved primer(s).

The assembly of the yoke-shaft connection without first cleaning away the paint from underneath the block seating locations was considered a major factor in the loss of sufficient bolt tension and hence the looseness of the connection. The compressibility/conformability of most paint coatings allows applied bolt tension to be progressively lost and hence renders such coatings unsuitable for use within stable, load bearing bolted connections.

The assembly of the yoke-shaft connection using a jointing or similar compound in lieu of the specified primer(s) is also considered to be a factor contributing to the looseness of the connection. The oily, anti-friction properties shown by the compound would have acted to reduce the surface friction within the connection thus increasing the loads transmitted through the bolts and bolt holes. The non-drying properties of the compound also prevented the effective "lock-up" of the connection when assembled, allowing the surface movement, fretting and bolt interference.

The potential exists for the similar factors described above that led to the failure of the examined R22 shaft to exist within the R44 fleet.

The yoke assembly covers the initiation and propagation site of the failure and therefore it cannot be easily observed in its early stages. Inspection of the yoke-to-shaft mating area during a daily inspection, by either a pilot or a maintenance engineer, is unlikely to discover evidence of the crack. By the time the crack has propagated to a length where it is visible outside the yoke, complete failure is imminent.

Output text

Safety Recommendation

The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority mandate a one-off inspection of the Australian R22 fleet and if considered necessary, the R44 fleet to:

a) inspect the A166 clutch shaft for evidence of fretting where it mates with the A907 yoke, and;
b) inspect the shaft to yoke attachment bolt holes for fretting cracking or other wear, and;
c) identify and remove paint from beneath the yoke assembly bearing block plate, and;
d) identify and remove from service any instances of a non-approved mating compound on the A166 shaft to A907 yoke for the R22 fleet and the C166 shaft to C907 yoke for the R44 fleet.

Initial response
Date issued: 22 December 2003
Response from: Civil Aviation Safety Authority
Action status: Closed - Accepted
Response text:

I refer to your letter dated 6 November 2003 regarding the release of Air Safety Recommendation R20030211 involving Robinson Helicopter Company R22 pre--impact failure of the clutch shaft.


The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority mandate a one-off inspection of the Australian R22 fleet and if considered necessary, the R44 fleet to:

a) inspect the A166 clutch shaft for evidence of fretting where it mates with the A907 yoke, and

b) inspect the shaft to yoke attachment bolt holes for fretting cracking or other wear, and;

c) identify and remove paint beneath the yoke assembly bearing block plate, and

d) identity and remove from service any instances of non-approved mating compound on the A166 shaft to the A907 yoke fro [sic] the R22 fleet and the C166 shaft to C908 yoke for the R44 fleet.

In response to the release of the Recommendation, the Civil Aviation Safety Authority advises the following:

CASA has issued two Airworthiness Directives (copies attached) in response to the matters raised by the Australian Transport Safety Bureau.

The Airworthiness Directives require the inspection of the main rotor yoke and clutch shaft joint for evidence of fretting, cracking, paint and the use of a non-approved jointing compound. If the inspection shows any of these signs, the yoke and shaft must then undergo a magnetic particle inspection procedure before being re-installed in the aircraft.

Airworthiness Directive AD/R22/51 became effective on 12 November 2003 and AD/R44/51 [sic] became effective on 3 December 2003.

Please feel free to contact me should you require any further information.

Last update 03 April 2012