Investigation progress update
On the morning of 22 December 2020, the pilot of a Robinson R44 helicopter was conducting aerial agricultural spray operations on a property 13 km south-east of Clare Valley Aerodrome, South Australia. After completing numerous spray runs throughout the morning, the pilot was preparing to land the helicopter adjacent to a loading vehicle for replenishment of chemical product by a ground crewman when a loud bang emanated from the rear of the helicopter.
The pilot reported that, following the noise, the helicopter descended rapidly and there was significant resistance from the flight controls. The helicopter collided heavily with the loading vehicle, coming to rest on its side (Figure 1). The pilot and crewman were uninjured. The operator’s preliminary on-site assessment of the substantially damaged helicopter identified that a mechanical disruption had occurred to the drive system.
The ATSB’s preliminary metallurgical examination of the drive train components identified that the clutch shaft forward yoke had fractured (Figures 2 and 3). The fracture occurred at a bolt hole on the yoke lug that connected with the forward flex plate and was due to the development of fatigue cracking that progressed almost entirely through the yoke cross‑section.
The fracture resulted in the loss of engine drive to the main rotor system (Figure 4). Corrosion product and fretting damage were identified in the vicinity of the bolt hole adjacent to the fatigue fracture surfaces.
Source: Operator supplied image and Robinson Helicopter Company R44 maintenance manual, annotated by the ATSB
Image description: Light surface corrosion and a degree of fretting was present on the contact surfaces of the lug
Source: Robinson Helicopter Company illustrated parts catalogue, annotated by the ATSB.
The airworthiness of the yoke is not limited to a total time in service (no life-limit) and it is required to be inspected at every 100-hour, or annual, inspection. The opportunity to conduct a detailed examination of the yoke contact surfaces for defects is generally limited to those occasions when the bolts are removed and the yoke is separated from the forward flex plate. That is only scheduled to occur during 12 year/2,200 hour overhaul inspections.
A general visual inspection of the assembled clutch shaft yoke during the 100‑hour (or annual) inspection may not identify defects such as corrosion, fretting and/or cracking.
The investigation is continuing and will include:
- a review of the helicopter maintenance records and maintenance schedule
- an assessment of components from the hydraulically-assisted flight control system
- further examination of the main rotor drive train assembly.
Should a safety critical issue be identified during the investigation, the ATSB will immediately notify relevant parties so appropriate and timely safety action can be taken.
The information contained in this update is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from the initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB's understanding of the accident as outlined in this update. As such, no analysis or findings are included.
The pilot of a Robinson R44 had been conducting aerial spray operations when on approach to land heard a loud bang emanate from the rear of the helicopter, followed almost immediately by experiencing difficulty with the flight controls.
A witness located at a refuelling vehicle adjacent the intended landing zone heard the noise and also observed the tail rotor had ceased operating. The helicopter descended and collided heavily with the refuelling vehicle, coming to rest on its side after the impact. The pilot and witness were uninjured. The helicopter was substantially damaged.
The evidence collection phase of the investigation will include an examination of the helicopter and maintenance records, and interviews with the pilot and any witnesses to the accident.
A final report will be released at the conclusion of the investigation. Should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant parties, so that appropriate safety action can be taken.
The investigation is continuing.
|Date:||22 December 2020||Investigation status:||Active|
|Investigation level:||Defined - click for an explanation of investigation levels|
|Location:||approximately 13 km south-east of Clare Valley Aerodrome||Investigation phase:||Final report: Drafting|
|State:||South Australia||Occurrence type:||Collision with terrain|
|Report status:||Pending||Highest injury level:||None|
|Anticipated completion:||4th Quarter 2022|
|Aircraft manufacturer||Robinson Helicopter Co|
|Aircraft model||R44 II|
|Type of operation||Aerial Work|
|Damage to aircraft||Substantial|