What happened
On 29 June 2006, a Robinson R22B helicopter, registration ZS-RTM, was engaged in a ferry flight from Wonderboom Aerodrome (FAWB) to Nelspruit Aerodrome (FANS). The aircraft had recently undergone a mandatory periodical inspection at an aircraft maintenance organisation (AMO). Shortly after departure, the pilot noted an unusual, soft noise originating from the engine area. Believing the sound might be related to recently replaced exhaust components, the pilot continued the flight. However, as the noise intensified ten minutes into the flight, the pilot performed a precautionary landing.
Following this landing, the pilot inspected the engine and, acting on instructions from the AMO, attempted a return flight to Wonderboom. During this second flight, only three minutes after takeoff, a loud bang and heavy rumbling occurred, forcing the pilot to execute a successful autorotation landing. Upon inspection, the pilot identified a failure in the shaft responsible for turning the engine's rear fan. Engineers from the AMO subsequently performed on-site repairs, allowing the aircraft to fly back to the aerodrome. There were 0 fatalities and 0 injuries during the incident.
The investigation
SACAA AIID investigators examined the failed components through metallurgical analysis. The investigation also reviewed the maintenance history of the aircraft, noting that the incident occurred only 12 minutes after the completion of a mandatory periodical inspection. The inquiry also looked into the regulatory compliance of the pilot and the AMO regarding incident reporting and maintenance protocols.
Findings
- The primary cause of the incident was the failure of the shaft and spider connecting the engine fan due to a fatigue crack.
- Metallurgical analysis indicated the crack resulted from fretting damage caused by two surfaces moving against each other. This movement may have been caused by an improper taper dimension match, an insufficiently tightened securing nut during assembly, or water contamination at the joint.
- The pilot failed to notify the Commissioner for Civil Aviation (CCA) of the incident as required by regulations.
- The AMO permitted the pilot to conduct an engine inspection despite the pilot not being a qualified engineer.
- Neither the pilot, the engineers, nor the AMO reported the occurrence to the CCA, violating mandatory reporting regulations.
Safety action
- It is suggested that mating taper surfaces should be coated with an anti-scuffing agent to prevent fretting caused by surface movement.
- Due to concerns regarding the quantity and condition of lubrication in the bearing, the investigation recommends shortening the re-lubrication interval or defining it by calendar time in addition to operational hours.