What happened
On 6 February 2012, a Guimbal Cabri G2 helicopter, registration SE-JNE, was conducting a training flight near Jönköping-Axamo Airport when the pilot experienced a sudden, loud bang from the engine and transmission area. This was immediately followed by intense vibrations and a rattling sound that increased rapidly in amplitude. The pilot, flying solo, responded by reducing airspeed and establishing an autorotation.
During the emergency descent, the vibrations were so severe that the pilot's headset and sunglasses fell off, and the cockpit instruments became difficult to read. After declaring a 'Mayday' to air traffic control, the pilot identified two potential landing sites. Due to the presence of power lines at the first site, the pilot chose a second field. During the attempt to reduce the descent rate, the vibrations intensified, causing the aircraft to yaw significantly. The helicopter touched down on a snow-covered field with a forward speed of approximately 20 knots, making contact with the tail skid first before sliding on its skids. The pilot was uninjured.
The investigation
SHK investigators examined the engine mounting system, which uses a single row deep groove ball bearing as a primary mounting point. The investigation focused on the failure of this bearing and the difficulty of maintaining it. The engine is connected to the rotor system via a drive belt tensioned by a hydraulic cylinder and an eccentric mechanism.
Technical analysis of the hardware revealed that the ball bearing had suffered catastrophic failure. The outer bearing race had begun rotating within the engine pulley, and the extreme friction caused severe heat damage to the eccentric component. Investigators also scrutinized the maintenance procedures, specifically the method of lubricating the bearing via a grease nipple that is difficult to inspect visually, requiring the use of a mirror.
Findings
- The primary cause of the incident was the failure of the deep groove ball bearing located in the engine pulley.
- The failure was driven by insufficient lubrication or fatigue fractures in the ball cage caused by vibrations.
- A significant contributing factor was the difficulty in verifying that grease had actually penetrated the bearing during maintenance. The existing requirement to weigh a grease gun to ensure a 10-to-20-gram injection was deemed an unreliable and unsatisfactory method for ensuring proper lubrication.
- The helicopter lacked any onboard systems, such as accelerometers, to monitor vibration levels during flight.
Safety action
Following the investigation, the manufacturer, Hélicoptères Guimbal, issued a Mandatory Service Bulletin to reduce the lubrication interval from 500 to 100 flying hours. SHK has recommended that EASA ensure the manufacturer amends the lubrication procedure to allow for certain verification that sufficient grease has been applied to the bearing.