What happened
On May 18, 1978, an Enstrom F-28A, registration HB-XED, departed Lausanne-Blécherette for Basel-Mulhouse to undergo a scheduled 50-hour inspection. Approximately ten minutes into the flight, the pilot experienced vibrations and a sudden zigzagging motion. As the aircraft began to oscillate vertically, a smell of burning rubber filled the cabin, and engine RPM fluctuations were noted.
Recognizing a critical failure, the pilot initiated an emergency autorotation. While the pilot had sufficient altitude to maneuver, the subsequent landing in a plowed field was heavy. The impact caused the main rotor to strike the rear of the fuselage and resulted in the loss of the left skid. The pilot escaped the crash uninjured.
The investigation
The investigation focused on the failure of the main rotor drive system. Examination of the belt-drive mechanism revealed that the front bearing of the belt tensioner pulley had failed. This tensioner pulley, which also functions as the system's clutch, relies on two ball bearings.
Investigators found that both bearings were completely dried out. The bearing cages and balls had disintegrated, and the raceways showed clear signs of overheating. The failure of the front bearing allowed the pulley axis to displace, leading to a loss of belt tension and grip.
Records showed that while the bearings had been replaced in 1976 and had 438 hours of service, they had not been properly maintained. During the previous 100-hour inspection in March 1978, the technician responsible for the maintenance admitted to not applying new grease. The technician had only inspected the rear bearing and assumed the front bearing was sufficiently lubricated because it was harder to access.
Findings
- The primary cause of the transmission failure was the breakdown of the ball bearings due to a lack of lubrication.
- The failure of the front bearing caused the drive belt to lose tension and slip.
- The maintenance technician failed to follow lubrication protocols, relying on an assumption of adequate grease rather than performing a physical check of the difficult-to-reach front bearing.
- The heavy impact during the landing was likely due to an error in altitude estimation during the final approach.