On Christmas Eve 1990 one of our Bell 214ST’s was carrying out night shuttle flights in the East Shetland Basin when at 500 feet and 110 knots the crew became aware of a low amplitude vibration with a frequency somewhere between a “buzz” and a “one per rotor revolution”. There was no associated noise, handling problem or flight deck instrument indication to indicate a malfunction of any aircraft systems.
On reducing speed through 100 knots a pronounced lateral cyclical movement was experienced and persisted until the speed fell through 90 knots. At 80 knots the vibration had reduced to negligible amplitude and this speed was maintained whilst a diversion was set up to the Safe Gothia, alongside Brent Bravo some 20 miles away, where Company engineers and a hangar were available. 80 knot winds were forecast for later in the night.
Cracks were found in the fin root structure around the 42 degree gearbox, in both the left hand side skin and the inclined front spar web fitting, known as the Banjo fitting. The crack in the front spar web had progressed across about
80% of the spar width.
After the event I was told that the maintenance crews on the Safe Gothia were so concerned with the lateral movement of the vertical fin in the heavy seas being experienced that they secured ropes either side of the fin to the hangar walls in case the fin fell off as a result of continued motion of the sea perhaps weakening the front spar web further!
AAIB Bulletin
September 1991