Background
On 15 November 2004, a Robinson R22 Beta helicopter, registration G-DERB, was involved in an incident during a training flight at Biggin Hill, Kent. The helicopter was manufactured in 1989 and powered by a single Lycoming O-320-B2C piston engine. The crew consisted of two persons: a commander holding an Air Transport Pilot's Licence and a student pilot. No passengers were on board, and no injuries were reported.
History of the Flight
The commander completed a daily inspection before departing from Manston to Biggin Hill. Shortly before arrival, an unusual vibration was detected. After landing, the commander discussed the issue with another company pilot, and it was decided to hover taxi to a nearby maintenance organisation experienced with the R22. During the hover taxi, the vibration intensified, prompting the commander to land and shut down the helicopter. Subsequent inspection revealed that one main rotor blade was cracked from the trailing edge through approximately 75% of its chord, as far as the spar at about the one-third span position. Examination of the rotor system also found that the teeter hinge was extremely stiff.
Aircraft and Maintenance Details
G-DERB had accumulated 2,989.9 total hours. The main rotor blades, part number A016-2, were fitted on 6 October 1999 and had 1,205.6 hours of airborne time. This blade type was being phased out due to a history of cracking, often from corrosion. The manufacturer had issued Service Letter SL-54 in support of FAA AD 2004-06-52, requiring replacement before 10 years calendar life and additional track and balance for blades with over 1,000 hours or five years. A subsequent Service Bulletin SB-94 mandated withdrawal by 1 December 2005.
Maintenance records showed that on 24 March 2004, rod ends were renewed. On 31 March, the helicopter was flown to another organisation to comply with the AD, but worn bearings in the main rotor head prevented successful track and balance. The blade spindles and head were to be reworked, but delays led to the helicopter being transported by road back to its base. On 2 and 3 June 2004, the main rotor head, blades, and spindles were reassembled. An apprentice mechanic, under supervision of the Chief Engineer, assembled the head without shims, as none were available. The pull-off load was initially unacceptably high but after re-assembly, the Chief Engineer recorded values within limits (15 to 22 lbf). The system was checked with a strobe light and signed off. The incident occurred 20 flying hours later.
Engineering Investigation
The rotor head was disassembled and examined. The teeter bolt's cadmium-plated surface was damaged, and the thrust washers and bushes in the main rotor head were deformed. No shim washers were found. The teeter bolt pre-load was correct, but the absence of shims prevented proper clamping of the bushes, causing excessive load on the teeter head and unintended motion between the bolt and bushes. The required shim thickness was calculated to be 0.059 inches.
The cracked blade was examined by a specialist laboratory. The crack initiated in the trailing edge of one skin by fatigue, progressed slowly forward, and then rapidly to the spar. The origin was not from damage, corrosion, or material defect, but from higher than normal stresses at a stress concentration near an internal doubler.
Analysis
Initial assembly was rejected by the mechanic due to stiffness. Plastic deformation of thrust washers and bushes likely occurred during initial tightening, altering clearances upon re-assembly. The recorded pull-off loads remained higher than normal due to missing shims, but fell within limits. Over the subsequent 20 hours, the teeter hinge progressively stiffened, culminating in extreme stiffness after the incident. Cadmium from the bolt surface likely flowed under heat and pressure, increasing friction. In normal flight, the teeter head allows rotor disk attitude changes with minimal coning hinge motion. However, a stiff teeter hinge forces blade flap at the coning hinges, causing cyclic movement of the blade center of gravity and inducing higher stresses. These stresses led to fatigue cracking in the blade.