AWB 63-2 Issue 2, Bell 206 One-Way Clutch Failures - Lubricating Oil Orifice
Bell 206 One-Way Clutch Failures - Lubricating Oil Orifice
AWB 63-2 Issue 2, 1 November 2004
Bell 206 Jet Ranger helicopters.
Advise operators and maintenance personnel of maintenance procedures for the one-way sprag clutch that will enhance its reliability, and reduce the possibility of torsional yield failure of the main rotor mast.
The Bell 206 one-way clutch requires a continuous stream of cooling and lubricating oil during operation. This oil is provided under pressure from the main transmission oil system, and is delivered to the sprag clutch via a calibrated restrictor. This restrictor can become partially or fully blocked thus reducing or stopping the flow of lubricating oil, causing the clutch to fail.
When the restrictor becomes partially blocked, the one-way clutch seems to operate for approximately 200 hours before failing. Clutch bearing failures may also be an early warning of reduced or low oil supply to the clutch.
Defect report investigations indicate that the restrictor can become blocked with rubber fragments, fibres or other particles. These contaminants may be introduced into the transmission oil by component failure or maintenance action. Inspecting the chip plugs in this area for evidence of correct oil flow may be ineffective. Due to normal residual oil in the area, when chip plugs are removed, they may often look “wet” even though the oil flow to the one-way clutch may be lacking.
A properly operating overrunning clutch should be locked or engaged from the moment the engine begins to start. The rotor blades should begin to move within the first few seconds of starter wind-up. When the clutch initially slips and then suddenly engages after the engine has already gained significant RPM during start (approximately 25%), the sudden shock of connecting the rotating engine to the high inertia of the stationary main rotor overloads many components, including all drives between the engine and the rotors.
Severe overload of this kind commonly results in a torsional yield failure of the main rotor mast. Clutch failure may also be indicated by severe vibration, and a squealing / grinding noise coming from the clutch during operation.
Since there is currently no Bell requirement to periodically check or clean the one-way clutch oil supply orifice, CASA recommends that the operator take action to change the maintenance program for the Bell 206 series to include a check of the one-way clutch lubricating oil orifice for cleanliness every 600 hours, whenever the main transmission is changed, and whenever the one-way clutch is removed or changed for any reason.
Operators should also check that the chip detector in this area is connected to the correct warning light in the master caution panel.
Enquiries with regard to the content of Airworthiness Bulletins should be made via the direct link e-mail address included on the Airworthiness Bulletin web site, AirworthinessBulletin@casa.gov.au or in writing to Maintenance Standards Branch, GPO Box 2005, Canberra ACT, 2601