Oil supply failure - Propeller:
To prevent an overspeed caused by an oil supply failure, one of two methods may be employed:
• Hydraulic lock valve The pitch lock mechanism houses a valve called the 'hydraulic lock valve'. This is a spring loaded ball valve opened by fine pitch oil pressure and closed by a spring. In the event of an oil supply failure the hydraulic lock valve closes and acts like a non-return valve to trap coarse pitch oil in the pitch change cylinder. The oil trapped in the cylinder prevents centrifugal twisting movement (CTM) from turning the propeller blades towards fine pitch without affecting the feathering ability if the propeller.
• Mechanical locking device A rachet mechanism allows the propeller to move to coarse pitch and to be feathered. The Pawl has to be withdrawn by P.C.U. action before the blades can move to a finer pitch.