Insufficient Brazing Could Lead to Wholesale Replacement of Turbine Shaft

An emergency Airworthiness Directive surrounding a roster of General Electric branded turbine engines has been published following "at least four reports of failures" of a drive shaft assembly.

The issue results in a failure of the 'torque reference tube magnetic insert braze joint' located in the 'power turbine drive shaft assembly'. The problem, if not sufficiently addressed, "could result in improper torque and engine speed indications, which in combination with specific phases of flight, could create an unacceptably high flight crew workload in maintaining control of the aircraft, and result in consequent loss of control of the aircraft."

The Directive cites a previous Service Bulletin which specified procedures for a phased array ultrasonic inspection of the tube's joint for inadequate brazing coverage. If necessary, the service even specifies repair or replacement of the power turbine drive shaft assembly where necessary. The Directive makes that ultrasonic inspection mandatory in order to continue use of all General Electric Company (GE) Model CT7-2E1, CT7-2F1, CT7-8A, CT7-8E, CT7-8F5 engines, as well as various restricted category helicopters with GE Model T700-GE-700, -701A, -701C, -701D/CC, -701D, -401, -401C, CT7-2D or CT7-2D1 engines.

The AD is considered an interim action since General Electric and the FAA are investigating the root cause of the insufficient braze joint. Should anything be found that would require additional work from operators and owners, then the FAA may issue further guidance.

FMI: www.faa.gov