Incident Summary
The crew of a BAe ATP, registration G-MANU, reported that during the first of three scheduled sectors, while descending through Flight Level 70, the right engine torque and propeller RPM began to fluctuate. The fluctuations ceased when propeller synchronisation was deselected, and the remainder of the approach and landing were normal. After completion of two more sectors, the right propeller de-ice system was recorded as inoperative.
Engineering Inspection
An engineering inspection revealed that part of the right propeller spinner was missing, with evidence that bolts had been projected forward from the back plate mounting position. All six blades of the right propeller assembly were damaged, including signs of contact with the damaged spinner during blade pitch changes. Removal of the spinner showed that all six bolts, which engage in thread inserts in the propeller hub and secure the back plate, were missing. The loss of the bolts and resulting lack of security of the spinner led to disbonding of the propeller de-ice slip-rings, causing the recorded failure of the right propeller de-ice system. One bolt had penetrated the fuselage ice guard and was only prevented from passing through the fuselage skin at cabin floor level by impact at a stringer position.
Operator Actions
The operator instigated a check of the rest of their fleet, to be completed by 5 July 1998. This check identified a number of loose bolts, including one propeller with only one of the six bolts beyond finger tight. The operator issued an instruction for the correct torque to be achieved (50 to 65 lbf/in) on all installations by 19 July, with a repeat check at the next A check (A check at 300 hour intervals). A propeller hub thread insert 'run down torque' check is to be carried out at the next base maintenance input (E, 600 hour or C, 3600 hour check), and witness marks are to be applied to the bolts following refitting to allow for visual inspection for any backing off. A visual inspection is to be carried out at each Intermediate A Check (300 hours), and torque checks are to be repeated every E check.
Manufacturer Consultation
The aircraft and propeller manufacturers have been consulted, and the maintenance instructions reviewed. The relevant section of the aircraft maintenance manual is to have minor amendments incorporated at the next scheduled revision. The propeller manufacturer, Hamilton Standard, is to issue a Service Information Release (SIR 6/5500/F:011) to all operators clarifying the requirement to check the bolt/thread insert rundown torque for each bolt and, provided it is within the specified limits (9.5 to 80 lbf/in), to then torque the bolts to 50 to 65 lbf/in plus rundown torque. They will also issue a Temporary Revision (T/R 61-1) to the ATP Propeller Maintenance Manual, pending incorporation at the next scheduled revision due to be published in November 1998.