In descent, the crew reduced the engine power to the minimum possible in flight, by positioning the levers at Flight Idle (FI). The speed of the aeroplane was 246 kt, close to the maximum speed in operation (VMO) of 250 kt. The crew then felt strong vibrations which were followed by a warning associated with the Propeller Electronic Control (PEC) of the right propeller.
After the flight, the drive shaft of the right engine AC wild generator was found sheared and replaced. A maintenance team carried out tests on the two engine/propeller assemblies. No vibration or abnormal operation was revealed.
The flight the next day proceeded normally. During the landing run, the crew reported a loud vibration noise when they moved the power levers from the flight idle to ground idle position.
Following this flight, various maintenance operations were undertaken. Three ground tests of the engine/propeller assemblies were carried out and did not reveal any abnormal operation. A component of the right propeller pitch-change mechanism (propeller valve module) was replaced. A fourth ground test was started, during which the power levers were moved to the reverse position. Vibrations appeared and the engines were immediately shut down. After the engine shutdown, blades No 1, 2, 5 and 6 of the right propeller were in the feather position while blades No 3 and 4 seemed to stay in the reverse position. The disassembly of the right propeller blades revealed, among other points, the rupture of the blade No 4 trunnion pin as well as damage to the propeller blade actuator plate.
The circumstances and damage observed were similar to that which had been observed in an investigation into a serious incident on 18 September 2013 in Indonesia, involving an ATR 72-212A registered PK-WFV. An investigation was opened by the Indonesian investigation authority, the NTSC, who issued an immediate safety recommendation to the operator of the aircraft concerning the verification of the condition of the propeller blade trunnion pins and the search for crack indications on part of the fleet.
On 30 November 2014, a new similar incident occurred in Sweden to an ATR 72-212A registered SE-MDB, for which an investigation was opened by the Swedish investigation authority, SHK. Shortly after this incident, the BEA issued four safety recommendations to EASA. These concerned in particular:
- informing pilots that there had been severe vibrations during descent at a speed close to VMO with the power levers in the flight idle position and that heavy damage to the propeller pitch change mechanism and, in one case, to the engine mounting brackets, had been observed;
- the planning and carrying out of flights so that operation close to VMO at flight idle is avoided;
- informing maintenance personnel in the cases where pilots have felt strong vibrations during the descent at a speed close to VMO with the power levers in the flight idle position;
- developing an appropriate operational procedure dealing with severe propeller vibrations and including this procedure in the operators’ operational documents.
At the end of the investigation into the incident of 30 November 2014, the SHK issued a safety recommendation to EASA that it “Consider introducing temporary limitations in the manoeuvring envelope, or limitations of the power ranges within the latter, until the problem is resolved and rectified.”
In total, seven cases of vibration phenomena on the ATR 72-212A have been reported in the last few years. In almost all of the cases, the rupture of a trunnion pin of one of the blades and damage to the propeller blade actuator forward plate were observed. The BEA investigation revealed the existence of alternating overloads causing damage to the plates and of a final overload in one direction resulting in the rupture of the trunnion pin. It was not possible to determine the cause of these overloads and the precise chronology of the damage and vibrations. Nevertheless, several elements may have contributed to it:
- a retention force caused by a ball bunching phenomenon;
- significant loads caused by the trunnion pin striking the ear of the plate on forward plate cyclic loading appearing when the aeroplane speed was close to VMO and the power levers in the flight idle position;
- unplanned operation of the control loop of the propeller pitch change mechanism affected by the forward plate cyclic loading and friction phenomena.
The investigation also revealed that the maintenance operations carried out on 9Y-TTC following the vibration phenomena did not identify this damage.
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