In descent, the crew reduced the engine power to the minimum possible in flight, by positioning the levers in 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 electronic propeller control (PEC) of the right propeller.
After the flight, it was found that the drive shaft of the right engine AC wild generator had ruptured and it was 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 1, 2, 5 and 6 of the right propeller were in the feather position while blades 3 and 4 seemed to stay in the reverse position. The findings on the disassembly of the right propeller blades included the rupture of the blade 4 trunnion pin and damage to the propeller blade actuator yoke 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 similar new 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 in order that it, in particular:
- ensures that all pilots have been informed that there have 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, has been observed;
- ensures that pilots planning and carrying out their flights avoid operations close to VMO at flight idle;
- ensures that pilots report to maintenance personnel if they have felt strong vibrations during the descent at a speed close to VMO with the power levers in the flight idle position;
- ensures that an appropriate operational procedure is developed 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, asking that it “Consider[s] 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 yoke plate were observed. The BEA investigation has revealed the existence of alternating overloads causing damage to the yoke 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 ball bunching;
- significant loads caused by the trunnion pins striking the ears of the yoke plate on the occurrence of cyclic loads on the forward yoke plate, 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 forward yoke plate cyclic loading and friction.
The investigation also revealed that the maintenance operations carried out on 9Y-TTC following the vibration phenomena did not identify this damage.
As a result, the BEA has issued several safety recommendations to EASA and the FAA. These concern:
- continuing the analysis of the cyclic load phenomenon on the forward yoke plate revealed at flight idle and at a speed slightly above VMO in order to confirm that the ATR72-212A flight envelope provides sufficient margins to prevent this phenomenon from causing damage to the propeller pitch change mechanism;
- continuing research in order to understand the sequence of damage to the propeller and the cause(s) of the overloads and that pending the outcome of this research, revising the ATR 72-212A manufacturer’s recommended operating procedures for descent to prevent any flight between 240 and 250 kt at flight idle;
- installing vibration level indicators for each propeller?engine assembly in the cockpits of commercial air transport aircraft equipped with turboprop engines;
- carrying out an in-depth study into the actual vibration behaviour of each propeller in flight idle with speeds around VMO, during the initial certification of the propellers.
Download the report(s) by clicking on the link(s) below: