What happened
On 4 May 2014, an ATR 72-212A, registration 9Y-TTC, operated by Caribbean Airlines, was performing a scheduled flight from Tobago to Piarco, Trinidad. During the descent phase, the aircraft was traveling at approximately 246 knots, approaching its maximum operating speed (VMO) of 250 knots. As the crew positioned the power levers in the flight idle position, the aircraft experienced intense vibrations accompanied by a warning from the right engine's electronic propeller control (PEC).
Following the initial event, the aircraft continued service for another day. However, during the landing roll of the subsequent flight, the crew reported loud vibrations when transitioning the power levers from flight idle to ground idle. Subsequent maintenance testing, which included moving the levers to the reverse position, triggered further severe vibrations, necessitating an immediate engine shutdown. Upon inspection, investigators discovered that several propeller blades had failed to move into the reverse position correctly, and significant internal damage was found within the propeller pitch change mechanism.
The investigation
The BEA examined the damage to the right propeller, which revealed the rupture of a blade trunnion pin and damage to the propeller blade actuator yoke plate. The investigation also reviewed several similar vibration events involving ATR 72-212A aircraft in Indonesia, Brazil, Tanzania, and Sweden. Technical tests were conducted on the engine/propeller assemblies, including examinations of the propeller valve module, the pitch change actuator, and the interaction between propeller blades.
Findings
The investigation established that the damage was caused by alternating overloads acting on the propeller pitch change mechanism. These loads resulted in the rupture of the trunnnion pin and damage to the yoke plate. Several contributing factors were identified:
- Significant loads were generated by the trunnion pins striking the edges of the yoke plate when the aircraft was operating at speeds near VMO with power levers in flight idle.
- Potential retention forces caused by "ball bunching" within the mechanism.
- Unplanned fluctuations in the control loop of the propeller pitch change mechanism, potentially exacerbated by friction and cyclic loading.
Maintenance inspections following the first vibration event failed to detect the underlying structural damage to the mechanism.
Safety action
The BEA issued several safety recommendations to EASA and the FAA, including:
- Revising manufacturer-recommended operating procedures to prevent flight between 240 and 250 knots while at flight idle.
- Implementing vibration level indicators in the cockpits of turboprop commercial aircraft.
- Conducting further research into the certification criteria and the specific sequence of damage during high-speed, flight-idle operations.