What aircraft was involved and where did it happen?

The accident on 2014-11-30 involved a aircraft, at Incident involving an aircraft of the type ATR 72 on the way from Stockholm to Visby, SE.

What was the probable cause of the accident?

The investigation was unable to establish the exact cause of the incident, but identified the rupture of a propeller trunnion pin and subsequent engine damage as key mechanical failures.

Severe vibrations lead to engine shutdown on flight to Visby

A scheduled flight from Bromma to Visby experienced intense aircraft vibrations during descent, resulting in the shutdown of one engine and an emergency declaration.

What happened

During a scheduled flight from Bromma to Visby, the flight crew experienced significant vibrations while descending through approximately 7,000 feet. At the time, the aircraft was traveling at an indicated speed of 250 knots with the power levers set to idle. As the descent continued, the intensity of the vibrations increased to a level that made it difficult for the crew to read cockpit instruments and hindered the ability of cabin crew to move through the cabin.

Flight recorder data indicates that the left propeller momentarily feathered before the right propeller also feathered, after which the right engine was shut down. The aircraft continued the flight using only the left engine. During the process of managing the right engine, the commander attempted to move the propeller control to the feather position but found it could not reach the fuel shut-off position. After cycling the control through the auto position, the commander successfully reached the fuel shut-off position, which caused the vibrations to subside. The co-pilot declared an emergency with Visby tower, and the aircraft landed without further incident.

The investigation

The investigation focused on the mechanical failures and the crew's response to the vibrations. Physical inspections of the aircraft revealed significant damage, including a ruptured eccentric trunnion pin on the second blade and a severely bent front propeller pitch change actuator plate. Additionally, the engine's compressor housing was found to be cracked along half of its circumference, the engine mounts had sustained damage from metal contact, and the AC generator shaft had ruptured.

Regarding crew performance, the investigation found that communication between the pilots lacked explicit confirmation regarding which engine's power levers were being manipulated. Furthermore, several warning signals activated during the event were not reset by the crew during the acute phase of the incident.

Findings

The rupture of the eccentric trunnion pin on blade no. 2 was a primary component of the mechanical failure.
The vibrations were severe enough to impact both cockpit instrumentation readability and cabin crew mobility.
There was a lack of clear verbal confirmation between the pilots concerning engine control movements.
The investigation could not definitively establish the underlying cause of the mechanical failure.

Safety action

Because similar incidents have occurred under comparable circumstances, SHK has recommended that EASA consider implementing temporary limitations on the maneuvering envelope or power ranges for this aircraft and propeller type until a permanent solution is identified. The investigation noted that extensive engineering initiatives are required from the aircraft and propeller type certificate holders to resolve the issue.