The Swedish Accident Investigation Authority (SHK) recommends EASA to limit ATR-72 power ranges in specific conditions following several unsolved engine vibrations incidents.
The SHK published their final report of a serious incident involving a Braathens Regional ATR-72-500 in November 2014. The aircraft operated on a domestic flight from Stockholm-Bromma Airport to Visby in Sweden with 55 persons on board.
The vibrations increased in intensity and the commander reduced the rate of descent to 2,500 feet per minute.
The vibrations became so severe that the cabin crew had difficulties moving in the cabin and that there were difficulties reading the instruments in cockpit.
Information from the flight recorders shows that the left propeller was first feathered momentarily. The right propeller was feathered thereafter, after which the right engine (PW127M) was shut off. The flight continued with the left engine in operation. The information also reveals that the communication between the pilots did not include confirmation of which engine’s power levers were manoeuvred. A number of warning signals were activated during the sequence of events. The signals were not reset during the acute phase of the event.
When the commander moved the right propeller control to feather position, he was unable to push it all the way to fuel shut-off position. The control was therefore returned to the “auto” position and then pushed back via the feather position to fuel shut-off, whereby the vibrations subsided.
The co-pilot explained the situation to the air traffic controller in the Visby tower and declared an emergency situation.
The approach and landing were executed without problems.
The SHK was unable to determine the cause of the serious incident. Since it noted that there had been six similar incidents, a recommendation was issued to EASA to “consider to impose temporary restrictions to the flight envelope or similar limits of the power ranges of the same until the problem is resolved and rectified.”
- May 2014 incident involving Caribbean Airlines ATR-72-600
- Sept. 2013 incident involving Wings Air ATR-72-500