Braathens AT72 near Visby on Nov 30th 2014, engine shut down in flight

Last Update: October 20, 2016 / 15:52:46 GMT/Zulu time

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Incident Facts

Date of incident
Nov 30, 2014

Classification
Incident

Cause
Engine shut down in flight

Airline
Braathens Regional Airlines

Flight number
DC-929

Departure
Stockholm Bromma, Sweden

Destination
Visby, Sweden

Aircraft Registration
SE-MDB

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A Braathens Regional Express Avions de Transport Regional ATR-72-212A on behalf of Malmo Aviation, registration SE-MDB performing flight DC-929/TF-929 from Stockholm Bromma to Visby (Sweden) with 51 passengers and 4 crew, was on approach to Visby when the crew needed to shut the right hand engine (PW127) down due to vibrations. The aircraft continued for a safe landing about 4 minutes later.

The aircraft was carrying the women's basketball champion Northland Basket to their match against Visby.

The airport reported the crew reported vibrations at the mechanism that controls the propeller blade angle.

The Northland Basketball Team, who won their match later the day, reported that strong vibrations occurred just prior to landing.

Sweden's Haverikommission (SHK) announced they have opened an investigation into the occurrence stating that the aircraft had a technical problem with one of the propellers and landed on single engine.

On Oct 19th 2016 the SHK released their final report concluding the probable causes of the serious incident were:

SHK has been unable to establish the cause of the serious incident.

The SHK reported that the aircraft had been enroute at FL140 and was descending through 7000 feet on approach to Visby, when small vibrations were felt. At that time the aircraft was descending at 3200fpm and 250 KIAS with the power levers at idle. The commander pushed the left hand lever forward first, then the right hand hoping the vibrations would cease that way as in past experience, however, there was no effect. The vibrations increased in intensity, the crew received a PEC (propeller electronic control) fault message. The commander reduced the rate of descent to 2500 fpm. The vibrations became severe, the master caution illuminated. The captain moved the right hand propeller control into the feather position, but then was unable to pull further back into the fuel shut off position, hence returned the lever to the AUTO position, then pushed the lever through the feather position into the fuel shut off position. The vibrations ceased.

The SHK wrote: "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 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."

The SHK listed following damage was found after landing:
- The eccentric trunnion pin on blade no. 2 was ruptured.
- The front actuator plate was severely bent on all six positions.
- The engine mounts had received damage from contact with metal.
- The engine's compressor housing was cracked along half of its circumference.
- The shaft of the AC generator was ruptured.

After disassembly of the propeller it was found that the 5 other trunnion pins had crack indications on both sides.

The broken trunnion pin showed indications of multiple bilateral overload. The ball bearings and ball bearing spacers were found in airworthy condition. The crack in the compressor housing was found occurred due to the overloads caused by the propeller.

The SHK reported:

An investigator from SHK participated as an observer on a flight from Bromma to Visby Airport. The flight was conducted with a sister aircraft with registration SE-MDC. The purpose was to gain knowledge of the flight operations environment in the cockpit.

Following take off from Bromma and during the climb to cruising altitude, weak vibrations were detected, which produced deflections in the form of movement to the left and right on both control wheels. During descent to Visby, with the power levers on idle, the vibrations increased in intensity when the speed approached 245 kts. At this point, the vibrations could be felt in the feet and on the sidewall to the door post. The vibrations ceased when the power levers were increased slightly and the engines' torque (Tq) increased to a reading in the region around 7-8 %.

The operator attempted to resolve the problem by performing a dynamic balancing of the propellers. This was however unsuccessful. The propeller blades were disassembled with the intention of weighing them in order to check for a potential source of the fault. In conjunction with this, a play in the trunnion pins' bearings was discovered.

The bearings were replaced, after which the propellers could be successfully balanced, meaning the vibrations did not return.

The SHK analysed that the damage, described above, occurred most likely during the incident flight after the power levers were brought to idle and stated: "It has not been possible to firmly establish the logic and the mechanics behind the damages, but it cannot be ruled out that they may have been a result of aeroelastic vibrations."

The SHK analysed: "the propeller type certificate holder has assessed that the likey cause of the incident was the development of excessive friction in the blades' retention bearings. An improved ball bearing separator has been designed and introduced as a correcting action."

The SHK summarized analysis:

SHK is able to establish that the incident occurred during descent at high speed when the power levers were reduced to idle and only the right engine (engine no. 2) was affected. The investigations of similar incidents carried out by other safety investigation authorities reveal that they too occurred under similar circumstances. Via the measures taken by BEA, EASA and the aircraft type certificate holder, the operators of the aircraft type have been informed of this and a procedure has been developed for identifying and shutting off the affected engine in similar situations.

SHK does not share the propeller type certificate holder's opinion on what caused the incident. According to SHK, it is not likely that the measures taken by the propeller type certificate holder are such that they sufficiently prevent the occurrence of a similar incident.

SHK's opinion is that further extensive engineering work in the form of thorough calculations and tests is required to find the cause of the incident.

The motive for this includes matters such as the registered and unidentified frequency of 39 Hz. It has also been established that the trunnion pins' bearing may be in contact with the front actuator plate during normal operation.

The propeller type certificate holder has indicated that the trunnion pin bearing by design should typically be against the aft plate protuberances during normal operation.

The forward plate should normally be loaded during isolated operating conditions, such as reverse operation.

The propeller type certificate holder has explained that the wear observed on the forward actuator plate protuberances is due to increased friction in the retention bearings.

SHK has established that both the aircraft type and propeller type have undergone a number of small changes since their original certification. There is nothing to suggest that any particular change has constituted the cause of the accident. It would however be valuable to investigate in greater detail whether the combined effect of the changes which have been made are such that they have a negative impact on the aircraft type's properties.

SHK considers such further investigative measures to be the responsibility of the aircraft and propeller type certificate holders, under supervision of the certifying authorities.