Darwin SB20 at Lugano on Nov 28th 2013, engine shut down in flight
Last Update: December 11, 2017 / 14:05:51 GMT/Zulu time
Milan Malpensa Airport confirmed the aircraft diverted to Malpensa due to a technical problem, the crew requested the longest runway available.
The airline reported the crew received indication of failure of the right hand engine prompting the diversion to Milan. The remainder of the flight was cancelled, the passengers were rebooked onto other flights. The indication may have been false, the aircraft is being examined.
On Nov 29th 2013 Switzerland's SUST issued notification that the occurrence was rated a serious incident, an investigation has been opened.
On Dec 11th 2017 the SUST released their final report concluding the probable cause of the serious incident was:
The serious incident is attributable to the fact that the total failure of the beta feedback transducer (BFT) for measuring the propeller blade angle led to a loss of control of the right engine due to a loose connector, so the engine had to be shut down by the crew.
The SUST wrote: "Just as the copilot was busy reading off the take-off time from the flight management system, the crew noticed that HB-IZW distinctly yawed twice to the right and back. According to the observations of the crew, the amber master warning light illuminated together with an aural tone plus the indications R ENG FAULT and R GEN FAULT, which appeared on part of the central screens in the cockpit. Furthermore, at 06:39 UTC, the caution R PROP CTL FAULT was logged. At this time HB-IZW was climbing at FL 110 over the municipality of Muzzano/TI, east of Lugano regional aerodrome." The propeller did not automatically feather, the first officer (25, CPL, 732 hours total, 531 hours on type) moved the condition lever from run to stop, after the engine had run down to below 100 rpm the first officer manually released the feather button.
The crew decided to enter a hold and work the related checklists including attempting an engine restart, the engine however did not reach a stable condition, hence the captain (45, ATPL, 8,600 hours total, 8,600 hours on type), pilot flying, aborted the attempt and decided to divert to Milan, where the aircraft landed without further incident.
The SUST wrote: "Based on the DFDR data it can be stated that both FADEC channels recorded a fault on the beta feedback transducer (BFT), resulting in the FADEC being unable to measure the right propeller blade angle. Visual inspection of the BFT unveiled that the left connector was loose and worn out (cf. Figure 6). Damage was found to the four not sufficiently tightened mounting bolts and on the housing. This indicates that unprofessional work had been carried out in this area."
The SUST analysed:
Since the engine start, the right hand engine’s redundant digital controller for all engine functions (full authority digital engine control - FADEC) was active on channel A. That means that FADEC channel A was measuring the propeller blade angle via the right connector of the beta feedback transducer (BFT) and its windings. In the recordings of the DFDR data, an instant reduction in the propeller blade angle from 38° to -5.4° was recorded at 06:39:45 UTC. Due to the layout of this hydraulic system, this is technically not possible. Therefore a measurement fault within the BFT must be assumed. A possible explanation is that the short circuit in the left connector interfering with the propeller blade angle measurement through the right connector. This resulted the FADEC switching to fault accommodation mode. Subsequently, the FADEC regulated the right engine to idle and generated the amber caution R PROP CTL FAULT.
As the examination of the BFT unveiled, a short circuit in the left connector between connector pins 3 and 6 existed. Inside the left connector, the plastic guide of the connector pins was spoiled. This defect was presumably caused by vibration. This only became possible by the loose mounting bolts of the connector.
Conclusively, it can be asserted that the existing fault could not be compensated by the system design, resulting in a total failure of the BFT in the right engine.
The SUST analysed that the crew's decision to not return to Lugano but divert to Milan was reasonable, the crew set the priorities correctly. The crew had no possibility to detect that an engine restart was not possible.
No safety recommendations were released as result of the investigation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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