What happened
On 28 November 2013, a Saab 2000, registration HB-IZW, was climbing through FL 110 near Muzzano, Switzerland, en route to Zurich. During the ascent, the flight crew noticed a distinct yawing motion. Shortly after passing FL 100, the engine indicating and crew alerting system (EICAS) triggered an amber caution for a right propeller control fault.
Following the onset of the malfunction, the crew identified an engine failure and proceeded to shut down the right engine. After an unsuccessful attempt to restart the engine, the crew elected to divert from their original destination to Milan-Malmalpensa. The aircraft landed safely on runway 35R with one engine inoperative. All 25 passengers and three crew members disembarked without injury.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the aircraft's digital engine control (FADEC) and propeller control systems. Flight data recorder analysis revealed an instantaneous, physically impossible drop in the recorded propeller blade angle from 38° to -5.4°.
Investigators focused on the beta feedback transducer (BFT), which is responsible for measuring the propeller blade angle. Upon inspection of the BFT, investigators discovered a short circuit between pins 3 and 6 in the left connector. This electrical fault was traced to a loose mounting bolt, which allowed vibration to damage the plastic guide of the connector pins, ultimately causing the short circuit and interfering with the measurement signal on the redundant channel.
Findings
- The primary cause of the incident was the total failure of the beta feedback transducer (BFT) due to a loose connector.
- A short circuit in the left connector of the BFT caused the FADEC to enter fault accommodation mode, which automatically regulated the right engine to idle.
- The loose mounting bolts on the connector allowed vibrations to compromise the internal plastic components of the connector.
- The existing system design was unable to compensate for this specific type of measurement failure, leading to the loss of control of the right engine.