What happened
On November 28, 2013, a Saab 2000, registration HB-IZW, was climbing through FL 100 near Muzzano, Switzerland, when the crew experienced a significant yawing motion. Shortly after, the Engine Indicating and Crew Alerting System (EICAS) triggered an amber warning for a right propeller control fault.
Upon analyzing the situation, the crew determined that the right engine had lost control and was effectively idling. Following standard emergency procedures, the crew shut down the right engine and decided to abort the flight to Zurich, opting instead to divert to Milan-Malpensa. Despite an attempt to restart the engine, the failure persisted. The aircraft landed safely at Milan-Malpensa with the right engine secured, and all 25 passengers and 3 crew members disembarked without injury.
The investigation
SUST investigators examined the aircraft's flight data recorder (DFDR) and cockpit voice recorder (CVR). The data revealed an impossible instantaneous drop in propeller blade angle from 38° to –5.4° at 06:39:45 UTC. Investigators concluded this was not a physical movement of the blades but a measurement error caused by an electrical fault.
Technical examination of the Beta Feedback Transducer (BFT), which measures propeller blade angle, revealed a short circuit between pins 3 and 6 in the left connector. The plastic guide for the pins had been destroyed, likely due to vibrations exacerbated by loose mounting screws. This short circuit interfered with the signal on the right connector, causing the FADEC (Full Authority Digital Engine Control) to enter fault accommodation mode and command the engine to idle.
Findings
- The primary cause of the incident was the complete failure of the Beta Feedback Transducer (BFT).
- A loose connector and subsequent short circuit caused the FADEC to lose control of the right engine's propeller pitch.
- The FADEC system was unable to compensate for this specific type of transducer failure, leading to the engine being forced into idle.
- The crew's decision to divert to Milan-Malpensa was appropriate given the challenging approach procedures at the local Lugano airport.