What happened
On May 17, 2012, an ATR 72-500 operated by an Italian airline departed Munich International Airport for Venice, Italy, carrying 58 passengers and 4 crew members. Approximately nine minutes into the flight, a flight attendant reported smoke in the cabin, suggesting an electrical cause. The pilots immediately declared an emergency and initiated a return to Munich.
During the descent, the crew identified an engine fire in the right engine. They activated the fire extinguishing system, which successfully extinguished the flames, and subsequently shut down the number 2 engine. As the aircraft prepared for landing on runway 26L, the pilots performed various checklists, including an evacuation plan discussion.
Just before touchdown, the pilot noted that the rudder felt stiff. The aircraft touched down at approximately 98 knots, but shortly after contact, the plane began a significant leftward yaw. The aircraft veered off the runway, striking an antenna of the visibility measurement system. The nose gear collapsed upon hitting a bump in the terrain, and the aircraft came to a stop in a grassy area. The crew ordered an immediate evacuation of all 62 occupants via the right-side doors. Six passengers sustained minor injuries.
The investigation
The BFU examined the aircraft's flight data recorder (FDR) and cockpit voice recorder (CVR), as well as the physical wreckage. Investigators focused on the rudder control system, specifically the Travel Limiter Unit (TLU), which restricts rudder movement at high speeds.
Testing revealed that the TLU was stuck in 'High Speed Mode,' which limited the rudder's travel. While the pilots had performed the 'Final' checklist, the investigation found that the TLU 'green light' indicator was not visible to the crew. This was because the Bus Tie Contactor (BTC) had been set to the 'ISOL' (isolation) position to manage the electrical smoke, which effectively cut power to the Caution Advisory Panel (CAP) that would have displayed the TLU status.
Furthermore, the investigation of the right engine, conducted in coordination with Canadian authorities, identified that a broken turbine blade in the second stage of the high-pressure turbine was the cause of the engine fire.
Findings
- The primary cause of the engine fire was the failure of a turbine blade in the second stage of the engine's high-pressure turbine.
- The aircraft veered off the runway due to the rudder being restricted by the TLU in high-speed mode.
- The crew was unable to identify the TLU malfunction because the electrical isolation procedure (performed to mitigate cabin smoke) had deactivated the warning panel responsible for displaying the TLU status.
- The aircraft's rudder pedals were physically restricted because the TLU actuator had not transitioned to low-speed mode.