Rudder control problems appear to have caused an Air Dolomiti ATR 72-500 to veer off the runway during an emergency landing at Munich airport in May.
Five occupants among the 58 passengers and four crew sustained minor injuries when the turboprop (I-ADCD) returned to the hub due to an engine fire soon after departure.
The nose-gear collapsed during the incident, which left the aircraft heavily damaged, according to an intermediate report by German air accident investigation bureau BFU.
The aircraft had departed Munich for a flight to Venice. Around 15min after departure, as the aircraft climbed through 13,000ft, a flight attendant alerted the pilots to smoke and the smell of an electrical fire and burnt plastic in the cabin.
Shortly afterwards there was an explosion in the starboard Pratt & Whitney Canada engine, says BFU. The pilots shut down the powerplant and discharged a fire bottle - which turned off the fire warning lights in the cockpit - then declared an emergency and returned to Munich.
The aircraft landed on runway 26L - there was wind from 240° at 5kt - but it veered off to the left after touching down. It departed the runway around 1,290m (4,230ft) from the threshold, collided with visibility-measurement equipment, and came to a halt in the grass after around 300m. The accident occurred on 17 May, coincidentally as Munich's operator celebrated the airport's 20th anniversary.
BFU says that the pilots noticed during the final approach that neither was able to operate the rudder. Later inspection showed that the rudder travel limiter unit, an actuated component in the empennage to limit rudder deflection at high speed, was in the position for high-speed flight. The reason has not been determined.
Investigators were able to correctly operate the limiter with on-board battery power by manually selecting the respective switch in the cockpit. Under ground power, however, the system showed inconsistencies when the switch was in the 'auto' position. The unit moved between the high- and low-speed positions depending on whether or not the aircraft's two main electrical buses were connected via a bus-tie contact switch.
After the aircraft had come to a stop, the pilots were unable to shut down the port engine using the standard procedure, and instead used emergency controls for engine fire.