Propeller overspeed almost wrecked both powerplants after Widerøe crew pulled levers below flight-idle setting
Norwegian investigators have revealed that a Widerøe Bombardier de Havilland Dash 8-100 came close to double engine failure after the pilots inadvertently retarded the throttle levers to below the flight-idle position.
Both propellers were subjected to overspeed as the crew drew the levers back beyond a safety stop mechanism while reducing thrust in heavy turbulence.
The "uncontrollably high" rotation speeds, exceeding 1,500rpm, severely damaged the right-hand Pratt & Whitney PW121 engine, says Norwegian investigation board SHT, and only "mere chance" prevented similar damage to the remaining powerplant. SHT adds that the overspeed generated a "deafening" noise in the cockpit, rendering communication impossible.
During the incident the aircraft - which had been at 8,870ft (2,700m), descending towards Sørkjosen - lost 760ft in altitude before the crew regained control and shut the right-hand engine down. SHT says the pilots had to run through the shutdown procedure twice after missing a crucial propeller feathering step.
Protections include triggers under the levers and a warning sign
Flight-data recorder information shows the aircraft had banked more than 58˚ to the right, and pitched down by nearly 20˚, before returning to stable flight. It also drifted 30˚ off course. The crew opted to return to Tromsø, the departure point, and executed a single-engined landing. None of the 17 passengers and three crew members was injured.
Widerøe short-runway operations often require pilots to pull the throttle levers behind flight-idle on landing. The levers have a safety stop - a trigger which must be lifted 5mm - to prevent accidental selection during flight.
In its inquiry into the 21 February 2006 event, SHT has recommended Bombardier introduces "measures" to prevent propeller overspeed on the type.
Since February 2010, Widerøe has been modifying certain Dash 8s to reduce the risk of recurrence. The airline also points out that its Q400s have a different power lever function, which offers better protection than earlier Dash 8 variants.