Investigators have determined that the pilots of a crashed Airlines PNG de Havilland Dash 8-100 had been reacting to an airspeed warning when they inadvertently pulled the throttles below the flight-idle setting.

The aircraft (P2-MCJ) was being flown manually, rather than by autopilot, because of an unserviceable yaw damper.

As the Dash 8 approached Madang, Papua New Guinea, the pilots commanded a steep descent – reaching 4,200ft/min, with the propellers at 900rpm – in order to dip below cloud cover.

Without the autopilot to assist stability, the pilots allowed the airspeed to creep, unnoticed, beyond the maximum operating limit as the aircraft descended through 10,500ft.

Neither pilot was alerted to the excessive speed until a warning sounded. The captain reacted to the warning by quickly pulling back the throttle levers, dropping them into the critical beta-range below flight-idle – a setting which leads to propeller overspeed if selected while in flight.

Both engines on the Dash 8 were subsequently driven to speeds more than 60% above their 1,200rpm limit. The left-hand powerplant was severely damaged while an uncommanded feathering of the right-hand propeller rendered it unusable.

With neither engine delivering power a forced landing was “inevitable”, says the Papua New Guinea accident investigation commission.

It points out that the crew did not respond with any standard emergency procedures or checklists.

Although the right-hand propeller had feathered, albeit without a command, the left was not feathered for around 3min – creating heavy drag, as well as asymmetric control, and increasing the descent rate. The descent averaged 2,500ft/min and, at one point, exceeded 6,000ft/min.

Deploying the landing gear and flaps to reduce the airspeed, and perhaps buy time to consider options, was “not considered” by the pilots, says the inquiry into the 13 October 2011 accident.

“If the landing gear and flaps had been extended, the impact could have been less severe,” it adds. The inquiry suggests that, had the proper procedures been followed, the crew might have been able to glide the aircraft to a point close to Madang.

It struck the ground, tail first, at 114kt and slid for 300m before being consumed by fire. Of the 32 occupants only the two pilots, the flight attendant and a single passenger survived.

Airlines PNG chief Muralee Siva notes that the aircraft did not have a lock-out function, to prevent inadvertent selection of beta-range throttle settings. This function will be mandatory on all Dash 8s by mid-June 2016.

The carrier says the accident inquiry is “well-researched and thorough”, and adds that the airline implemented all the safety recommendations.

Investigators believe the crew’s perception of the time available to handle the emergency might have been affected by stress. The pilots told the inquiry that they had insufficient time to carry out any formal response procedures, although the investigation determined that 4min 18s elapsed between the onset of the crisis and the ground impact.

Source: Cirium Dashboard