The PNG Accident Investigation Commission released the final report of their investigation into a fatal DHC-8-100 accident near Madang, Papua New Guinea. It was concluded that both propellers oversped during descent after having been put into beta range. A forced landing was necessary.
On October 13, 2011, the DHC-8-100 turboprop operated on Airlines PNG flight CG-1600 from Port Moresby (POM) to Madang (MAG) via Lae-Nadzab (LAE).
The pilot-in-command was hand-flying the aircraft on the final leg because the autopilot was unserviceable. He conducted a low power, steep descent towards Madang with the propellers set at 900 revolutions per minute in an attempt to get below cloud in order to be able to see across the ocean to Madang. Neither pilot noticed the aircraft’s speed increasing to its maximum operating speed; when this speed was reached, a warning sounded in the cockpit.
The pilot-in-command pulled the power levers backwards, through the flight idle gate and into the ground beta range. Moments later, at 10,090 feet, both propellers oversped simultaneously and exceeded their maximum permitted revolutions per minute (rpm) by in excess of 60 percent.
The overspeeding propellers back-drove the turbines in the engines (instead of the engine turbines driving the propellers) and this caused severe damage to the left engine. The right engine was not as badly damaged; its propeller feathered due to a system malfunction so that the propeller was no longer back-driving the turbine. It was not possible, however, for the pilots to unfeather the right propeller and generate useable thrust from the right engine, which meant that a forced landing without power was inevitable.
Use of the landing gear and flaps was not considered by the pilots, although the flaps could have been extended until the engines were shut down before impact and the landing gear could have been extended at any time. If the landing gear and flaps had been extended, the impact could have been less severe.
The aircraft thus force-landed gear-up on sparsely timbered terrain on the northern side of the Buang River, 33 km south east of Madang township. During the impact sequence, it was severely damaged while colliding with trees and the ground, and an intense fuel-fed fire began. Of the 32 occupants three crew member and one passenger survived.
Contributing safety factors:
- The Pilot-in-Command moved the power levers rearwards below the flight idle gate shortly after the VMO overspeed warning sounded. This means that the release triggers were lifted during the throttle movement.
- The power levers were moved further behind the flight idle gate leading to ground beta operation in flight, loss of propeller speed control, double propeller overspeed, and loss of usable forward thrust, necessitating an off-field landing.
- A significant number of DHC-8-100, -200, and -300 series aircraft worldwide did not have a means of preventing movement of the power levers below the flight idle gate in flight, or a means to prevent such movement resulting in a loss of propeller speed control.
If a beta lockout mechanism had been installed on the aircraft, the double propeller overspeed would not have occurred when the power levers were moved below the flight idle range and in the ground beta range during flight. Installation of this mechanism is now mandatory on DHC-8 aircraft worldwide by 19 June 2016. If the pilots had followed the standard emergency procedures detailed in company manuals, they would have given themselves more time to manage the emergency, consider their options, and carry out the approach and forced landing.