Accident Overview
On November 1, 2008, about 1902 Alaska daylight time, a twin-engine Construcciones Aeronauticas SA (Casa) 212 turboprop airplane, registration N437RA, collided with tundra-covered terrain during a forced landing approximately 1 mile north-northeast of Toksook Bay, Alaska. The airplane sustained substantial damage to the fuselage, wings, and empennage. The two crew members, an airline transport captain and a commercial first officer, sustained minor injuries. The flight was operated as a visual flight rules (VFR) cargo flight under 14 CFR Part 135 by Arctic Transportation Services (ATS), Inc., Anchorage, Alaska.
Flight and Crew
According to the captain, the first officer was flying the trip to Toksook Bay. The flight originated at Bethel Airport, Alaska, about 1822, and was en route to Toksook Bay and Tununak before returning to Bethel. Initially, instrument meteorological conditions (IMC) prevailed, and an instrument flight rules (IFR) flight plan was filed. However, soon after departure, weather improved significantly and the IFR clearance was canceled. The flight continued to Toksook Bay in VFR conditions. During the approach to Runway 16, the first officer advanced the engine power levers to shallow the approach path, but there was no response from the right engine. The airplane began to yaw right, and the captain took control, initiated a go-around, and added full engine power. At about 500 to 600 feet above ground, the yaw intensified, and the airplane descended rapidly. The captain applied full left aileron and rudder but could not maintain altitude. The left engine torque meter indicated 100 percent, while the right engine torque meter indicated between 0 and 10 percent. The captain called for propeller feather; the stall warning horn sounded and the warning light illuminated. He pitched the aircraft forward to avoid a stall and selected a tundra-covered area for a forced landing. The airplane struck the ground with the right main landing gear and right front portion of the fuselage.
Engine and Systems
The airplane was powered by two Honeywell TPE331 turboshaft engines equipped with a Negative-Torque-Sensing (NTS) system, which limits negative torque from a windmilling propeller by cycling blades toward feather but does not automatically feather the propeller. The captain reported that the right engine NTS system did not reduce drag during the approach, and he was unable to increase power to that engine. The investigation revealed that the mechanical linkage connecting the cockpit engine power lever to the right engine propeller pitch control (PPC) was found disconnected, with the attaching bolt missing. The fuel control unit remained in the flight-idle position after the linkage became disconnected.
Maintenance and Inspection
At the time of the accident, the airplane had accumulated 26,005.9 flight hours. The right engine was a leased unit installed on August 26, 2008, 237.2 flight hours before the accident. The operator’s FAA-approved aircraft inspection program (AAIP) included phase inspections. Two inspections occurred after installation: an event 2A inspection on October 4, 2008 (114.3 flight hours before the accident) and an event 1A inspection on October 20, 2008 (21.2 flight hours before). The 2A inspection checklist included item 40: visual inspection of engine control rods and locking devices for security, alignment, and damage; and item 41: functional test for torque of bolts and screws attaching levers and arms of serrated shafts of the fuel control, PPC, and governor.
Propeller Examination
On January 27, 2009, the right propeller hub assembly was disassembled and inspected at Dominion Propeller, Inc., Anchorage. The impact-damaged hub had three of four composite blades broken at or near the root, and the fourth blade cut about six inches from the hub. All four blades remained attached to the hub with no evidence of slippage. Score marks on the blade grips and propeller dome corresponded to impact forces and indicated that all blades were at or near the flight-idle position at impact. No preaccident mechanical anomalies were found.
Aftermath
The National Transportation Safety Board did not take custody of the wreckage; no parts were retained. Following the accident, company management elected to install a bolt into the previously threaded splined shaft of the PPC to provide a secondary means of attachment. This modification, considered a minor repair or alteration by the FAA, was applied to all company Casa 212 aircraft immediately after the accident.