History of the Flight
The Cessna 208B (registration C-GRXZ, serial number 208B0469) departed Tofino, British Columbia, at 1353 Pacific standard time on an instrument flight plan. The flight was in visual meteorological conditions above broken cloud layers over a designated mountainous region. At approximately 1420 PST, while at 9000 feet above sea level, the engine failed. The pilot initiated a right turn, declared an emergency to Vancouver Area Control Centre, and requested a range and bearing to the nearest airport. The aircraft's rate of turn increased, and it rolled out on a heading toward Port Alberni Regional Airport, about 17 nautical miles northwest. The pilot, still in VMC, would have to descend into cloud. He then requested navigational information to avoid terrain. Communication was lost as the aircraft descended through 7000 feet asl. Radar data showed a tight left-hand 360° turn at about 6000 feet asl, with a rate of descent increasing to about 2500 feet per minute. The aircraft emerged from the turn at 4500 feet asl heading toward Port Alberni. Witnesses heard a Mayday call indicating an attempted forced landing on a logging road. The aircraft struck trees during a steep right-hand turn and crashed. Fuel leaked into the cabin but no fire occurred.
Engine Examination
The engine, a Pratt & Whitney Canada PT6A-114A (serial number PCE19352), was removed and examined at an approved teardown facility. All 58 compressor turbine (CT) blades were broken, fractured at various heights. One blade was fractured near the airfoil root platform, with fracture surface topography indicating fatigue cracking. The compressor seized due to bearing damage following the CT blade failure.
Metallurgical Examination
Examination at the TSB Engineering Laboratory determined that one CT blade failed from overstress extension of a fatigue crack. The crack initiated at a metallurgical anomaly in the parent blade material. It progressed from the trailing edge toward the leading edge in a high-cycle mode, eventually causing overstress rupture. The remaining CT blades and power turbine blades failed from impact damage from debris of the first blade. The number two bearing assembly failed due to imbalance from the blade breakage.
Engine History
The engine, manufactured in 1995, had a basic time between overhaul (TBO) of 3600 hours. It was overhauled twice. The first overhaul at 3528 hours was in the United States. A hot section inspection on 22 October 2003 at 7677 hours replaced all 58 CT blades with new ones. A TBO extension to 6000 hours was granted per P&WC Service Bulletin 1703. The second overhaul on 7 November 2005 at 9528 hours inspected and re-installed the blades. At the accident, the engine had about 140 hours since the last overhaul. The failed blade had about 1991 hours since new.
Engine Condition and Trend Monitoring
An engine parameter recording system and condition trend monitoring software were required per Service Bulletin 1703. Guidelines called for daily recording and processing at least every three days. Sonicblue Airways recorded parameters weekly. Review of recorded data from the accident flight and archives showed no exceedances or trend changes. Data from the final portion showed compressor speed at 0 rpm (seized compressor) and propeller rpm between 5 and 19 (feathered).
Commercial Single-Engine Instrument Flight Rules Operations
Canadian Aviation Regulations permit single-engine IFR operations with authorization. The aircraft and pilot met standards, including specific equipment and training requirements.
Engine Reliability
SEIFR approval is based on turbine engine reliability. The basic in-flight shutdown rate for the PT6A-114A in 2005 was about 0.0025 per 1000 hours, but total IFSD rate including all causes was about 0.01 per 1000 hours.
Mountainous Regions
SEIFR operations in designated mountainous regions were initially prohibited but the restriction was removed in December 2000.
Navigation Equipment
The aircraft had a King KLN 89B GPS with an aviation database over seven years out of date. The GPS could display range and bearing to nearest airports, meeting the regulatory requirement for rapidly determining nearest airport location.
Terrain Awareness Equipment
Canada had no requirement for terrain avoidance equipment in SEIFR operations at the time. The accident aircraft was not equipped with EGPWS or TAWS.
Aircraft Glide Performance
The Cessna 208B pilot operating manual includes a glide performance graph; specifics were not provided in the source.