History of Flight
On May 21, 2011, at about 1600 Pacific Daylight Time, a de Havilland DHC-6-300, registration N178GC, operated by Grand Canyon Airlines, experienced an uncontained engine failure in the left engine while flying at 5,500 feet mean sea level, about 3 miles northeast of Grand Canyon West Airport. The pilot made a safe landing at the airport. All 17 persons on board were uninjured. The flight was a Part 135 on-demand air tour from Boulder City Municipal Airport. Visual meteorological conditions prevailed. Post-flight inspection revealed engine parts had exited the casing and breached the cowl. No other aircraft damage was reported.
Engine Damage
The engine was a Pratt & Whitney Canada PT6A-27, serial number PCE 40304. Initial examination showed a 4 by 10 inch torn opening at the bottom of the combustor section in the axial plane of the power turbine (PT) wheel. The exhaust case was intact but had pock marks. The PT shroud was battered from internal particle impact, consistent with PT blade release. The gas generator case had a large breach between the 4 and 7 o'clock positions, aft looking forward. The compressor assembly rotated freely; all first-stage blades were present and undamaged. The compressor turbine was intact with nicks and dents.
All PT blade roots were in place, but all PT blades were fractured at varying radial locations from the root platform to about one-third span. Fracture surfaces showed only overload. The PT hub was intact but battered, and about three-quarters of the trim balance rim was fractured. The PT retaining bolt removal torque was significantly lower than normal. The No. 3 and No. 4 bearings were undamaged.
The propeller reduction gearbox (RGB) front and rear cases were intact. Eight of 12 aft PT shaft housing studs were fractured. Bright shiny metal flakes were found throughout oil-wetted surfaces, densest at the 6 o'clock position.
The first-stage planetary gear assembly showed extensive damage. The sungear shaft was fractured in multiple locations, separating into three pieces. The front gear had circumferential and axial fractures; two other gear teeth were fractured from the front to about half span. Several other teeth were spalled on the contact surface, and the gear material was discolored from overheating. The part was a non-original equipment manufacturer part, sold by Timken Alcor Aerospace Technologies (TAAT) and manufactured by Sungear, Inc.
The three first-stage planet gears were damaged and discolored, with fractured aft corners and spalled tooth load surfaces. The second-stage planetary gear assembly was intact and undamaged.
Laboratory Analysis
NTSB laboratory analysis found that the average surface roughness of the root fillet radius of the sungear measured 22.68 micro-inches, coarser than the specified 8 micro-inches maximum. Fractures of the sungear were the result of a fatigue crack that emanated from the root fillet radius of the spline portion.
Corrective Action
Between May 2010 and August 2011, five additional sungear failures occurred, none resulting in uncontainment. The FAA issued Emergency Airworthiness Directive 2011-20-51 on September 15, 2011, mandating removal of certain TAAT sungears within 15 operating hours or 15 days. After five more failures, the FAA issued Immediate Airworthiness Directive 2011-25-12 on December 28, 2011, removing all serial numbers of TAAT manufactured P/N E3024765 from service.