N178GCDehavilland DHC-6-300 · 1980

On May 21, 2011, at about 1600 Pacific Daylight Time, a de Havilland DHC-6-300, registration number N178GC, operated by Grand Canyon Airlines (GCNA), experienced an uncontained engine failure in the left engine (No. 1) while flying at an altitude of 5,500 feet mean sea level (MSL), about 3 miles northeast of Grand Canyon West Airport (1G4). The pilot made a safe and uneventful landing at the Grand Canyon West Airport. All 17 persons on board including the crew were uninjured. The flight was operated under Title 14 Code of Federal Regulations (CFR) Part 135 as on-demand air tour flight that departed from the Boulder City Municipal Airport (BVU) for a local sightseeing tour of the Grand Canyon. Visual meteorological conditions (VMC) prevailed throughout the flight. Post flight inspection of the airplane revealed that engine parts had exited the engine casing and breached the engine cowl. No other airplane damage was reported.

Initial Visual Examination

The engine was not examined in the field by NTSB investigators. A Pratt & Whitney Canada (P&WC) field engineer and a local FAA inspector shared their initial photographs of the engine in the nacelle, which depicted a 4 inch X 10 inch torn opening at the bottom of the combustor section in the axial plane of the power turbine (PT) wheel, confirming an un-contained event. The incident de Havilland DHC-6-300 was powdered by a P&WC PT6A-27, serial number (S/N) PCE 40304, turbopropeller engine. The engine was removed from the airplane and shipped to the P&WC facilities in Montréal where the powerplant group met with the purpose of tearing the engine down and determining the cause of the un-containment.

Engine Disassembly and Examination

Examination of the engine revealed an exhaust case that was intact; however, there were pock marks on both the exhaust exit ducts. Pock marks indicate that there was particle impact from the internal side of the engine. The PT shroud was battered due to internal particle impact, consistent with PT blade release. There was a large breach on the gas generator case in the axial plane of the PT wheel; the direction of the displaced case material was from the inside towards the outside. The circumferential location of the opening was between 4 and 7 o'clock position, aft looking forward. The maximum width of the torn opening in the engine axial direction was approximately 4 inches, occurring at the 5:30 o'clock location. The tear tapered down on both ends in a conical fashion.

The compressor assembly rotated and driveline continuity with the accessory gearbox (AGB) was confirmed. All 1st stage blades were present and were undamaged. The compressor turbine was intact; however, there were nicks and dents on the leading edges of all the blades. The compressor turbine wheel was removed; however, no further disassembly of the gas generator was performed. The AGB was intact, still attached to the compressor assembly and undamaged. The driveline continuity was tested and confirmed and therefore no further disassembly was performed on the AGB.

All the PT blade roots were still in location and locked by the retaining rivets; however, all the PT blades were fractured at varying radial locations from the blade root platform to approximately 1/3 span. A visual examination of the fracture surfaces found only evidence of overload. The PT hub was intact; however, it was battered and scored on the up-stream face and approximately ¾ of the trim balance rim was fractured. There was heavy rotational scoring on the up-stream face of the blade platforms as well as the leading edges of the remaining blades. The PT retaining bolt removal torque was significantly lower than normal. The PT wheel and PT shaft master splines were undamaged. The PT shaft was intact and undamaged. The aft air seal was rotationally scored with a 1-inch long section completely worn through. The seal was blackened, and the fracture surfaces were rounded, consistent with overheating. The No. 3 and No. 4 bearings were undamaged, bright, shiny, and rotated freely and still oil wetted.

The propeller reduction gearbox (RGB) front and rear cases were generally intact. Eight of the 12 aft PT shaft housing studs were fractured. The 1st stage carrier bushing sleeves were undamaged. Bright shiny metal flakes were found throughout all the oil wetted inner surfaces. The densest accumulation of the metal flakes was at the 6 o'clock location.

The 1st stage planetary gear assembly consists of one 1st stage sungear, three 1st stage planet gears, one 1st stage carrier, and a 1st stage ring gear. The sungear shaft is not located or supported by running bearings; instead, it is suspended on the aft end by the PT shaft splined coupling and on the front end by only the inter-mesh of its spline with the three planetary gears. The accurate radial locating and fixing function of the front sungear spline is highly dependent on the critical tight tolerances of it and the three planetary gears. Additionally, the hardness and surface finishes on the gear surfaces are critical to long life. Upon assembly, the four gears are rotationally matched with indexed markings.

The 1st stage sungear shaft was fractured in multiple locations, resulting in the sungear separating into 3 pieces: the front gear, the mid-shaft section, and the aft conical section. The front gear was fractured circumferentially and axially. The axial fracture allowed a sliver of the gear and tooth material to separate. Two other gear teeth were fractured from the front to approximately ½ the span length. Several other gear teeth were fractured at the aft end corners. The remaining gear teeth were spalled on the tooth contact surface. All the tooth crowns were battered and severely deformed. The gear material was discolored, consistent with overheating. The part markings on the gear were as follows: E3024765 Rev C FAA-PMA MFR 1L9D9 S/N PC5-15 indicating the part was sold by Timken Alcor Aerospace Technologies (TAAT), Incorporated, but manufactured by Sungear, Inc, San Diego, California.

The three 1st stage planet gears were damaged and discolored. The aft corners of all the gear teeth of the three gears were fractured. The size of the missing corner chips was between 1/8 inch and ¼ inch. One gear exhibited a fractured gear tooth of about 1/3 of the tooth span from the front. The tooth load surfaces were spalled and scored. The crowns of all the teeth were battered. The gear material was discolored, consistent with overheating. The common part marking on the three 1st stage planet gears was as follows: ASSY E3101455-02 REV A FAA-PMA MFR 1L9D9 SET EE-134, indicating that the parts were sold by Timken Alcor Aerospace Technologies, Incorporated.

The 2nd stage planetary gear assembly was intact, and all surfaces were bright and shiny. All gear surfaces were generally undamaged, exhibiting only minor scores and dents. All gears rotated freely and were oil wetted. It was not further disassembled.

Related Failures of Pratt & Whitney Canada PT6A-27 Engines

A review of the engine's repair documentation revealed that a new non-original equipment manufacturer (OEM), parts manufacturer approval (PMA), 1st stage planetary gearset, sold by Timken Alcor Aerospace Technologies, Incorporated (TAAT) and manufactured by Sungear, Inc. had been recently installed in the RGB. Discussions with the local FAA manufacturing inspection district office (MIDO) and TAAT representatives revealed that, prior to this event, two planetary gearset failures had occurred on P&WC PT6A-27 engines with low time TAAT planetary gearsets. Neither of these events resulted in an uncontained PT failure and were therefore not reported to NTSB.

A summary of the NTSB laboratory report findings indicated that the average surface roughness of the root fillet radius surface of the sungear measured 22.68 micro-inches, which is coarser than 8 micro-inches, maximum, specified in the sungear engineering drawing. Fractures of the sungear were the result of a fatigue crack that emanated from the root fillet radius of the spline portion of the sungear.

Between May 21, 2010 and August 1, 2011, five additional sungear failures occurred, none of which resulted in an un-containment. The FAA subsequently issued an Emergency Airworthiness Directive, EAD 2011-20-51 on September 15, 2011, mandating the removal of a limited set of S/Ns of TAAT 1st stage reduction sun gears P/N E3024765 that were manufactured by Sungear, Inc. from the RGB assemblies of P&WC PT6A-15AG, -27, -28, -34, -34AG, -34B, and -36 series turboprop engines within 15 operating hours or 15 days. These gears were identified by TAAT as being suspected of having surface finish discrepancies as described in the NTSB laboratory report.

After five more failures were reported, the FAA issued Immediate Airworthiness Directive, AD 2011-25-12 on December 28, 2011, which removed all S/Ns of TAAT manufactured P/N E3024765 from service.