NTSB Factual Report

On August 12, 2007, about 19:45 mountain daylight time, a Bombardier Learjet 60 airplane, N243F, operated by D&D Aviation, experienced a loss of power in the left engine, a Pratt & Whitney Canada (PWC) PW305A turbofan, while the airplane was climbing through flight level (FL) 380 (38,000 feet mean sea level) after departing Salt Lake City (SLC), Utah. The pilots reported that as the airplane was climbing through FL 380, they heard a loud bang and the airplane began to shake severely. The pilots further stated that about 10 seconds later, the cabin filled with smoke and fumes. The pilots stated that they donned their oxygen masks and declared an emergency. The airplane returned to SLC for an overweight, single-engine landing without further incident. The airplane was operating on an instrument flight rules flight plan under the provisions of 14 Code of Federal Regulations Part 91 from SLC to Wichita, Kansas. The two pilots on board were not injured.

The on-scene examination of the engine revealed that two fan blades were fractured transversely across the airfoils adjacent to the blade root platform. All of the other fan blades were in place in the hub, but all were damaged with nicks and gouges on the leading edges.

The left engine was removed from the airplane and shipped to PWC's Bridgeport, West Virginia maintenance facility for disassembly and examination. Because the investigation of a previous PW305A fan blade fracture had found that several of the opposite engine's fan blades were cracked (refer to NTSB investigation ENG07IA018), the right engine's fan rotor was also removed and returned to PWC's Bridgeport facility for examination. The two fractured fan blades, Nos. 8 and 22, were removed from the fan hub and submitted to the PWC's Materials Laboratory for metallurgical examination under the oversight of the Transportation Safety Board of Canada. The left engine's remaining 22 fan blades and the right engine's 24 fan blades were checked with the PWC airfoil thickness gage that found the left engine's fan blade airfoils were below the minimum thickness, but the right engine's fan blades were acceptable. It was noted that the below-minimum thickness fan blades were from heat code (H/C) MCBWF, the same heat code batch lot that had been identified as being below the minimum airfoil thickness in the previous PW305A fan blade separation incident. The remaining fan blades from the left engine along with all of the fan blades from the right engine underwent a fluorescent penetrant inspection (FPI). The FPI revealed five blades from the left engine had crack indications in the convex side of the airfoil adjacent to the blade root platform, the same location of the crack indications in the fan blades from the previous incident. The right engine's fan blades, which were from a different batch lot, did not have any FPI crack indications.

PWC's metallurgical examination determined that fan blade No. 22 fractured due to a fatigue crack on the convex side of the airfoil adjacent to the blade root platform about 1.3-inches forward of the trailing edge. PWC's examination also determined that fan blade No. 8 had fractured from overload, although the fracture coincided with a preexisting fatigue crack. The cracks were of the same orientation and morphology as the cracks that noted in the fan blades from the previous fan blade separation incident. The metallurgical examination revealed that the fan blades' material conformed to the requirements and that there were no indications of any preexisting material defects.

PWC, using a coordinate measuring machine (CMM), accomplished a dimensional inspection of the left engine's intact fan blades. The intact fan blades from the previous PW305A fan blade separation incident were also dimensionally inspected. The dimensional inspection revealed the fan blades' airfoils that were all associated with H/C MCBWF were, on average, 0.091-inches below the nominal dimension of 0.273 inches in the area where the fatigue fracture and cracks occurred. According to PWC, that area of the fan blade was the highest stressed part of the airfoil and under-minimum thickness condition would result in stresses that exceeded the material properties.

The PW305 fan blades are produced for PWC by Blades Technology Limited (BTL) of Israel. According to BTL, at the time the fan blades that fractured and were found cracked were manufactured, the forging dies had to be aligned manually. BTL stated that its records for the H/C MCBWF fan blades indicate the forging dies had been shimmed during the setup and that it believed the thin airfoil condition was the result of an improperly aligned forging die. At the time the H/C MCBWF fan blades were produced, PWC did not have a dimensional inspection requirement for the fan blades' airfoils. PWC stated that shortly after the H/C MCBWF fan blades were produced and well before the previous fan blade separation occurred, it had added a dimensional inspection requirement for PW305 fan blades.

According to PWC and BTL records, there were 624 H/C MCBWF PW305 fan blades produced. PWC developed an inspection gage to measure the airfoil thickness with the fan blade still installed in the hub, although the hub had to be removed from the engine. PWC issued Alert Service Bulletin 24588 that required all H/C MCBWF fan blades in service be inspected. According to PWC, of the 529 H/C MCBWF fan blades that were still in service (the others having been removed from service due to damage from FOD, bird strike, etc.), 519 were found to be under the minimum thickness and were removed from service. PWC stated that it made a corporate decision to also removed from service the 10 H/C MCBWF fan blades that passed the airfoil thickness inspection.

NTSB Probable Cause

The fatigue fracture of the fan blade due to the blade vendor's improper setup of the forging dies, which resulted in fan blades with airfoils that were below the minimum thickness in the highest stressed area of the airfoil and resulted in operating stresses that exceeded the material's capability. Contributing to the incident was the engine manufacturer's lack of a dimensional inspection of the fan blade's airfoil.