What happened
On a flight departing Kelowna Airport, British Columbia, a WestJet Airlines Boeing 737-200, registration C-FTWJ, experienced a significant engine failure shortly after takeoff. While climbing through 3,300 feet, the crew heard a loud bang similar to a compressor stall. This was immediately followed by a rapid drop in the number 1 engine's RPM and a spike in exhaust gas temperature. The pilots successfully shut down the Pratt & Whitney JT8D-17A engine and leveled the aircraft at 6,000 feet. After declaring an emergency, the crew returned to Kelowna, landing safely on Runway 15 with no injuries and no fire.
The investigation
Investigators examined the engine and found extensive mechanical damage. While the initial stages of the compressor appeared normal, the 3rd stage showed significant damage, and the turbine section contained melted hardware. Metallurgical analysis by the TSB Engineering Laboratory revealed that multiple airfoils in both the 3rd and 8th stages had suffered from fatigue cracking due to cyclic loading.
Maintenance records showed that the engine had experienced several surging events during taxiing the previous day. In response, maintenance personnel replaced a pressure ratio bleed control (PRBC) valve. Because the manufacturer's troubleshooting manual did not require it, no boroscopic inspection was performed to check for internal compressor damage. Two primary scenarios were considered: one involving a failure of an 8th stage airfoil retaining pin, and another involving liberated material near the 3rd stage rotor. Both scenarios pointed to a compromised gas path causing airflow instability.
Findings
- The compressor gas path was compromised by either a failed 8th stage airfoil retaining pin or a liberated stator piece near the 3rd stage rotor.
- Airflow instabilities caused vibratory excitement, leading to widespread fatigue cracking in the rotor airfoils.
- The fatigue-weakened airfoils eventually failed during the climb, causing the engine power loss.
- The replacement of the PRBC valve, combined with a lack of requirement for a boroscopic inspection, masked the underlying compressor instability.
- The maintenance manual lacked warnings regarding potential fatigue cracking resulting from compressor surges.