Bombardier Inc CL 600 2C10 N715SK incident near Chicago, Illinois

Probable cause

A Bombardier CRJ700 No. 1 engine failure caused by buckling of the combustion outer liner due to fatigue cracks. The liner buckling resulted in airflow disruption and reduced engine performance. The engine fuel control unit attempted to compensate for the decrease in engine performance by increasing the fuel flow rate. The combustion liner damage and increased fuel flow led to incomplete combustion of the fuel injected into the combustion chamber. Unconsumed fuel then ignited downstream of the combustion chamber (delayed ignition) and resulted in high pressure turbine thermal damage, and the eventual separation of a high pressure turbine 2nd stage nozzle vane segment. The separated vane segment subsequently caused low pressure turbine case burn through and catastrophic impact damage to the high and low pressure turbine stages.

NTSB analysis

The No. 1 engine failure was caused by combustion outer liner buckling due to low cycle (thermal) fatigue. An axial crack that was about 2.5 inches long in the outer panel connected with a circumferential crack that was about 12 inches long between the 10 o'clock and 12:30 positions. At the junction of the two cracks, the outer liner was displaced (buckled) about 1.5 inches inward toward the engine centerline. The liner deformation caused an airflow disruption that prevented complete combustion of fuel in the combustion chamber and decreased engine performance margin. In response to the decrease in performance, the engine control system increased fuel flow in an attempt to compensate. The combination of increased fuel flow and incomplete combustion of the fuel injected into the combustion chamber led to delayed ignition of unconsumed fuel downstream of the combustion chamber (flame holder). The delayed ignition resulted in thermal damage to the high pressure turbine (HPT) hardware. The first stage HPT blades and vanes receive a high volume of cooling airflow and remained intact, but the HPT 2nd stage nozzle was thermally damaged and a nozzle vane segment at the 10 o'clock position eventually separated. After separation of the HPT 2nd stage nozzle vane segment, all four stages of the LPT experienced severe impact damage as separated fragments moved through the gaspath. The HPT 2nd stage nozzle separation also resulted in LPT case burn through at the 10 o'clock position that allowed hot gases to escape and contact the engine fire detection loop, which triggered a fire warning. There was no evidence that high energy fragments exited the LPT case.

Combustion liner cracks are a normal occurrence on high time parts. The engine line maintenance manual (LMM) includes inspection criteria with limits based on crack length and location.

SkyWest received a total of four GE customer notification reports (CNRs) for the incident engine related to reduced hot day performance margin. SkyWest complied with the CNR recommendation to perform a core engine water wash after receipt of each CNR. Although the first water wash was effective, the third (April 1, 2017) and fourth (May 6, 2017) attempts did not improve engine performance margin. The CNRs all also included a recommendation to perform a borescope inspection (BSI) of the HPC and HPT (recommendations 3 and 4). SkyWest performed a HPC BSI on June 6, 2017 and no anomalies were reported. They then elected to cease further maintenance on the engine and instead scheduled it for removal and replacement. The HPT 2nd stage nozzle thermal damage would likely have been visible if an HPT BSI had been performed.

Reduced engine margin and the two ITT exceedances were both strong indicators of a potential core engine issue, but SkyWest did comply with all AMM inspection requirements leading up to the engine failure. GE CNRs are non-mandatory diagnostic tools, so both the completion of the recommended maintenance actions and the order the maintenance actions are completed are at the discretion of the operator.

Text from the NTSB. Open the NTSB docket for ENG17IA028 for the full record.