LOS ANGELES — Bombardier says a suspected problem in the low pressure turbine (LPT) led to a sudden loss of power and uncontained failure of a Pratt & Whitney PW1500G on the first CSeries CS100 test aircraft during ground runs in Mirabel, Quebec on May 29.

Pratt, however, declines to confirm the LPT as being the source of the problem and says "we are not going to discuss that level of technical detail." The engine maker adds it has "disassembled the engine and Pratt & Whitney and Bombardier have reviewed it in-depth. We understand the issue and have developed a plan with Bombardier to resume ground and flight testing."

The event, which occurred on the No.1 (left) engine, also caused damage to the airframe and led to the grounding of the CSeries test fleet. According to the Canadian Transportation Safety Board "the crew immediately shut down the engine, observed smoke, and declared an emergency." The engine was subsequently removed for inspection and the aircraft returned to Bombardier’s nearby assembly facility for repair.

Analysts at UBS Americas Equity Research, who met with Bombardier’s CEO Pierre Beaudoin, comment that "while root cause analysis is ongoing, Bombardier emphasized that the failure was unrelated to the gearbox, and also suggested that a manufacturing defect (rather than a design flaw) may have been the cause. The subject engine was known to have problems, and Bombardier had considered sending it back to Pratt prior to incident on May 29. The engine was instead repaired at Bombardier and the failure occurred during subsequent ground-testing."

Details of what caused the apparent LPT failure, and which part of the three-stage assembly was affected, remain unknown. In the PW1500G, as with all versions of the PW1000G geared turbofan family, the low-pressure spool is de-coupled from the fan by the gear system, which enables it to rotate at around twice the speed of conventional non-geared turbofans for increased efficiency. The higher rotational speed has raised questions over potential maintenance and wear-and-tear issues, but Pratt says the speeds are comparable to well-proven earlier engines such as the JT8D. The engine maker has also focused on new flange and hub designs, as well as blade, stator and outer shroud configurations to handle the running conditions. MTU is responsible for the LPT assembly as well as the first four stages of the low pressure compressor.