What happened
On July 29, 2022, a Pilatus PC-12/47E operated by the Swiss Confederation was performing an IFR flight from Bern, Switzerland, to Saarbrücken, Germany. The crew consisted of a pilot in command, a second pilot, and two passengers.
During the initial takeoff roll, the pilot noted unusually high Interstage Turbine Temperature (ITT) and decided to abort the takeoff. After reviewing engine parameters and following the necessary checklists, the pilot attempted a second takeoff. During this second attempt, the pilot again observed high ITT and a lack of expected engine performance, prompting a decision to abort the flight and enter the local traffic pattern. During the final approach, the crew observed sparks emitting from the engine exhaust. The aircraft landed safely at Bern-Belp Airport with no injuries, though the aircraft sustained light damage.
The investigation
The BFU investigation, delegated by the Swiss safety authority SUST, examined the engine' and aircraft history. The investigation included a borescope inspection of the engine at Bern and a detailed teardown of the Pratt & Whitney PT6A-67P engine at the manufacturer's facility in Canada.
Investigators analyzed flight data from the onboard LDR, which confirmed ITT peaks exceeding 800°C during both takeoff attempts and during the final approach. The investigation also reviewed maintenance records, noting that the aircraft had undergone significant maintenance, including a compressor wash and inspections, just days prior to the event. The investigation also looked into recent reports of an "ENGINE NG" caution light appearing during ground operations following the maintenance work.
Findings
Technical analysis of the engine revealed that a stator vane from the first-stage compressor had fractured at the braze joint and was missing. This failure caused aerodynamic disturbances that subsequently led to the fracture of a second-stage rotor blade via high-cycle fatigue.
The primary cause of the engine malfunction was the fatigue failure of a first-stage compressor stator vane, which was attributed to a known design weakness. The investigation found that the part in question was an older design that had not yet been replaced under the manufacturer's existing Service Bulletin (SB14535), which recommended upgrading the assembly to prevent such brazing failures. No evidence was found to link the recent maintenance activities directly to the component failure.