LAX04IA311 · Sikorsky S-70A

The twin-engine helicopter's number 2 engine experienced an uncontained engine failure and liberated debris punctured the helicopter's firewall while in cruise flight. The pilot made an uneventful landing at a nearby airport. Examination of the engine revealed segments of the stage 2 aft cooling plate (S2ACP) exited the power turbine case. Further investigation determined that the gas generator rotor's forward curvic seal was not installed during the operator's replacement of the gas generator turbine (GGT) module 92 hours prior to the event. Manufacturer's analysis of the missing curvic seal revealed it would have caused the reversal of the secondary airflow in the B-sump, which eventually led to auto ignition of the B-sump oil and exposure of the B-sump hardware to elevated temperatures. The elevated temperatures resulted in the separation of several B-sump components, including the separation of the compressor shaft. The compressor shaft separation resulted in the GGT rotor to rub into the stator and extensive secondary damage, including the separation of the S2ACP. The GGT module was replaced 209 hours prior to the event. According to the manufacturer, the forward curvic seal is part of the engine assembly rather than part of either the GGT rotor assembly or the high pressure compressor rotor assembly. According to General Electric (GE), if the curvic seal was removed from the engine during removal of the previous GGT rotor-stator assembly, and either discarded, or returned to GE's service facility with the rotor, a new seal would not have been included with the new GGT rotor-stator assembly, and the GGT could have been reinstalled without it. The manufacturer also reported that the maintenance manuals did not call for the verification of curvic seal installation during GGT replacement. This military engine was not a Federal Aviation Administration (FAA) certificated engine, but did share similar design features of other FAA certificated engines. The manufacturer reported that the civilian certificated engine maintenance manuals do call for the verification of curvic seal installation following a GGT replacement. The engine was removed from the event helicopter 92 hours prior to the event and was shipped to an engine service center for an internal leak at the number 3 labyrinth seal (located in the front frame of the engine). The service center repaired the inlet guide vanes, which, according to the manufacturer, was a typical repair when oil is observed leaking in the flow path in front of the compressor. However, their analysis of the missing curvic seal indicates that it too would have resulted in an oil leak in the front frame.

Verbatim NTSB analysis and probable cause from the NTSB dataset