Hiller UH 12E N7173S accident near Dinosaur, Colorado

Probable cause

Failure of a S4 compressor blade due to a fatigue crack that likely initiated at a corrosion pit. Contributing to the accident was maintenance personnel’s failure to detect the corrosion pits during maintenance.

NTSB analysis

The commercial pilot and two passengers were searching for lost cattle during an air taxi flight. While at low altitude, the pilot heard a grinding noise and noticed a loss of engine power. The pilot made a forced landing, which resulted in substantial damage to the tail rotor.

The engine examination revealed compressor damage consistent with a S4 blade separation during operation. The debris released from the S4 failure caused severe downstream damage, resulting in a loss of compressor function and a total engine power loss.

Metallurgical examination of the S4 wheel found corrosion on the forward rim face and at both the suction and pressure sides of the blade stubs. Scanning electron microscopy examination of areas of S4 blade stub fracture surfaces not obliterated by secondary damage found cracks initiating in corrosion-damaged areas on the suction side of the blade crown and evidence that the cracks progressed in fatigue across the blade thickness until failure in overload.

Review of the operator’s aircraft service records found that the compressor was installed in 2012 with unknown time since new and zero time since overhaul. The records indicated that the compressor module was overhauled in 2006.

Exposure to a corrosive environment is required to initiate corrosion. The operator reported that the compressor was not operated in a corrosive atmosphere since the 2006 overhaul. No compressor module service records before 2006 were available.

The Rolls-Royce 250-C20B Overhaul Manual requires that the S4 compressor wheel satisfy overhaul inspection criteria during overhaul. The Rolls-Royce 250-C20B Operation and Maintenance Manual requires a periodic detailed compressor inspection whenever engines are operated in corrosive environments. The manual includes a CAUTION regarding this requirement.

Text from the NTSB. Open the NTSB docket for CEN20LA073 for the full record.