On September 9, 1990, a Bell 206B (registration N385EH) operated by Era Aviation Inc. was involved in an aviation accident near Kokhanok, AK. Investigators recorded the probable cause as: The air compressor scroll tube assembly compressor end fitting breaking due to fatigue, influenced by variations in tube bend development and linear measurements from engineering drawings. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 10 related events involving the same aircraft type or operator are linked below.
A turbine engine experienced a spool down during operation. Investigation revealed the air compressor scroll tube assembly had failed due to fatigue, with manufacturing deviations contributing to the break.
What happened
The pilot reported experiencing a spool down on the turbine engine during flight operations. This event necessitated an immediate examination of the powerplant to determine the root cause of the mechanical failure.
The investigation
A thorough inspection of the engine was conducted following the incident. The examination revealed that the air compressor scroll tube assembly compressor end fitting had broken. A metallurgical analysis of the failed component showed no material irregularities. However, the tube bend development and linear measurements were found to vary from the engineering drawings. The failure occurred in a fatigue mode, with the origin located on the tube outer diameter approximately 0.020 inch below the 'B' end flare radius.
Findings
The investigation concluded that the air compressor scroll tube assembly failed due to fatigue. Contributing factors included deviations from engineering specifications regarding the tube bend development and linear measurements.
Probable cause
The air compressor scroll tube assembly compressor end fitting breaking due to fatigue, influenced by variations in tube bend development and linear measurements from engineering drawings.