Metallurgist Inspects Helicopter For Alleged Mechanical Failure

This case involves a tourism helicopter crash in Alaska that was allegedly caused by a mechanical part failure. The helicopter was cruising with a pilot and 4 passenger tourists at 800 feet when the pilot felt an impact. As the pilot slowed down for landing, she noticed a heavy mechanical sound and a strong vibration. The sounds and vibrations continued to worsen making it difficult for her to control the helicopter. During the landing, the main rotor blades contacted the tail boom and one of the tail rotor blades resulting in separation of the tail rotor gearbox support structure. As a result of the crash, several of the tourists suffered whiplash injuries and ongoing emotional distress. It was suspected that fatigue fractures in the attachment studs may have contributed to the accident. An expert metallurgist with aircraft expertise was sought to review the case and determine what parts may have failed and how.

Question(s) For Expert Witness

1. What are fairly typical failure mechanisms you have seen in TRGB attachment studs?

Expert Witness Response E-172745

I have invested a 30-year career in assessing and understanding the strength of aerostructures and their performance under loads. This has included testing and failure analysis and repair. I have never served as an expert witness. Depending on the details of the attachment, the TRGB studs can be subjected to significant shear and tensile stresses. The rotation of the tail rotor causes these stresses to be cyclic (repeated), and the number of repetitions of these cycles can be quite high. This means the bolts can experience fatigue failure at loads that would otherwise appear benign. If there were any small defect in the studs or other aspects of the attachment, the fatigue strength could be greatly reduced. Damage tolerance analysis is one way of assessing the propagation of damage, and to estimate the reduction in the number of cycles to failure. Things that can induce damage in the studs include environmental effects, corrosion, occasional exceedance of design loads, careless handling, and poor maintenance practices.