Incident Overview
On March 29, 2009, at approximately 1820 central daylight time, a crack was discovered on one main rotor blade of a Bell 206 L-3 helicopter, registration N708M, operated by Air Evac EMS, Inc. The helicopter had just returned from a mission and landed without incident at its operations base in Abilene, Texas. Visual meteorological conditions prevailed during the mission and subsequent repositioning flight, which was conducted under Title 14 Code of Federal Regulations Part 91 without a flight plan. The pilot and two medical crewmembers were not injured. According to the operator, the preflight inspection for the mission revealed no anomalies. The mission and repositioning flights were turbulent but otherwise unremarkable. During the post-flight inspection, the pilot discovered a crack in one of the main rotor blades. The pilot stated that he was unable to detect any vibration due to the turbulence.
Blade Examination
The operator removed the blade and sent it to the National Transportation Safety Board (NTSB) Materials Laboratory in Washington, DC. On July 15, 2009, an NTSB investigator and an engineer from Bell Helicopter examined the blade. The crack was observed in the top and bottom skin of the trailing edge, running chord-wise approximately 8.75 inches from the trailing edge toward the leading edge. The crack then branched into two longitudinal cracks running in opposite directions: one measuring 9.5 inches inboard and the other 7.75 inches outboard. Examination of the fracture surface revealed features consistent with a fatigue crack that initiated from a pit along the leading edge of the trailing edge strip. The initiation site exhibited a yellow discoloration consistent with corrosion. Further examination revealed a region of interconnected porosity that formed a channel through the adhesive layer bonding the lower skin to the trailing edge strip.
Manufacturing Background
According to Bell Helicopter, during the manufacturing process, the blade undergoes two separate leak checks designed to detect porosity in the epoxy that bonds the upper and lower skins to the trailing edge strip. Assembly records indicate that these leak checks were conducted on October 23, 2005, and October 26, 2005, respectively. No notes in the assembly records indicated that a leak was observed; however, quality assurance stamps confirmed that the checks were performed. Bell Helicopter noted that if a leak is detected, additional adhesive is applied, and an entry in the assembly records is not necessarily made.
Previous Vibration Issue
Engineers and investigators from Bell Helicopter reported that trailing edge blade cracks are rare. If not found during normal maintenance and pilot checks, the crack will eventually become large enough for the pilot to detect a one-per-revolution vibration in flight. They stated that when such events occur, the pilot can easily detect an imbalance situation, and the condition does not progress beyond the imbalance and vibration. These cracks are typically discovered during post-flight inspection while diagnosing the imbalance. According to the operator and company maintenance records, a one-per-revolution vibration had been experienced, and tracking and balancing were performed on the blade, after which the vibration was no longer noted.