LABELLE, FL, USA
N70696
Bell UH-1B
A main rotor blade had been changed during troubleshooting of the main rotor system. While in a hover to track and balance the rotor system, the other main rotor blade and grip separated. Examination showed the main rotor blade and grip separated due to fatigue cracking of 40 out of 49 main blade strap laminates. The blade that had been change and did not separate was found to have 26 out of 49 main blade strap laminates separated due to fatigue. The fatigue initiated at the strap pin hole. The main blade straps have a life limit of 1,200 flight hours. Logbook records showed the main blade straps were installed in the rotor hub assembly during overhaul in Singapore, 640.4 flight hours before failure. The main rotor straps were reported to be new when installed.
On March 11, 1997, about 1000 eastern standard time, a Bell UH-1B, N70696, registered to Helicorp Inc., crashed at Labelle, Florida, after separation of a main rotor blade, while on a Title 14 CFR Part 91 maintenance test flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was destroyed and the commercial-rated pilot and mechanic-rated passenger received minor injuries. The flight was originating at the time of the accident. The pilot stated he brought the helicopter to a 4 to 5 foot hover in order to track the main rotor blades. He heard a loud bang and the helicopter struck the ground. The mechanic-rated passenger stated a main rotor blade had been changed the previous day and he and the pilot were attempting to track and balance the main rotor blades. They had made two previous runups and shutdown to make adjustments. They started up and pulled the helicopter into a hover. The helicopter felt smooth but suddenly the airframe shuddered. As he looked out the forward window he heard a loud explosion. The helicopter began to come apart and fell to the ground. Postcrash examination of the helicopter at the crash site was performed by an FAA inspector. The "white" main rotor blade and grip separated from the helicopter after failure of the main rotor strap. The blade came to rest about 200 feet southeast of the helicopter. The stabilizer bar separated and was found about 300 feet west of the helicopter. The main rotor hub and second main rotor blade separated and was found lying adjacent to the helicopter. The transmission and remainder of the rotor head separated from the helicopter and struck an automobile about 150 feet from the helicopter. Examination of the main rotor hub and separated main rotor blade was performed under FAA supervision at Bell Helicopter, Fort Worth, Texas. The "white" main rotor grip and blade separated as a result of fatigue cracking of 40 of the 49 laminates that make up the main rotor strap. The fatigue initiated at the strap pin hole. The remaining nine laminates separated due to overstress and had no evidence of fatigue cracking. Examination of the main rotor strap from the "red" main rotor grip and blade, which did not separate, showed 24 of the 49 laminates had fatigue cracking which initiated at the strap pin hole. None of the other 25 laminates had cracks. (See the Bell Helicopter report.) Maintenance records for the helicopter showed the main rotor hub assembly was overhauled in Singapore on February 23, 1989. Records showed that the main rotor straps were installed at that time and were reported to be new upon installation. The records show the main rotor straps have a life limit of 1,200 flight hours. The main rotor hub assembly was installed on N70696 on July 25, 1994, at aircraft total time 7021.2. At the time of the accident the main rotor hub assembly had accumulated 640.4 flight hours since overhaul. On March 10, 1997, .3 flight hours before the accident, the logbook records showed the "red" main rotor blade was changed "for troubleshooting main rotor" system. (See logbook records.) The aircraft wreckage was released to A. T. Turner, Director of Maintenance, Colony Helicopters/Helicorp, Inc., Labelle, Florida, on March 19, 1997. Components retained by NTSB for examination were returned to Colony Helicopters/Helicorp, Inc.
Company maintenance personnel's failure to properly evaluate a main rotor system problem resulting in main rotor straps with multiple laminates separated due to fatigue cracking being allowed to continue in service until failure and separation of a main rotor blade.
Source: NTSB Aviation Accident Database
Aviation Accidents App
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