Bell 525 investigation looking at vibration and resonance

It was leaked to R&WI that the NTSB is looking at an unusual vibration and frequency resonance in the airframe and controls of the Bell 525 as the cause of the crash over Texas. The #1 prototype was at 183 knots, with 92% power, performing a single engine never exceed speed test, when a vibration at 6 hertz vibration appeared in the tail rotor, followed by a significant drop in the main rotor RPM system, which allowed the rotors to drop out of plane, striking the nose and tail boom.

The cockpit voice recorder was unavailable for analysis, because in an insane regulation, the CVR was not powered. The FAA allows test flights to operate without the CVR operating.

www.rotorandwing.com...

The NTSB formally approved the probable cause of the crash as a severe vibration of the helicopter, followed by the inabilitiy of the crew to maintain sufficient rotation speed of the main rotor. This allowed the rotors to flap, and make contact with the airframe. The vibration was caused by a very high airspeed and a sustained low rotor RPM, and was a condition that hadn't been seen before. Aural clues to the problem sounded, but the same aural warning sounded for 21 other conditions that required alarms, so it wasn't anything to give the crew an indication that the rotor RPMs were dropping.

www.rotorandwing.com...

Sound like a rotor stall with them being out of balance...Not enough stiffness in the blades?