The NTSB noted that existing visual alerts to low rotor rotation speed may have been affected by the severe vertical vibration; because no distinctive aural warning tone was available and the increased collective control friction may not have been detected, the crew may not have realized the rotor rotation speed was low and did not respond accordingly, concluding that "the lack of an automated safeguard in the modified one-engine-inoperative software used during flight testing to exit at a critical [rotor rotation speed] threshold" and "the lack of distinct and unambiguous cues for low [rotor rotation speed]" further contributed to the inability to maintain control.