Originally Posted by
TTSN
This is as close to RHC has got to recording a rotor divergence accident - it’s audio only and from 1992 so 30 years ago. Rotor head design exactly the same then as it is now. Seemingly no control inputs from instructor or student and in level cruise flight when it occurred (no low RPM horn etc either). Reproduced from an article headlined Undetermined Reasons in Vertical October 2016.
On June 29, 1992, a flight instructor and his pre-solo student took to the air in a Robinson R22 helicopter over northern California’s San Francisco Bay Area. The instructor was relatively experienced, with about 2,000 hours of R22 flight time. The student had only four hours of flight time, all in the R22. She had brought along a microcassette voice recorder, which was set up to tape the cockpit and radio communications during her lesson.
The helicopter’s low rotor r.p.m. warning horn was checked on the ground before takeoff; it operated normally. And nothing appeared to be out of the ordinary during the 17-minute flight to a local practice area near Richmond, where the student, at the instructor’s request, executed a shallow left turn.
The U.S. National Transportation Safety Board (NTSB) described what happened next: “Seconds later, while cruising at 2,200 feet, the CFI [certified flight instructor] began talking. In mid-sentence an undetermined event occurred which interrupted his speech. A wind-like background noise started, and the student exclaimed, ‘Help.'”
Radar data confirmed witness reports that the helicopter’s tail boom and main rotor had separated in level flight. The aircraft plunged into the San Pablo Bay, killing the instructor and student. Examination of the wreckage revealed that the aircraft had experienced “mast bumping” — severe contact of the rotor hubs with the mast, a phenomenon that is often associated with low-G maneuvering. The main rotor blades had diverged to strike the tail boom, which can occur as a result of mast bumping or low r.p.m., leading to rotor stall.
...
Since SFAR 73 was enacted, Robinson loss of main rotor control accidents have occurred less frequently in the U.S., but they haven’t stopped entirely. And in at least one country, New Zealand, they have continued to occur at a high rate, with New Zealand’s Transport Accident Investigation Commission (TAIC) citing at least 12 such accidents or incidents since 1996, despite the relatively low total number of Robinson helicopters in the country.
In a very small number of these accidents, there is eyewitness testimony or other direct evidence to suggest improper handling by the pilot. But in most cases, investigators are no closer to being able to explain these accidents than they were 20 years ago. For almost all of them, the probable cause statements are essentially the same: “the divergence of the main rotor from its normal plane of rotation for an undetermined reason.”
IIRC, there was no evidence of any break in the control system for the swash plate to controls, or pitch links, and the same for the T/R control system. An uncommanded yaw could result in compromising TPP-fueslage clearances, but there should have been some evidence of a break in continuity, or an aerodynamic cause of such an event. Something like a loose stab, vertical or horizontal should show up in the wreckage path and have witness marks of the failure modes... So that remains a very odd event. That the IP was in mid sentence suggests that a surprise input did not occur from that source. For the 22, around that time the MRB had a change in mass that affected the Locke number (IIRC, can't recall the dates off hand) which may have masked the effectiveness of the RHCSC vs an improved inertial damping of the MRB...