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.
Before the Richmond crash, 23 other Robinson R22s had experienced similar losses of main rotor control — events which are almost always fatal and, in the absence of recording devices, leave few clues as to their causes. Investigators trying to piece together circumstances after the fact had chalked up many of these accidents to low-G maneuvering or low rotor r.p.m., implying mishandling by the pilot.
But the recorded evidence in the Richmond crash simply didn’t support the usual explanations. Neither pilot had voiced any concern about the operation of the helicopter before the breakup. The low r.p.m. warning horn didn’t activate before or during the breakup sequence, and spectral analysis of the audiotape indicated that the aircraft was being operated at a normal main rotor r.p.m. Radar data showed that the airspeed was normal for cruise flight, and there was nothing to suggest low-G maneuvering.
With no easy way to explain the Richmond crash, the NTSB launched a special investigation into R22 loss of main rotor control accidents. Meanwhile, the U.S. Federal Aviation Administration (FAA), which had already conducted two special certification reviews of the R22, initiated a third. It also convened a technical panel to study R22 loss of main rotor control accidents, and commissioned the Georgia Institute of Technology (Georgia Tech) to conduct simulation studies of the R22 main rotor system.
In addition to several airworthiness directives and bulletins, in February 1995, the FAA issued Special Federal Aviation Regulation (SFAR) 73, which created specific training and proficiency requirements for Robinson R22 and R44 helicopters. When the NTSB issued its own special investigation report the following year, it was still unable to explain the Richmond crash and many similar accidents, but it was encouraged by the fact that no loss of main rotor control accidents had occurred since the SFAR was enacted.
Although the Safety Board cannot conclude that the operational changes will eliminate all in-flight rotor strikes, the absence of such accidents since these actions were implemented suggests that they have been effective,” the NTSB wrote. “The absence of such accidents also supports the proposition that most of the accidents were caused by large, abrupt control inputs and the corrective actions taken should help prevent such accidents.”
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.”