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.
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.
As a guy whose flown several different R22s over San Pablo Bay over the years, I couldn't help but think about this one from time to time.
I sometimes wonder if maybe spacial disorientation had something to do with it, as the water in that bay is so still and glossy that I always had to force myself to not look down, as it would mess me up pretty quick.