TTSN,
On November 3, 1987, at approximately 1338 Pacific standard time, N8475A, a Robinson R22 Alpha operated by Helicopter Adventures, Inc., broke up in flight during a dual instructional flight near Moraga, California. The CFI and commercial helicopter pilot student, who was preparing to revalidate his CFI helicopter certificate, were killed. Witnesses reported observing the helicopter hovering at 400 feet agl when they heard the engine noise suddenly decrease, and a loud pop. The witnesses said they observed parts, "like sparklers," fall off the helicopter as it fell to the ground. The flight had originated 8 miles southwest, in Concord, California, about 1230, after having been refueled with 100LL fuel.
Hovering, ( no airspeed) 2 up and full fuel.
They are looking at rotor divergence at speed when light.
Not the case this time.
Every reported case states the machines were being opperated well within Franks limits, in mild conditions.
Quite a few witnesses state a tailwag or left right left right rolling before breakup, but none say a wild left roll ( low/0G) before breakup.
Sounds more like a main rotor phase issue, followed but a serious -AOA input to the MR.
Just an uneducated guess by an interested party.