To: hover Lover
As surprising as it might seem, I can't say Robinson messed up. The use of incorrect material is quite common in the aircraft industry. It shouldn't happen but it does.
I have to assume that the process engineers specified the material used to make the yokes and that they considered all of the environmental and stress related conditions in the selection of the material. What this seems to be, is a quality problem at the manufacturer of the yokes assuming it was a bought out spec part.
However, Robinson makes most of the pieceparts that go into the makeup of the 22 and 44. If it was in fact made by Robinson and assuming that the spec for the base metal was very tight, then they did screw up, as they made several mistakes in their material selection for that particular batch of parts and their quality control in allowing the use of non spec material. But, it happens everywhere. Sometimes it's a faulty process or faulty manufacturing.
As an illustration,when I worked in Iran, Bell Helicopter shipped 43 B-214s to our facility for assembly and checkout. Of the 43 helicopters, we found 18 that had improperly heat treated gears in the transmissions. Bell built the trannies and made the gears.
Sikorsky improperly overhauled two S-61 rotorheads and caused the crash of two LA Airways Helicopters with all passengers and crew lost including the grandson of the owner of the airline.
It happens but it shouldn't
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The Cat