SAS,
The first accident was in the Falkland Islands in 27/2/1987.
It’s been discussed here before but unfortunately the BOI report has been archived by MOD so I can’t find it.
That aircraft was on a post maintenance check flight and nosedived in almost vertically from about 1,000 feet, with the loss of five POB, I believe.
The second was the more well known and much discussed Mull of Kintyre accident.
The first accident made me determined, as the father of a young family, to avoid getting posted to Chinooks if at all possible. I terminated my commission less than a year before the second accident, which made me think I’d definitely made the correct decision.