Brian
We have been here before I believe; the BOI is the Convening Officers BOI. As, however, the AOC has many other matters to attend to (including other BOI's or Unit Inquiries) he appoints an Investigating Board to inquire into the occurrence and produce a report containing the salient facts. It is normal to invite the Board to give their opinion in respect of human failings. That opinion is in no way binding on the AOC; or on the CinC who will subsequently review the proceedings.
As you will be aware, this particular BOI was also reviewed by the then CAS who did not see fit to alter any of the findings.
Best wishes