Why is it a slur? Lets take this BBC news story as a line in the sand.
AAIB ready to publish report on Clutha helicopter crash - BBC News
In November 2014 the statement from the AAIB was:-
The statement said: "This, together with further examination of the aircraft and subsequent tests, has now been completed, to the extent that the investigation team may reach its conclusions.
"Several weeks are still required to complete the draft final report and it is expected that this draft will be circulated to 'interested parties', as defined by the Regulations, for comment early in 2015.
"The final report is expected to be published in the middle of 2015."
So describe the process that now follows and maintains the highest level of integrity that befits the AAIB. I struggle to see how an independent board investigating the facts of the matter need to circulate drafts...
Either the team were able to reach conclusions or as it has published many times in the past it is unable to conclude matters.