Normally the 2-man team AAIB send out to a site consists of one Engineer and one Pilot. What I meant is that there are a few specialist Engineers there who do lab or ADR analysis and don't get involved in that side of thing - but they are in the minority.
The pilots who fly with the airlines do enough to stay current in a right hand seat - usually a couple of days a month, and they aren't employed by the airline. It is entirely so that AAIB keeps professional knowledge current, they are firmly there as inspectors first, pilots second.
G