Airmail travels in a number of different ways:
- in a full freighter aircraft (either in "bins" or bulk loaded)
- as hold freight on a standard PAX service (see the
Royal Mail stickers on the rear of British Airways' A319/320s for instance)
- in a "combi" aircraft, as mentioned by
finncapt (not too common these days, although some larger aircraft have combi versions - Tu-204, B747, ...)
- in a "Quick Change" aircraft (737-300QC, BAe 146-200QC, ...) as done by EAP, Mistral Air, Titan Airways, Jet2, etc. - again either bin or bulk loaded
- in the cabin of a PAX-configured aircraft (bulk loaded); as mentioned, the weight limit is lower, with each mailbag restrained only by the regular passenger seatbelt.
There may be only a few mailbags for a particular destination every day, so a separate mail/freight flight might not be profitable. Unlike express freight, airmail does not usually bring a great deal of revenue, but bear in mind that for the airline, an empty volume on a return service is potentially lost revenue... As to why Air Berlin load mailbags in the cabin rather than the hold, your guess is as good as mine! (Unless of course it was a Dash 8)
Cheers
