Quite some time ago there was a thread on the earliest known auxiliary power units (APU) in aircraft (here:
viewtopic.php?f=3 ). For the purposes of the discussion, I'm suggesting they need to be on-board, fixed engine starting systems, with other possible tasks.
Recently, watching a bit of period film at the Airways Museum film night, was saw a four engine airliner being tickled to life with an APU starter. It was a Handley Page HP-42, so puts the date back to 1930.
http://en.wikipedia.org/wiki/Handley_Page_H.P.42Can anyone suggest an earlier APU?
Regards,