I've been reading a 1910 Tom Swift book - Tom Swift and his Airship, in
which the dirigible is also outfitted with "planes" - i.e., wings.

And of course that's where monoplane, biplane and triplane come from,
though I confess I'd never connected the two.

So, can anyone tell me when an aeroplane's 'plane' started being
referred to as a wing?

Well, Tom Swift books are fiction-- figments of the author's
imagination.

I believe the Wright Brothers called them "wings." They spoke of
achieving lateral control by means of "wing warping."

vince norris