From an engineering point of view, planes would be close to the bottom of the list of fossil fuel uses to transition to hydrogen/other "X-fuel" [0].

Still, my cynical part expects that we will see renewable-powered flight as a relatively early (premium) use of power-to-X for the sole reason that it will allow the affluent and/or powerful to fly while claiming to be climate neutral.

[0] Our society uses insane amounts of hydrogen. One fundamental use is for synthetic fertilizer which supplies around half the nitrogen input for agriculture, without which we would be hard pressed to feed the world. This hydrogen is currently produced from fossil fuels.