Whatsapp will handle it via wifi. (if your recipients have installed Whatsapp)
iMessage
gogo
Google hangouts
Viber
... (some USA based cell carriers, on select systems.)
search flyertalk for some hints to find the gateway / portal on certain airlines. Often you want to initiate the connection / texting recipient as soon as you board (while on the ground). There are some tricks to helping assure your success (and it is never 'assured'...) Airlines seem to have significant IT challenges (too...
)