Translation:These airplanes are taking off, and those are landing.
szállni is difficult to translate directly and it's meaning changes with the coverb.
leszállni - land (e.g. plane, bird, helicopter)
felszállni - take off (e.g. plane, bird, helicopter)
kiszállni - to disembark / get off (e.g. from a bus or train)
beszállni - to board / get on (e.g. from a bus or train)
That's correct, a minor thing to note: száll in isolation usually refers to gliding (without propelling): birds, gliders, paper airplanes, frisbies, etc. Also, things blown by the wind: dust, fog, smoke...
Eg. only the more general repül can be used for "flying in (one) place"