***Not a teacher***
I believe both are acceptable usages, with almost identical meanings, but slightly different viewpoints.
'To promote' - draws attention to the fact that this is an aim of the airline company, and views it from a 'distance'
'Promoting' - draws attention to the practise of promotion, and views it more from 'inside' - i.e. you are more aware of the airline company putting the promotion into action
Interested in Language