In a moment - momentary - momentarily
Momentary is an adjective, meaning 'for a moment', in a repeated sense: 'As we drove through the wood, we caught momentary glimpses of the castle'. This translates to the adverbial usage: 'Momentarily we could see its roof in the flashes of lightning'.
It does not mean 'in a moment' or 'soon'. Don't use it that way - unless you are in America, where, AWE believes, 'I momentarily expect to be promoted' means 'I expect any moment now [to hear the news of] my promotion'. This isn't acceptable in British academic circles - unless, of course, your teacher happens to be an American. As always, the good writer will adapt her/his material to suit the audience.
The pilot of an aircraft who announces that 'We will be taking off momentarily' may mean to reassure the passengers that they will soon be in the air; but pedants, and all careful writers of English, will worry that they are to proceed in a series of short hops, like an early aeroplane struggling to take off, and will not be reassured at all.