Hey @Cherrypie,
As long as your are on airplane mode, you aren't connected to any network and therefore no usage would occur.
Rest assured, roaming is only billed when an outgoing usage is validated through your Fido SIM card, such as texting, using data, answering/placing a call, retrieving your voice messages.
You can learn everything you need to know about Fido Roam right here.
We have plenty of other articles on roaming, feel free to browse the Community. 
