Do airlines allow passengers to take an unrelated trip within a return flight?

Question

I'm planning to visit Japan and then Mongolia in September.

An airline I'm thinking of using to get from Australia to Japan doesn't offer flights to Mongolia itself, though it's possible one of its partner airlines does.

Are there any airline rules against booking a return trip from Japan to Australia, and ordering a return trip with an unrelated airline that goes from Japan to Mongolia and back to Japan? In case it's dependent upon carrier, I'm looking at using JetStar to go from Australia to Japan.

The main concern I have is that the aircraft carrier may look upon it with suspicion in the same way as fuel dumping or hidden city ticketing is.

Answer 1

There's no problem with this. People do it all the time. In contrast with other "creative booking" techniques, you're not abandoning any part of any ticket.

Answer 2

What you describe is perfectly fine (it doesn't violate any airline terms and conditions). However, be careful to allow sufficient time between the flights. In your case, you'd want the separate Japan-Mongolia-Japan flights to return you to Japan with ample cushion before your Japan-Australia flight. Otherwise, if your Mongolia-Japan flight is delayed or cancelled, you might miss your Japan-Australia flight. I'd suggest at least an overnight between your Mongolia-Japan flight and your Japan-Australia flight.

Another option that in some cases is less expensive is to book an open-jaw and a one-way ticket, for example, Australia-Japan-Vietnam and Vietnam-Australia.