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

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.
Best Answer
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.
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Can you take only the return flight?
The practice you're referring to is known as \u201cthrowaway ticketing.\u201d Most of the major airlines, including Alaska, American, Continental, Delta, Northwest, and US Airways, prohibit throwaway ticketing (or \u201cthe purchase and use of round-trip tickets for the purpose of one-way travel only,\u201d according to Continental's ...Can one airline book you on another?
It is also possible to request a seat on another airline if a seat is available. "However, there are no federal regulations requiring airlines to put you on another airline's flight or reimburse you if you purchase a ticket on another airline," the DOT says.Does return flight mean there and back?
What is a Return Ticket? A return ticket is a flight to a destination and back to the original departure point. For example: Brisbane to Sydney and return back to Brisbane.Can you miss a flight on purpose?
While it may come as a surprise to incredibly conscientious travelers who typically arrive at the airport three hours prior to their scheduled departure time, some travelers do in fact miss flights on purpose. Though not common, this can be done as a way to score cheap flights.Watch Flight Attendant Bully a Woman, Kick Her Off Plane
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