Q. I have read that some airlines, in the event of a flight delay or cancellation, will agree to fly passengers on a competing airline if that airline can arrange to fly the passenger to his destination sooner than the original airline. However, one thing that I can’t seem to find out is with whom exactly do certain airlines have agreements in case you need to be put on a competing airline.  Based on United Airlines' Rule 240 in its contract of carriage, it says they will transport you on "another carrier or combination of carriers with whom UA has agreements for such transportation."

However, we never get to find out who these “other carriers” are. Does it vary by route, by airport? I imagine since United is in the Star Alliance, those partner airlines are included. However, what others exist?  

My wife and two infant girls (along with an entire United 757) were cancelled on the tarmac in San Diego due to a maintenance issue. I asked to be rerouted on Virgin America or Southwest, which flew the same route we were scheduled to fly on.  The response was always “sorry, we don’t have an agreement with them” though United did offer to fully refund my money and then use that to buy a “walk-up” last minute fare on Southwest.

So where can I find information about which airlines will work with whom?  

A. First of all, most airlines either never had one or have done away with "Rule 240" in their contracts of carriage. Newer airlines such as Virgin America and JetBlue never had a Rule 240, which was a regulation required by the Civil Aeronautics Board when airlines were regulated by the U.S. government prior to 1978. United and Alaska Airlines are the only airlines I know of that still have the rule printed in their contracts. In general, the so called "legacy" carriers that flew during the era of airline regulation (American, US Airways, United, and Delta) are the only domestic U.S. carriers that regularly "interline" with each other and offer accommodation on each other in the event of a flight irregularity, but that doesn't mean that they will always rebook passengers on another airline. It doesn't hurt to ask, no matter what airline you're flying, however, since exceptions are sometimes made.