Airlines can be fairly lenient as well. A colleague with his familywas heading off to Nigeria from LHR last week. They are dual British/Nigerian Nationals.

One of his children could not find his British Passport on the day of the flight. (He's not a baby and must be in his Mid 20's!!!)All he had was his nigerian passport which had expired several years ago.They decided to try their luck at LHR and were allowed to fly without a valid passport. (Using the expired Nigerian passport!!!)

And I thought that you could'nt fly internationally without a vaid passport