Okay thanks. I wondered if it was plane-by-plane or flight-by-flight. Guess it's the former. 50/50 might be enough to be worth carrying the adapter. Is it just a money-saving thing or do they disable them when they need service? Not that it matters.

Is United two-prong or one-prong audio? SeatGuru doesn't say and I remember it being the former (AA being the latter).

Thanks for both your recent answers to my questions.

=aw