It is impossible to answer your question without knowing all the details, however, here's a guess:
Upon arrival from PHX AA825 was sked to depart A23(the Narita gate), then moved to C3.
Gate changes happen all the time (BTW, no gate is a specific destination gate, just what AA
prefers to use if all goes well). Why can be maintenance issues with planes (not necessarily the inbound or outbound acft, but virtually any plane within an hour or so of DFW
), terminal equipment, ground equipment, etc.
At C3, I overheard the gate agent tell the captain the aircraft had been inoperable and at the gate since 900am with a tank balance problem.
Which means the plane is currently
not released by maintenance for flight, however...
We boarded F, groups 1 and 2...
This indicates maintenance folks fully expected to have the problem fixed in time to depart using that airplane. Fuel tank imbalance problems are often fixed on the ground by redistributing the fuel to within proper limits [MD80s do not have tank-to-tank transfer capabilities except when on the ground]. On a short flight [DFW
] I would expect maintenance to rebalance the fuel load and we would depart.
...and were told to get off, the plane was not flying and proceed to gate C21.
Oops, the problem was not a simple fuel imbalance, but rather something more serious and/or another unrelated problem [gate agent didn't know about?].
Why would we board a plane declared inop?
Because it was not "declared inop" prior to or when you boarded. AA
maintenance provides "decision time" to passenger service [airport & gate agents] and Flight Ops [pilots & F/A's] when they do not know if a plane will be usable or not. That's the time maintenance says it will make a final (sometimes not so final) decision if they think they can fix the problem or not. Boarding proceeds based upon maintenance saying when they'll have the problem fixed, but sometimes the "fix" can be worse than the original problem.
I overheard 4 or 5 passengers venting that AA has been doing cancels, rebooks, standbys due to MX issues.
We hear this a lot, but most folks don't fly those planes day-in and day-out. AA
averages 1-3 cancellations per day system-wide (out of 2,400+ daily flights). So no, there are not that many cancellations on a normal day; however, when a flight is cancelled, that creates a chain reaction of rebookings, standbye's, etc., etc., etc., and it becomes a major incident for those folks affected (understandable, but not a regular event by any means).
Is this normal? Wouldn't the captain found this out from OPS?
No and probably not [he's a lot closer to the involved maintenance folks than S.O.C. will ever be]. A little pro-active Captain's involvement, especially in keeping customers informed, is never a bad thing --but many captains don't like talking to people who they think might be angry at them. :-(