Interesting back-up info from flyertalk.com’s forum (in spoiler box due to length). Explains a lot about how the airline got their backs so hard into the corner with the air crew transport:
[spoiler]Even if we don’t get to know from the airline or unofficial info from those involved what happened exactly, we can make inferences from public info. Not sure if somebody else has posted this yet but this is what I found.
• This happened on Sunday 4/9.
• On 4/9, Flight UA4600 from DEN to Louisville was delayed 130 minutes, arriving at 12:31 AM Monday instead of 10:21 PM Sunday, having left at 8:04 PM instead of 5:40 PM. This was on an Embraer 145 aircraft with registration N14153 and operated by Trans States Airlines.
• That would mean that the UA4600 crew would not be allowed to show up for their next flight before 10:31 AM Monday, so probably not allowed to fly before 11:30 AM or so considering the mandatory rest time as well as the flight planning and preparation time in advance of departure.
• On Monday 4/10 there were UA flights scheduled out of Louisville on Embraer 145 aircraft at 6:05 AM, 7:30 AM, 9:20 AM, 2:45 PM and 3:45 PM Eastern. All operated by Trans States Airlines.
• Most likely the crew of UA4600 on Sunday was scheduled to operate the 9:20 AM flight on Monday, because even with their delay they would have been allowed to operate the later ones, and if they had been on schedule they would not have been allowed to operate the earlier ones.
• For a crew to operate the 9:20 AM flight on Monday they would have had to be in Louisville at 10:20 PM or so.
• The flight of the incident in the news, UA3411, was scheduled to leave ORD at 5:40 PM Central and arrive at 8:02 PM Eastern, though it actually arrived at 10:01 PM.
• The incoming delayed flight, UA4600, was not coming from cascaded delays from earlier flights, because the aircraft N14153 got to Denver on time as UA4680 having arrived from Grand Junction at 4:53 PM Mountain, and all earlier flights for this aircraft were on time as well. So no way for the airline to predict early enough that UA4600 would get delayed so much.
• UA3411 was scheduled to depart at 5:40 Central, 4:40 Eastern: 9 minutes after UA4680 landed at Denver (4:49 Mountain so 5:49 Central).
• However, UA3411 estimated departure was changed at 5:32 Central (4:32 Mountain, so 17 minutes before UA4680 landing). It was changed to 6:00 PM instead of 5:40. Afterwards the estimated departure time was pushed more and more many times.
• There was another Chicago-Louisville flight on AA a bit later than UA3411 (6:40-8:54 PM).
• Though there were delays at DEN around the scheduled time for the departure of UA4600, there were also many flights on time and the majority of delays were short, so unlikely that there was bad enough weather to massively delay all flights.
So if I’d had to bet on the sequence of events on the operations side I’d bet on this:
• UA4680 crew from Grand Junction to Denver found something wrong with the aircraft while en route
• They notified the airline en route so aircraft checks could be done before the next flight for this aircraft, UA4600 Denver-Louisville, and perhaps let the mechanics begin preparing for these checks before landing
• UA4600 was consequently delayed
• It became evident that the UA4600 crew would not be able to operate the Louisville-Denver 9:20 flight the next day
• An ERJ145 crew from Trans States Airlines that could operate that 9:20 flight from Louisville (UA4766 to DEN) was found to be available in Chicago
• With so little time to spare UA considered that sending this crew to Louisville on AA was more risky schedulewise than sending them on their own flight, or they might have considered it but the AA flight was full
• UA found that UA3411 was the only acceptable way to get an ERJ145 crew from Trans States Airlines to Louisville on time
• UA decided that inconveniencing 4 passengers from UA3411, who in case they had an urgent necessity to get to Louisville that same night could get there by road in 5 hours, would be less worse than inconveniencing up to 70 passengers or so from the 9:20 flight to Denver, making most of them lose their connections and having to rebook them on other flights, compensate, refund, etc. as well as cascading the delay to the rest of the flights this aircraft had to do on the day, with the same consequences for every flight
• UA3411 was already boarding or boarded when the UA4600 delay was known and a decision was made, and the aircraft was held so the crew they found for the 9:20 flight could get to it
• Then happened what we all know from the news
• UA3411 departed with a delay caused both by waiting for the UA4766 replacement crew and the incident, but arrived on time for them to get the mandated rest
• On Monday, UA4766 left for Denver on time at 9:20 with the crew that came in on UA3411
Apologies if this seems confusing but the story involves different crews, flights and aircraft with simultaneous events in different time zones. Hope it makes sense from an operations point of view that if events went this way the operational decision to take 4 passengers out and put 4 crew in instead was reasonable and correct not only in economic terms of the costs involved in the alternatives but in order to minimize impact on passengers as well; and also that the airline could not have known, in advance of the UA3411 boarding, that they would need to reposition crew there last minute.
Sources - Flightradar24, FlightStats, ExpertFlyer, FlightAware [/spoiler]