Originally Posted by
FlightlessParrot
I just booked flights on Air New Zealand. The seat allocation pages clearly showed which types were supposed to operate each flight, and distinguished between 777-200ER and 777-300ER, which mattered to me because of the arrangement of seats. In the other direction, I chose the A321neo, which meant a slightly different class of booking. People book online a lot these days, and it probably suits the airlines to get seat allocation done in advance. In these circumstances, it is possible to avoid an aircraft which is faintly dodgy, even if one recognizes that an exceptionally unlikely event is only made slightly less unlikely.
Yes, you're right, I know. DIY booking and seat selection systems give you that opportunity. But then again, within the airline, the ops centre (Maintrol or whatever) and its maintenance provider you have other priorities and forces at work, that mean that on the day of your flight the model/variant that was supposed to fly won't be available so another type will be substituted. There may have been a catastrophic delay somewhere in the system that meant that aircraft and crews must be shuffled around, or an AD may have required a hangar visit for the whole fleet, or scheduled maintenance may mean that the aircraft that would have operated is on a major check on the day of your flight and a substitute is pulled in, or any one of many other disruptive factors.
In short, the day's Mayfly (or its modern electronic equivalent) never looks quite like the programme entered into the reservation system 6 months before. So while it's very likely that you'll get the aircraft type and seat you expected, it ain't guaranteed, whatever they may tell you. You'll only know when you walk down the airbridge and see what's attached to the other end of it.