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If you go to http://flightstats.com and enter the airline and flight number, you can select a particular day, then select the event timeline tab. On that, you may see an aircraft equipment designation (AEQP), e.g. 77W (Boeing 777-300ER) for QR557 yesterday and 788 (Boeing 787-800) for the QR1369. You can see this for a few days prior to today but have to register an account to see earlier dates.
For future dates, you can usually see what the anticipated aircraft is when you allocate your seat or buy your ticket, but this is subject to change. Viewing the flights on your route might show you how likely the anticipated aircaft is.
Upvote:8
The plane being used for a trip will almost always stay the same; the airline will have made precise calculations on how to spread out their fleet, and a change would cause issues with load capacity, passenger capacity, pilot training and is even potentially limited by whether an airport is able to support the plane (the Airbus A380 being an obvious example for this).
TripAdvisor's Seat Guru matched with Flight Aware can be used to find the model of plane that has historically flown under a given flight number (and even future scheduled flights) and then match it to a seating plan. It appears that Seat Guru has even recently updated to allow you to do this from one single site, but I can't do that without knowing when you fly.
Using these sites, we can discover from your flight data that the code QTR557 relates to a Boeing 777-300ER (twin-jet). If comparing this to the Qatar fleet in Seat Guru, we can then see the seating layout for that plane. Your only issue here is that the Qatar fleet has 3 different seating layouts for the B777-300; using your flight details should take you to the correct one however.
I have flown in the Qatar 777 from Doha to Bangkok, and can inform you that I prefer Quatar Airway's Airbus 330s (I prefer the 2-4-2 layout), but the 777 is still a great plane to fly in. Do try and avoid seats A, D and K with the media computer in the footwell however; these are recorded on Seat Guru.