Originally Posted by
riku2
there are normally two PNR's for this kind of flight. If you go to BAEC and try to do the seat reservation it will show the second (AY) PNR just before redirecting you to the AY site.
At this point go to AY site yourself and check the booking using that PNR. You should get the seat map.
This is how it works for me (BA ticket, BA flight number but AY operated) and it even recognizes my gold card and allows seat selection free of charge.
There aren't two PNRs anymore. About a month ago AY decided to get rid of their PNR and just rely on the BA issued ones - for those who had a separate AY PNR they will find it no longer works on the AY app or AY MMB, however you should be able to add the BA PNR to the AY app.
One option is to go on AY MMB using the BA PNR and use the chat function to select seats.