Originally Posted by
MADPhil
This is the case, but you add the KTN on the AA site when you have the AA PNR. You will not get Pre-Check on the BP if it is printed by BA but you can reprint the BP on an AA kiosk at the port of entry.
Not so easy when leaving T5, but I'm based at MAD and will just pop over to the AA desks in T4 when I have an IB issued boarding pass for an onward connection and I get the precheck just fine.