Unfortunately PW you appear not to understand the law reagrding TUPE. This is quite different from merely changing your supermarket allegence. The law is there to protect workers on an existing contract when that contract is awarded to another supplier. Therefore ALL Flightline employees who have been involved in the IAC contract must by law be taken on by Eastern. The employer (and the employees) have no option. If a Flightline pilot, engineer, check in staff etc... do not want to be TUPE'd they must resign without any redundancy - and I think that extremely unlikely. If Eastern do not want to employ them then after a specific period of time they must give the employee notice and pay them redundancy.
As I understand it both parties have accepted that TUPE applies and the IAC have insisted that it is observed by Eastern. How Eastern deal with this and the potential issues it will no doubt raise with it's existing employees only time will tell - but I wish them the best of luck.