Normally the statute of limitations is five or six years (I think it's different between Scotland and the rest of the UK). The reason for the limitation is that if your employer asks for money back, you obviously should be able to defend yourself, for example by proving that you never received that money. After five or six years it is assumed that you wouldn't be able to provide any such evidence, so nobody can ask for the money back anymore.
That's not specific to overpaid wages but quite general.
In addition there is the question whether the pilots should have known they were overpaid. For example, I'm quite happy with my salary, but if it was less, I would look for and find a different job that pays better. If the company claimed in five years time that I had been overpaid all the time, then I would say that if they had given me the "correct" lower payment, I would have found a better job elsewhere, so asking me to repay the money seems quite unfair.
(Why do you need to defend yourself? Maybe your salary was £3,000 per month. Someone in the right position records that they are paying you £4,000 but puts £3,000 into your account and £1,000 into their own. Then that person has an accident and their replacement finds that you were overpaid according to their records.)