In response to chopabeefer's query, the clause is within the Scheme's Perogative Instrument which can be found on the Defence Intranet/Admin/Armed Forces Pensions page. It reads:
(c) where condition C is met, any day not falling within a continuous period of at least five years throughout which the member’s pay was calculated using the professional aviators’ pay spine;
(condition C is being on the PA Spine)