I retired nearly 10 years ago and the rules may have changed... Back in the day, as commissioned aircrew, you were paid your standard rank salary plus flying pay which was a considerable proportion of your overall pay. Your initial contract with the RAF was until age 38 - if you had been picked up for promotion to Sqn Ldr and beyond, before age 38, then this extended to age 55. However - if the RAF wanted to keep you after age 38 then you could sign on to be "Specialist Aircrew" which again extended your service to age 55 but instead of extra pay going onto your salary, the increments went on to your flying pay. This was fine until you retired, because your pension was calculated on your salary and your flying pay was not considered. However, the flying pay was a regular taxed payment, and therefore under EU rules, it should have been added into the pension calculation. The result was the PAS spine which then current SA aircrew could sign into (which is what I did). In effect, you no longer received flying pay but it was "converted" into normal standard salary, and it is this that has "inflated" the rank pay and given a really good pension - especially on the top increment. (Before comments come in - this is a somewhat simplified version - it was a little more complicated than that).
The other change between PAS and SA was the promotion system. Again, back in the day, the SA spine could lead to promotion to Sqn Ldr with no real change to circumstances. It worked on a "dead man's shoes" principle and was quite common - if you were promoted then you remained as SA. However, with the PS Spine, you remain on the General List promotion system and if you do pick up Sqn Ldr then you go back into general List with everyone else. (However, General List Sqn Ldrs can apply to join PAS, but this is not always forthcoming).
The bottom line - PAS is a good salary for the rank that you hold. You will not get the "automatic" sqn ldr that used to happen with the old SA system. The final pension is good, but at the expense of a more senior rank.