Aeronut is correct.
Assuming AFPS 75:
Resettlement Commutation Resettlement Commutation means the surrender of part of a pension to obtain an additional tax-free lump sum, with the restoration of the pension in full at age 55. This scheme applies to officers and other ranks who gave service on or after 31 March 1978.
AFPS Commutation document
http://www.mod.uk/NR/rdonlyres/9DE7D...ommutation.pdf
You have to be nuts not to commute as much as you possibly can.