
AFAIK, Except for some Exit and bulkhead rows,
ALL aisle seat armrests raise in
WHY on ANY Qantas 747, 767, A330 & 737.
As
bigjobs posted there is a "little catch/switch towards the back of the aisle armrest". It is underneath the armrest, about 5cm from the base/hinge. It is normally a blunt alloy hook protruding about 1cm. You hold the armrest steady, slide the hook forward and lift the armrest up. (This can be fiddly as the hook is held by a strong spring. Don't try to lift the armrest before sliding the hook forward at least 1.5 cm).
I have often used this to slide out from underneath my tray table.