Aiya, it is not illegal to photograph aircraft in the air,
however, I believe it is forbidden to photograph whatever is in the
air force base without prior approval from the Mindef.
Whatever planes take off from the AFbases are visible to everyone,
citizen and foreign spys alike. If RSAF wants to hide some top
secret weapons, they have to do it in the dark, or covertly
overseas. Singapore is too small. Even the US Nevada Edward's AFB
cannot hide all the strunkworks (aka stealth projects).
There is a difference between spotting them in the sky in any part
of singapore or tom-pepping into any of the air force bases
especially Tengah and Paya Lebar. As for Changi AFB, whatever is
happeneing there is so visiable when you land at Changi
Airport.
I once saw the air base having a small fire (can't really tell what
happened), except there was a couple of fire engine trucks and
people (fireman/air base personnel??) trying to put off the fire
(could be an exercise though)... I saw this as I was touching down
in Changi... 
