wrote in message
...

Are you sure they're saying "clearence available upon request" and not
"clearance on request"?

Experience and context has told us that "clearance on request" means the
tower
has requested it, but it hasn't come through yet. Having said that, in
plain
English it means they have the clearance whenever he would like to request
it.

Well, plain English would be "Clearance RequesTED".