Is it ok or common to use "are you able" to make a request instead of using "can you" or "could you" when you know that the person is able to do the task and you are just requesting something? I suspect that "are you able" may sound as if the ability of the other person is in doubt. If the intention is just to request politely that the other person do a routinary task, is it ok/common to say "are you able"?
"Are you able" is much less used than "can you": ngram; however, in the domain of plain possibility no distinction is made between the two in OALD, Grammar Point can / could / be able to / manage.
"Can" or "be able to" are used to say that something is possible or that somebody has the opportunity to do something: Can you/are you able to come on Saturday?
It can't be used at all for requests.
Can you feed the canaries while we're gone ? ≠
Are you able to feed the canaries while we're gone?
Will you be able to feed the canaries while we're gone? (That is not a request, but the plain asking of whether it is possible to do the little task.)
Addition prompted by this comment (user gyerena)
This feeling that the ability of the person is in question when using "able" can become more of a certitude after taking in the more penetrating definition in the SOED, which I should have included. It indicates clearly that "able" goes beyond "can" and in what way.
II actively. 4 Having the qualification for, and means of, doing something; having sufficient power (to do); (pred. followed by to do esp. used with parts of be to supply the deficiencies of can)
No, it is not as common to say "when you are able", in fact it is comparatively rare, and this ngram shows that; however, this rarety is not to be imputed to the would be fact that it is not proper but rather to the fact that it is more suited to another context which happens not to have as great an incidence as those for which "can" is proper. Whereas the OALD definition speaks of possibility and opportunity the SOED mentions (correspondingly) "means"; time, for instance can be viewed as a means and so using "able" in such a context where time is the determining factor is not wrong but it is much less usual.