Even though I know how to do this , as I test I entered these terms in google. "john" "aircrack-ng" and "stdout" and guess what. The first entry came up with this.
Let me google that for you
You are never going to get anywhere with backtrack or linux if you cant use google. #failbus
john <john options here> --stdout | aircrack-ng -a 2 -b <mac> -w - /path/to/mycaptureddata.cap