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



thank you for googling that for me.
The last time i googles something on BT, i was allways re-directed here for the info....so i thought i had learned it was a good idea not to try google when exploring BT......