I've always wondered what this was on teh back end of 747's. I heard once that it was a small jet engine that ran the plane's air conditioning systems. Does anybody know for sure? Mistergus July 06, 2005

This is the exhaust of the Auxiliary Power Unit, a jet-fuel engine located in the tail, that runs the aircraft / cabin systems while the big engines are not running. So basically, between shut-down and start-up, but also during flight (don't know if standard or just in cases of emergency). It's not only a 747 that has an APU, but all (large) passenger aircraft. For example, look closely at the tails of 767 and 320. For further details: http://www.hs-powersystems.com/prodmain2.htm Regards, Klaas Sluijs The Netherlands July 06, 2005