Alex - pondweed probably has problems with his sexlife and feels the need to take it out on people in anonymous forums...... 'twas a great question and one that caught me out as a turboprop F/O once when taxiiing out!!! I asked my skipper what the open hatch was on the 757 up ahead and he didn't know so we called tower and "helpfully advised the crew" who sat there for a few minutes thinking about it and it turned out to be an outflow valve. So there you go - it's even caught out experienced turboprop crews in the past so yes it WAS a good question!
