[Tommy]

I have one comment and a story on this topic. First, for multi-engine aircraft, the weight versus air density calculation is done to assure the plane can keep flying if one engine quits at the most inopportune moment during takeoff. As long as everything works as planned, a few pounds won't be noticed.

Second, I was an instructor pilot working in a Runway Supervisory Unit controlling local air traffic at an Air Force pilot training base when a visiting KC-135 tanker taxied out to depart. We handed control of the runway back to the main control tower so they could handle the tanker. All the local aircraft were directed away from the runway and we heard the tower clear the tanker for takeoff. The big tanker taxied on to the runway and just sat there. After a minute or so, we called the tower to find out if there was a problem. We heard the tower ask the tanker pilot if there was a problem. He replied, "We calculated our fuel load based on the forecast for five knots of headwind. We're waiting for that wind." I glanced over at the windsock and saw it sitting lifeless in the still air. About a minute later a passing breeze stirred the sock and the tanker's engines started to spin up. The sock was still flapping a little when he released brakes and started the slowest takeoff roll I've ever seen. I saw the nose of the tanker begin to rise with about 1,500 feet of runway remaining, and he lifted off with about 1,000 feet of runway remaining.