PA25 pilot experiences momentary fuel starvation during the downwind turn due to low fuel level while towing a glider. A wing rock releases the glider and a quick turn to land on the intersecting runway is made. The aircraft holds 36 gallons of usable fuel and 35.7 gallons were required to fill the tanks.

Narrative

I had performed a total of 12 glider tows to various altitudes during the morning shift. Several of the tows were to higher than normal altitude; several to 4;000 FT AGL and two over 5;000 FT AGL. I knew I was getting low on fuel so prior to tow #11 I announced to the Line Chief that I would go for fuel after that tow. Upon returning the tow pilot that was scheduled to relieve me for the afternoon shift had taxied out to the line when he discovered his tail-wheel was low on air pressure. He asked me by radio if I could take one more tow. The pilot of the glider waiting for a tow was an instructor with a student. He announced by radio that it was a pattern tow. I knew I was low on fuel but I did not do a detailed calculation on my fuel supply. Neither the relief tow pilot; nor the instructor in the glider put any pressure on me to perform the tow. However; knowing that the student had been waiting and would be delayed if I declined; I felt a desire to perform the flight rationalizing that it was only a pattern tow. I performed the tow with the student and instructor on Runway 26. When turning from crosswind to downwind the engine momentarily lost power. I rolled the wings level and the engine power came back up to normal RPM. I rocked my wings to signal the glider to release which he did. At that point I was on a close in base leg for Runway 35. I reduced power to idle; made a normal landing on Runway 35 and taxied to the fuel pump. The glider made a modified pattern to landing on Runway 26. The cause of the power loss was the low fuel; while in the bank the fuel in lower wing tank un-porting the fuel line. The fuel feed resumed when I rolled the wings level. While towing gliders in the Pawnee I use a calculation of 1 gallon of fuel for every 1;000 FT of altitude. In the past this calculation has always been very conservative. After towing for 30;000 FT the aircraft normally uses about 27 or 28 gallons of fuel. I knew I was over the 30;000 FT mark in tows that morning; in fact I was at 35;000 FT. I did not add up the altitudes accurately and rationalized; based on my past experience; that I had a few gallons of buffer. I suspect that I had a slightly increased fuel consumption because of the high tows that morning. After landing I fueled the aircraft with 35.7 gallons of fuel. The aircraft has a fuel capacity of 38.5 gallons with 36 gallons usable. I am solely responsible for this incident. I was complacent in monitoring my fuel consumption. I created internal pressure to take the tow (in fact I should not have taken the previous tow) even though there was no external pressure to do so. For whatever reason; I chose to fly with low fuel rather than disappoint the student and instructor that had been waiting and anticipating their flight. I never thought I would fall prey to this sort of lapse in judgment. The 1 gallon per 1;000 FT calculation works for glider tows in this aircraft. This incident would have been easily prevented by closely monitoring fuel consumption; setting minimum fuel for takeoff and not violating the minimum for any reason.