On July 4, 1998, at 1245 Pacific daylight time, a Cessna 170A, N1296D, registered to and operated by the pilot as a 14 CFR Part 91 banner towing operation, collided with the terrain just after picking up a banner from the Renton Municipal Airport, Renton, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was substantially damaged and the airline transport pilot was not injured. The pilot reported that he was cleared by the tower for a low approach to pick up a banner. The pilot stated that after the banner pick-up, he applied full power and performed the "abrupt climb (normal procedure)." The pilot stated that he leveled the airplane at about 50 feet above ground level when the airplane experienced an "abnormal sink." The pilot failed to release the banner and the airplane continued to lose altitude. The airplane collided with the ground hard on the main landing gear. The left main landing gear collapsed and the left wing contacted the ground. The pilot stated that the winds were shifting and that he might have encountered a downdraft. A witness reported that he observed the airplane dive towards the ground in a steady descent. The witness stated that the airplane continued its descent with no pitch change and collided with the ground "square on the mains." The witness reported that the winds were variable and gusting.