two major problems were found and corrected during testing. the first was a problem with air circulating around the wing when operating at low speeds close to the ground, which had a serious effect on the spreading of the jet flow through the nozzle. this led to flow separation near the flap, and a decrease in effectiveness of the usb system. in response, boeing added a series of vortex generators on the upper surface of the wing, which retracted when the flap was raised above 30°. additionally, the tail surfaces were initially placed well aft in order to maximize control effectiveness. this positioning turned out to interfere with the airflow over the wings during usb operations, and a new tail with a more vertical profile was introduced to move the elevator forward.