During the war these bombers were initially highly successful due to the low-level bombing tactics used, with the air force chief-of-staff of the Zhili clique, General Zhao Buli (趙步壢) personally flying many of the missions. However, on 17 September, returning from a successful bombing mission outside Shanhai Pass, General Zhao's aircraft was hit by ground fire from the Fengtian clique in the region of Nine Gates (Jiumenkou, 九門口) and had to make a forced landing. Although General Zhao made a successful escape back to his base, the bombers subsequently flew at much higher altitude to avoid ground fire, which greatly reduced their bombing accuracy and effectiveness.