The Zveno-3 in which TB-3 carried two Grigorovich I-Z fighters under the wings presented a different challenge — the I-Z was a monoplane with fixed landing gear which touched the ground while it was suspended under the mothership. To accommodate the uneven ground during takeoffs, the fighters were attached via a floating frame that permitted vertical movement relative to the TB-3. However, immediately after takeoff the I-Z pilots had to push forward on the control sticks to lock the frame in the fixed bottom position — if the fighter-bomber was not rigid in flight, the bomber became extremely difficult to control. During one of the test flights, I-Z pilot Korotkov incorrectly timed the locking maneuver and the lift generated by his fighter broke the docking frame and crashed him into the bottom of the mothership's wing. As the bomber came in for an emergency landing with both fighters still attached, the slow landing speed of the TB-3 resulted in loss of lift for the I-Z which fell away, killing Korotkov. This was the most serious accident of the entire Zveno program despite the inherent complexity of carrying as many as five aircraft, performing mid-air launches and dockings, and damage to the motherships by propellers and landing gear of the fighters. Tests with various combinations of aircraft determined that top-mounted configurations presented the most difficulty in docking due to turbulent airflow coming off the mothership's wings. The problem of safe underwing attachment was solved with the appearance of the Polikarpov I-16 fighter with retractable landing gear. This permitted the use of the same rigid mounting frames as for the top-mounted aircraft.