What happened
During the flight, the pilot flying (PF) initiated a speed increase to 210 knots, a maneuver consistent with company procedures when tracking toward a destination. At approximately the same time the speed was increased, the autopilot began capturing the assigned altitude. This simultaneous occurrence caused the autopilot to revert to pitch mode. Because the aircraft was climbing at an unusually high rate—driven by the use of normal take-off power during departure—the altitude capture occurred earlier than anticipated.
Concerned that the autopilot was not correctly configured to maintain the assigned altitude, the PF disconnected the system to hand-fly the aircraft. Following a reconfiguration of the autopilot modes by the pilot monitoring (PM), the PF attempted to reconnect the autopilot, believing it would successfully maintain the aircraft at 5,000 ft. However, the reconnection attempt was unsuccessful. The PF was distracted by searching for conflicting traffic and did not realize the system had failed to engage, while the PM did not correctly verify the autopilot status. Consequently, LQG climbed through its assigned altitude, resulting in a loss of separation with OKF.
The investigation
The investigation focused on the interaction between the aircraft's automated systems and the flight crew's manual inputs. Investigators examined why the altitude capture occurred unexpectedly and how the autopilot transitioned into pitch mode. The inquiry also looked into the crew's monitoring processes during the period when they were managing conflicting traffic.