What happened
While descending toward ALDIM, the flight crew accepted an air traffic control request to transition from an overflight to a straight-in approach. To meet the requirement of crossing ALDIM between 3,6 and 4,000 ft, the crew utilized speedbrakes, flaps, and slats to increase their descent rate. They also utilized the flight level change (FLCH) mode to maintain a speed limit of 210 kt.
As the aircraft approached the waypoint, the crew extended the landing gear to further increase drag. Because the aircraft was in FLCH mode, the engines did not automatically compensate for the additional drag during a turn, causing the aircraft to pitch nose-down to maintain 200 kt. During this period, the pilot flying inadvertently changed the vertical control mode. The pilot monitoring, who was focused on monitoring terrain clearance, did not notice the change.
As the descent continued, the flight director transitioned to an altitude select mode to level off at 3,300 ft. The crew failed to detect this automation change. In an attempt to stop the descent, the pilot flying selected a higher altitude, which inadvertently disarmed the altitude select mode and caused the flight director to revert to a flight path angle mode of -13°.
This resulted in a rapid descent of 2,973 ft/min, reaching a minimum altitude of 1,381 ft AAL. The crew eventually disconnected the autopilot and raised the nose to correct the situation, eventually selecting a lower speed which allowed the autothrottle to transition to climb power.