What happened
During takeoff from O'Hare International Airport (ORD), the aircraft accelerated and rotated normally. At 800 feet above ground level, while performing the climb checklist, the captain discovered that the nose-down trim was non-functional. The first officer checked both his trim switch and the backup system, but neither responded.
The crew decided to return to ORD. Despite pushing both control yokes forward to their stops, the aircraft continued to climb until the crew managed to level the airplane at approximately 8,000 feet. After declaring an emergency, the crew followed the Pitch Trim 1 and 2 Failure checklists, which required lowering the flaps to 9 degrees. This configuration change caused the aircraft to pitch nose-high and become uncontrollable. The captain regained control by having the first officer retract the flaps.
Lowering the landing gear provided slight stability improvements. The crew attempted to reset the trim circuit breakers twice, but the issue persisted. While approaching runway 9R, the crew performed a 360-degree descending turn to lose altitude. Upon deploying spoilers to slow the aircraft, the airplane pitched abruptly nose-high. The crew responded by pushing both yokes forward, advancing the throttles, and retracting the spoilers. During this period, the aircraft banked left at an angle of 50 to 60 degrees before the crew regained control.
Following instructions from approach control, the crew transitioned to an approach for runway 4R. The airplane remained in a landing configuration with the gear down and flaps and spoilers retracted. The crew executed a long, shallow approach and landed the aircraft on the runway.
The investigation
An examination of the spoiler control unit showed that when the command was given to retract the spoilers, the unit failed to send an input to the horizontal stab control unit (HSCU) for one unit of nose-down stabilizer trim. Other examinations of the airplane systems and trim components revealed no anomalies.
Following the event, the manufacturer determined that the horizontal stab actuator was inadequate to move the horizontal stabilizer under all flight conditions. Specifically, if the crew fails to trim the airplane after takeoff before reaching a certain airspeed, air loads on the stabilizer can overpower the trim actuator.