What happened
On 7 November 2000, an Embraer EMB-145EU, registration G-EMBN, was performing a scheduled passenger service from Belfast International Airport to Manchester. Shortly after takeoff, the aircraft's stabilizer trim moved into a full nose-up position. This sudden change in trim caused the aircraft's pitch attitude to increase sharply, reaching a maximum of 25.7 degrees nose up while airspeed began to decrease.
The flight crew, which included a training captain and a first officer, faced a highly stressful situation. To manage the extreme pitch attitude, the crew applied significant nose-down pressure on the control columns. The aircraft entered a period of long-period pitch oscillations, known as phugoid cycles, where airspeed and pitch fluctuated significantly. During the climb, the crew operated the pitch trim cutout switches and eventually disconnected the elevator system to split control between the two pilots, attempting to manage what they initially believed might be a jammed elevator.
After the oscillations stabilized, the crew reduced engine power to mitigate the fluctuations. The aircraft eventually leveled off at 7,000 feet. Despite the difficult handling characteristics and the need for continuous manual input to maintain control, the crew elected to proceed to Manchester. The aircraft landed safely and without further incident at 1300 hrs.
The investigation
Investigators examined the aircraft's flight data and cockpit voice recorders, which provided a complete record of the event. The investigation focused on the cause of the uncommanded trim movement and the subsequent flight instability.
Technical examinations of the horizontal stabilizer actuator and the control unit revealed no mechanical or electrical failures that could have caused the trim to move uncommanded. While the aircraft's maintenance computer showed several fault messages related to the stabilizer actuator, these were found to be common occurrences linked to the use of the disconnect handle. Extensive testing of the trim systems by the manufacturer and the operator confirmed that the primary and backup trim systems were functioning normally.
Findings
- The investigation determined that the uncommanded nose-up trim movement was not caused by a mechanical or electrical failure of the trim system.
- The total trim change of 3 degrees occurred in two distinct stages, suggesting that the input was made manually by the handling pilot.
- The first officer's attention was likely diverted by a previous error involving incorrect aileron input during the takeoff roll, leading to a momentary lapse in concentration regarding the trim switches.
- The commander did not observe the EICAS trim display, which would have alerted him to the extreme nose-up trim setting.
- The subsequent pitch oscillations were exacerbated by the crew's uncertainty regarding the effectiveness of their control inputs during the phugoid cycles.