FTW00IA228

The airplane was descending through 14,500 feet when an uncommanded disconnect of the autopilot occurred, and the airplane "abruptly" pitched up from 5 degrees nose down to 15 degrees nose up (pitch reversion). Simultaneously, the captain and first officer applied forward pressure on their control columns to regain control of the airplane. The airplane was re-established in a descent and the flight crew found that the airplane was controllable in the landing configuration. Subsequently, the flight landed without further incident. Post incident examination of the airplane revealed that the right elevator trim control wheel was improperly installed. The installation allowed the autopilot trim servo to request nose down trim until the trim tab actuators had reached their physical limits. The autopilot trim servo continued to request nose down trim, loading the elevator trim system cables, until the autopilot disconnected. During the examination, it was also noted that the trim tab deflections and cable tensions did not meet manufacturer specifications. Review of maintenance records revealed two instances when the elevator pitch trim system underwent maintenance. On April 14, 1998, the right side elevator trim control wheel was removed and reinstalled "while aligning stop arm into wheel." Continental Express reported that the trim tabs were adjusted during the maintenance. On August 11, 2000, the following maintenance write-up was recorded in the aircraft flight log: "Elev trim will not go full forward, stops at two units." The flight log indicated that the following corrective action was taken; "Lubed trim wheel for elev trim." Continental Express reported that the trim wheel was not removed during this corrective action.

History of flight

On August 12, 2000, at 1303 central daylight time, an Embraer EMB-120ER twin turboprop airplane, N16718, operating as Jetlink flight 2931, experienced an uncommanded disconnect of the autopilot while descending, and proceeded to land without further incident at the San Angelo Regional Airport (SJT), San Angelo, Texas. The airplane was registered to the Chase Manhattan Bank and Trust Co., of San Francisco, California, and operated by Continental Express Airlines Inc., of Houston, Texas. The airline transport rated captain, airline transport rated first officer, and 20 passengers were not injured; the flight attendant sustained minor injuries. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan was filed for the 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight. The flight originated from the George Bush Intercontinental Airport, Houston (IAH), Texas, at 1145, and was destined for San Angelo, Texas.

According to the captain and first officer, the captain was the flying pilot. The flight was cleared by the Houston Air Route Traffic Control Center (HOU ARTCC) to descend from flight level 200 to 6,000 feet. The captain established the airplane in a 1,500 foot per minute descent, at 260 knots, with the autopilot engaged in the descent mode. The airplane was descending through 14,500 feet when the airplane experienced an uncommanded disconnect of the autopilot, and the airplane "abruptly" pitched up from five degrees nose down to 15 degrees nose up (pitch reversion). Simultaneously, the captain and first officer applied forward pressure on their control columns to regain control of the airplane. The captain stated that he observed the pitch trim in the "full nose down position," immediately following the event. The first officer reported that after the event, the captain stated that "he could not trim the aircraft and he needed more nose down than he had available."

The airplane was re-established in a descent; however, the captain had to maintain constant forward pressure on the control column to maintain the descent. The captain then retarded the throttle and slowed the airplane to 200 knots, and forward pressure on the control was no longer needed. The flight crew referenced the quick reference handbook (QRH), but did not find any section that was applicable to the event experienced. They decided to descend to 10,000 feet and execute a controllability check. The captain reported that with the airplane in the landing configuration he was able to control the airplane without applying abnormal force to the control column and with normal use of pitch trim. The first officer contacted the flight attendant following the event and she reported that her right leg had been injured; however, none of the passengers had been injured. The flight crew then declared an emergency and the airplane landed in San Angelo without further incident. Upon arrival in San Angelo the flight attendant was transported to a hospital, treated, and released the same day.

The flight attendant reported that, prior to the pitch event the first officer had made an announcement to the passengers indicating that the airplane had begun its descent for landing, and he illuminated the fasten seat belt sign. She stated that she was in the rear galley of the airplane, preparing for landing, when "the back of the plane dropped and [she] flew into the air." Subsequently, she landed on the galley floor, on her right ankle and leg.

Personnel information

The captain was hired by Continental Express on June 11, 1998. On May 8, 2000, he was issued an airline transport pilot certificate, and satisfactorily completed his most recent flight proficiency check. He held type ratings in the Embraer EMB-120 and the McDonnell Douglas DC-10. According to information provided by Continental Express, the captain had accumulated a total of 5,500 flight hours, of which 120 hours were in the EMB-120. Additionally, he underwent his most recent FAA medical examination, and obtained a first class medical certificate with no limitations, on July 1, 2000.

The first officer was hired by Continental Express on March 8, 1999, and satisfactorily completed his most recent flight proficiency check on May 11, 2000. He held an airline transport pilot certificate that was issued on July 5, 1996. He held type ratings in the Boeing 707 and Boeing 720. According to information provided by Continental Express, the first officer had accumulated a total of 3,811 flight hours, of which 815 hours were in the Embraer EMB-120. Additionally, he underwent his most recent FAA medical examination, and obtained a first class medical certificate with no limitations, on February 1, 2000.

The flight attendant underwent her initial operating experience (IOE) check on July 8, 2000, and was hired by Continental Express on July 17, 2000.

Aircraft information

The 1989 Embraer EMB-120ER airplane (serial number 120136) was delivered new to Continental Express. It was configured with 30 passenger seats, three crew seats, and was powered by two Pratt & Whitney PW-118 turboprop engines. At the time of the event, the airplane had accumulated a total of 22,727.4 hours.

The elevator trim tab system is a subsystem of the elevator system that enables the pilot, co-pilot, or the autopilot to adjust the pitch of the airplane. When the autopilot is in use, an autopilot trim servo automatically adjusts the elevator trim. The elevator trim may also be activated by the pilot or co-pilot via the electric switches mounted on the flight control columns, or via one of the elevator trim wheels located on either side of the central cockpit pedestal structure. The plastic trim control wheels are interconnected via a shaft. The inner side of the right trim control wheel is fitted with a spiral track, which is referred to as the cursor gearing. When the right trim wheel is inserted onto the interconnecting shaft, the physical backstop of the cursor gearing meets with the tip of the movable backstop. The movable backstop is mounted to the central cockpit pedestal. When the trim is adjusted, the tip of the movable backstop rides along the cursor gearing. The elevator trim wheels are connected to the elevator trim tab surfaces through a series of cables, chains, pulleys, sprockets, bell cranks, and rods. The elevator trim pitch down limit is set when the control wheel on the right side of the central cockpit pedestal (co-pilot's side) is installed.

The Embraer maintenance manual provides procedures for the installation of the elevator trim control wheels (MM 27-32-03). The left trim wheel is first positioned on the interconnecting shaft. The right trim wheel is then positioned on the interconnecting shaft so that the tip of the movable backstop fits into the cursor gearing. The tip of the movable backstop must be inserted so that it is flush with the pitch down physical backstop of the cursor gearing; simultaneously, the pitch trim scale on the pedestal must indicate the pitch down limit on the cockpit elevator trim scale. The pitch down limit may be two or three units, depending on modifications to the airplane. Aircraft 120136 was modified so that the scale's pitch down limit was three units.

According to Embraer, if the tip of the movable backstop is inserted into the cursor gearing correctly (so that the physical backstop limits the travel of the trim wheel at two or three units on the elevator trim scale), and the airplane is trimmed by the autopilot trim servo to the two or three unit limit, the autopilot will disconnect and a pitch reversion will not occur. However, if the tip of the movable backstop is not inserted into the cursor gearing correctly (so that the physical backstop will not be reached at the two or three unit limit on the elevator trim scale), the autopilot may request nose down trim un