On July 18, 2000, at 1845 hours Pacific daylight time, America West Express Flight 6088, a Bombardier Canadair Regional Jet (CRJ) CL-65, N97325, experienced an autopilot pitch trim master caution light followed by an autopilot pitch trim nose down master caution light, and a pitch trim malfunction. The flight crew elected to divert to the Fresno-Yosemite International Airport, Fresno, California. The airplane, operated by Mesa Airlines d.b.a. America West Express under 14 CFR Part 121, was not damaged. The 3 crew members and 20 passengers were not injured. The flight departed the Monterey Peninsula Airport, Monterey, California, at 1820, and was scheduled to terminate at the Phoenix Sky Harbor International Airport, Phoenix, Arizona. Visual meteorological conditions existed for the scheduled domestic passenger flight and an instrument flight rules (IFR) flight plan had been filed. Use your browsers 'back' function to return to synopsisReturn to Query Page
The pilot reported that while climbing through flight level (FL) 200 the autopilot pitch trim master caution warning light and the autopilot pitch trim nose down master caution warning light illuminated. The captain stated that he disengaged the autopilot in accordance with the airplane flight manual procedures in an attempt to resolve the problem. The flight crew said they had to maintain approximately 2 inches of forward pressure on the flight control column to maintain level flight. The first officer stated that they were able to manually adjust the stabilizer trim up and down, but at no point would the trim go below four units of trim.
Company maintenance personnel removed and replaced the horizontal stabilizer trim actuator (HSTA) and the associated motor control unit (MCU) per aircraft maintenance manuals. The system, with the replaced HSTA and MCU, underwent a series of diagnostic checks, as well as a functional flight test with no discrepancies noted. The airplane was returned to service.
During removal of the components, technicians heard a component loose inside the trim actuator unit and surmised that an internal failure had occurred. The HSTA and MCU were sent to the manufacturer's facility in Grand Prairie, Texas, for further examination.
Review of the Service Difficulty Report (SDR) database for the CRJ revealed that since 1995, there had been 66 HSTA events, where "loss of trim authority" was reported.
The captain held an airline transport pilot certificate with ratings for single engine and multiengine land airplanes. The captain held type ratings for the CL-65 and BE-1900. A first-class medical certificate was issued to the captain on March 9, 2000, with a limitation for corrective lenses - must wear lenses for distant - possess glasses for near vision.
Company records indicated the captain completed recurrent training on May 31, 2000. He had 5,000 hours total time, with 125 hours in the CL-65. He had flown 200 hours in the last 90 days, 100 hours in the last 30 days, and 5 hours in the past 24 hours.
The first officer (FO) held an airline transport pilot certificate with ratings for single engine land, multiengine land airplanes, and instructor ratings for single engine, multiengine, and instrument airplanes, as well as a ground instructor rating.
The FO held a first-class medical certificate issued on May 22, 2000, with no limitations or waivers. Company records indicated the FO completed recurrent training on August 28, 1999. The FO had 3,754 hours total time, with 775 hours in the CL-65. He had flown 167 hours in the last 90 days, and 46 hours in the last 30 days.
The CL-65 attains its pitch trim through movement of the horizontal stabilizer. The pitch trim can be operated in one of four modes; manual trim, autopilot trim, auto trim, and Mach trim. During the incident encountered by flight 6088, the airplane was operated in the autopilot mode.
The horizontal stabilizer is positioned to the desired pitch setting via the HSTA output jackscrew. The HSTA is mounted on the vertical stabilizer and is attached to the leading edge of the horizontal stabilizer's forward spar. The HSTA contains two electric motor assemblies, one jackscrew and gearbox module, four position sensors, and two speed sensors. The two motors in the HSTA are controlled by the MCU. The MCU is an electric unit, which controls the speed and direction of the motors in the HSTA. The MCU is mounted forward of the HSTA on the vertical stabilizer.
The MCU receives pitch commands from the horizontal stabilizer trim control unit (HSTCU). The HSTCU sends control and feedback signals though the active control channel. There are two feedback control channels, and the horizontal stabilizer control system is operated when either trim channel engage switch is activated (pushed in). When the flight crew selects both channels; Channel 1 operates as the primary system, and Channel 2 operates as the standby system. If Channel 1 is selected and Channel 2 is not selected, Channel 1 operates as the primary system, with no standby system available. If a Channel fault failure of the primary system is detected by the HSTCU, automatic changeover to the standby system (Channel 2) will occur. The flight crew will be notified via Electronic Information Control Advisory System (EICAS) that a STAB CH1 (CH2) INOP has occurred.
The flight crew reported that during the incident both switches were engaged and there was not a fault failure indication on EICAS. According to Bombardier, this may have occurred due to a detached bell gear, which drives the Channel 1 position sensor (dual channel). The HSTCU monitors and compares the two channels of position sensor for both the primary and standby systems. Bombardier further reported that the HSTCU monitors the channel position sensor and compares it to the actual motor velocity for each system independently. Should the compared values exceed a given threshold within a specific time period, the HSTCU will automatically switch to the standby system and post an EICAS message indicating a fault. Bombardier reported that in the case of America West flight 6088, a difference in the two values was not sensed, and therefore, no channel changeover took place. It should be noted that Channel 1 and Channel 2 position sensors are not cross-compared in normal operation. The sensors are cross-compared during a ground Built In Test (BIT), conducted as part of the first flight of the day acceptance checks performed by maintenance personnel.
COCKPIT VOICE RECORDER
The CVR was shipped to the Safety Board Vehicle Recorder Laboratory in Washington D.C. Examination of the CVR revealed that the extracted information was overwritten and did not contain pertinent data to the investigation.
FLIGHT DATA RECORDER
The flight data recorder was shipped to the Safety Board Vehicle Recorder Laboratory in Washington D.C. The Safety Board specialist reported that at 24,954 seconds (elapsed time) at an airspeed of 315.8 knots, and an altitude of 16,417 feet, the autopilot was disengaged. The autopilot was reengaged at 25,082 seconds. While the autopilot was disengaged, the pilot trim command was activated several times. The copilot trim command was only active from 25,042 to 25,048 seconds. Approximately 1 second after the autopilot was reengaged, the autopilot trim command became active. According to the data, the autopilot trim command was active multiple times until it remained inactive after 25,115 seconds. The maximum pitch attitude experienced by the airplane was 3.1 degrees at 25,027 seconds. The lowest value for the stabilizer trim was 0.9 degrees at 24,975 seconds. (See attached report)
TEST AND RESEARCH
On August 22, 2000, a Safety Board investigator examined the HSTA (model number 8396-3, serial number 496), and the MCU (model number 7062-3, serial number 313), at SFIM Inc., in Grand Prairie, Texas. Also in attendance were the FAA, Bombardier, and MESA airlines, who were parties to the investigation. AVIAC/SAGEM of France manufactured the above referenced parts.
Technicians from SFIM bench tested the HSTA to read out and verify the position sensor values. When the values are measured, all four sensors should indicate the same voltage. Channel 1 position sensors read 4.34 volts and 4.33 volts. Channel 2 position sensors both read 3.78 volts. SFIM indicated that when the position sensor values are not the same it is a result of a faulty bell gear in the HSTA.
The HSTA was disassembled for examination. The teardown revealed that the Channel 1 bell gear had separated from its four mounting tabs. In addition to the fractured bell gear tabs, two of the main spur gear bolts had backed out, and were loose and out of position in the HSTA. The other two main spur gear bolts in the HSTA were finger tight, but in place.
The bell gear and its mounting tabs were shipped to Bombardier's Materials Laboratory. The components were examined on November 9, 2000. Bombardier's report indicated that the mounting tabs failed as a result of fatigue. The report opined that the main source of the cyclic stresses, which caused the fatigue cracking, was probably due to vibrations. They further indicated that it appeared that the vibrations might have been involved in producing both the wear on the gear teeth and the fatigue cracking of the legs. Materials Lab personnel observed fatigue cracks in a "small radius" where the mounting tabs mate with the bell gear.
According to Bombardier, flight 6088's autopilot posted caution messages informing the pilots that pitch trim was not responding to given commands. Eventually the bell gear was moved to its limits, erroneously indicating that the actuator was at the full extend/retract limit, but still operating within the normal range of surface travel. The horizontal stabilizer's actual position, however, was not at its travel limits, but somewhere within its normal operating range. When the HSTCU receives the information that the position sensors are at their electrical extend/retract limit, it will stop moving the surface in that direction, and inhibit all inputs (autopilot, manual trim) without posting a message on the EICAS.
Service Bulletin 8396-27-02, issued January 12, 2000, was developed as a result of repeated bell gear failures. It called for replacement of the bell gear with a new bell gear part. Service Letter 8396-27-01, issued December 2, 1998, resulted from repeated main spur gear bolts backing out in service and required that Loctite be used with new bolts, and new torque values be applied to the bolts.
According to maintenance records submitted by Mesa Airlines, the HSTA had been installed on April 14, 1999. The HSTA had accrued 3,006.4 hours of total time, with 2,117 recorded cycles, and was identified as original equipment. The HSTCU had been installed on this airplane on April 19, 1999. The records did not indicate when the MCU was installed on the airplane. However, prior to the incident the MCU was removed because the HSTA Channel 1 circuit breaker popped repeatedly.