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On February 24, 1997, at 0636 mountain standard time (mst), a Dehavilland DHC-8-311, N430AW, operated as United Express Flight 7451, by an airline transport pilot, experienced an elevator control malfunction during initial climb to cruise altitude. The pilot declared an emergency and landed the airplane at Rapid City Regional Airport, South Dakota, without further incident. Visual meteorological conditions prevailed at the time of the incident. The flight was being conducted as domestic commuter air carrier service under 14 CFR Part 121. An IFR flight plan was on file. There were no reported injuries by the 2 flight crew members, 1 flight attendant or 42 passengers on board the airplane. The flight originated at Rapid City, South Dakota, at 0615 mst, and was en route to Denver, Colorado.
In his written statement, the captain said that he engaged the airplane's autopilot while passing through 10,000 feet mean sea level (msl). Approximately 10 minutes later, the crew noticed illumination of a "PITCH MISTRIM, TRIM NOSE DOWN" annunciator light. The captain attempted to trim the airplane using the manual elevator trim, but it was jammed and would allow only limited movement. The captain then disconnected the autopilot to see if it would free up the trim. This action was unsuccessful. The crew determined that they had a jammed elevator and referred to the Quick Reference Handbook checklist addressing elevator trim malfunctions. The checklist required the crew to use the standby elevator trim system. The captain said that he attempted to do this several times, but it seemed ineffective. The crew declared an emergency and returned to Rapid City, South Dakota. The crew landed the airplane without further incident.
The airplane, a Dehavilland DHC-8-311, serial number 279, was manufactured in 1991. The airplane had undergone a 50-hour line check at Grand Junction, Colorado, on February 16, 1997. At the time of the incident, the airplane had 13,374 total airframe hours. The airplane had flown 20 hours since the last line check.
A post-incident inspection was conducted by the Federal Aviation Administration and Mesa Airlines at Rapid City, South Dakota, on February 24, 1997. An examination of the aft fuselage compartment revealed that one of the two master links which hold together the chain and cable assembly to the electric elevator trim system had come apart. The chain had subsequently entangled itself in the elevator trim cables. No other elevator trim system anomalies were found. The electric elevator trim servo motor, chain pieces and cable were retained for further examination.
TESTS AND RESEARCH
The electric elevator trim servo motor, chain, cable, and pieces of the master link were examined by the investigator-in-charge at Grand Junction, Colorado, on March 4, 1997. It was revealed that the "C" clip which snaps to the two posts of the master link base plate, and holds the master link cover plate in place was missing. Examination of the electric elevator trim servo motor revealed no anomalies.
The NTSB investigator-in-charge (IIC) examined the electric elevator trim system chain and cable assembly in another Dehavilland DHC-8 airplane, at Grand Junction, Colorado. On that airplane, the IIC slid the "C" clip off of the cover plate with his finger nail.
An engineer for the National Transportation Safety Board examined the electric elevator trim system chain and cable assemblies in three production DHC-8 airplanes, in Toronto, Canada, on June 20, 1997. It was found that the "C" clips on the master links of these airplanes could not be removed without the use of specialized tools.
Parties to the investigation were the Federal Aviation Administration, the Transportation Safety Board of Canada, Bombardier, Incorporated, and Mesa Airlines.
The airplane was released and put back into service on February 26, 1997. The flight data recorder, electric elevator trim servo motor, chain, and cable parts were released and returned to Mesa Airlines.