On November 20, 1999, at 2107 Eastern Standard Time, a Boeing 767-200, N610UA, operated by United Airlines as flight 20, experienced an inoperative stabilizer trim system while executing an approach to runway 31R at Kennedy International Airport (JFK), Jamaica, New York. The airplane was not damaged. The 2 flight crewmembers, 6 flight attendants, and 145 passengers were not injured. Night visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the scheduled 14 CFR Part 121 passenger flight that departed the San Francisco International Airport, San Francisco, California.

According to the captain, while at 1,400 feet, and on final for runway 31R at JFK, the first officer stated he was having trouble trimming the airplane. The captain checked the trim indicator and noticed it was approximately 12.5 degrees nose up. The captain then attempted to trim the airplane via his yoke switch and the manual trim handle, with no success. The captain advised ATC they needed a go-around, and requested a block altitude of 2,000 to 3,000 feet.

After changing to departure control, the captain declared an emergency, and ran the unscheduled-stabilizer-trim-quick-reference checklist. The first officer flew the airplane and communicated with ATC. The captain completed the checklist, but control of the stabilizer trim was not regained. The captain then requested a phone patch to system aircraft maintenance control (SAMC) in San Francisco. While in communication with SAMC, the captain had the purser pull circuit breakers H11 and H20, and then reset them after a 30 second cool down period. The stabilizer trim was still inoperative. SAMC then requested that the trim cutout switches be reset. After resetting the switches twice, normal operation of the stabilizer trim was reestablished.

The captain took control of the airplane and had a flight attendant advise the passengers that an unsafe indication necessitated the go-around, and that everything was "now normal." While at altitude, the captain preformed a controllability check at approach speed, and then executed an approach to runway 31L at JFK without further incident.

The captain added that during the emergency he and the first officer did not observed any warning indications, which made troubleshooting the problem more difficult.

The first officer estimated that it required approximately 15 to 20 pounds of forward pressure on the yoke to keep the airplane level. In addition, he stated that the stabilizer trim indicator was 13 to 14 degrees nose up.

According to data retrieved from the digital flight data recorder, the stabilizer was at 11.46 degrees nose up, during the emergency.

According to the maintenance manual, stabilizer position and stabilizer trim control system faults are indicated in the flight compartment for the following reasons: Unscheduled stabilizer trim; faulty stabilizer trim drive rate; loss of power to the control stand stabilizer trim position indicators; loss of input signal to control stand stabilizer trim position indicators; and incorrect stabilizer position for takeoff.

Testing conducted by the operator of the stabilizer trim system found that the left stabilizer position transmitter, and the right stabilizer position transmitter were out of calibration. Also, the left stabilizer trim control module (STCM) was found to have a higher than normal leakage, and the connector for the corresponding shut-off valve was shorting across several of its pins due to skydrol incursion. The right STCM was also removed and tested ok.

According to the operator, the system should have operated at half rate with the above discrepancy, but did not. There has been no reports of difficulties with the airplane's stabilizer trim system since the initial event.

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