MINNEAPOLIS, MN, USA
N675UP
Boeing 747-123
While troubleshooting a fire warning indication with company maintenance control using the #2 VHF transceiver, the crew experienced several uncommanded 'kicks' of the yaw damper. The crew turned off the upper yaw damper, but not the lower. They continued to experience 'kicks' although not as severe. The flight departed to Minneapolis where it landed uneventfully. Inspection of the airplane after the incident revealed an open shield at the #2 VHF coax connector which is located near the yaw damper control box. The connector was replaced and the airplane was returned to service.
On May 18, 1998, at 0215 central daylight time, a Boeing 747-123, N675UP, operated by United Parcel Service (UPS) experienced uncommanded yaw damper inputs during cruise flight at FL350 while en route to Anchorage Alaska. The flight diverted to Minneapolis, Minnesota, where an uneventful landing was made. The captain, first officer, and flight engineer were not injured and the airplane was not damaged. The 14 CFR Part 121 cargo flight was operating on an IFR flight plan. The flight originated from Louisville, Kentucky, at 0159 eastern daylight time. While en route, the flight crew received an intermittent fire warning light and bell which they suspected was from the lower cargo area. They contacted UPS Maintenance Control using the VHF #2 transceiver and were unable to isolate the warning so they initiated a diversion to Minneapolis. While communicating with Maintenance Control the flight crew experienced several uncommanded "kicks" of the yaw damper. According to UPS, the flight crew turned off the upper yaw damper, but they did not turn off the lower yaw damper. They continued to experience "kicks" although not as severe. The flight landed at Minneapolis without incident at 0320 cdt. The crew write-up in the maintenance logbook regarding the uncommanded yaw damper "kicks" stated "Experienced rapid severe uncommanded flt control inputs in crz flt and then after with autopilots A or B or autopilot off, air divert to MSP." The crew reported during a debrief that the "kicks" occurred with either the "A" or "B" autopilots engaged. They reported turning the autopilot off at approximately FL150. A review of the Flight Data Recorder data indicated that approximately 45 minutes into the flight the #2 VHF was keyed at which time the upper rudder surface deflected left to a maximum of 3.3 degrees and the lower rudder surface deflected left to a maximum of 1.1 degrees. The data also shows that in addition to the yaw damper inputs, uncommanded movement of the control wheel occurred whenever the #2 VHF was keyed. The maximum control wheel movement was 16.9 degrees to the right. The control wheel deflections continued to occur after the upper yaw damper was disconnected as long as the autopilot was engaged. Maintenance inspection of the airplane after the incident revealed an open shield at the #2 VHF coax connector. The #2 VHF transceiver is located near the yaw damper couplers, and the pitch and roll computers. The connector was replaced and the airplane was returned to service. Inspection of the airplane did not reveal any evidence of a fire. The intermittent fire warning was corrected by maintenance personnel replacing the fire warning circuit card. On May 17, the day previous to this incident, a different flight crew who flew N675UP reported "When keying mic on #2 VHF (all three mics) Yaw Damper Kicks! Pulled #2 VHF CB [circuit breaker]." Maintenance inspection of the airplane after this event determined the #2 transceiver was unserviceable and it was replaced. Operational checks of the transceiver and yaw damper were conducted and the airplane was put back in service.
the open shielding on the #2 VHF transceiver coax connector which resulted in electromagnetic interference with the yaw damper system.
Source: NTSB Aviation Accident Database
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