NTSB Identification: CHI96IA207.
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Scheduled 14 CFR operation of NORTHWEST AIRLINES
Incident occurred Friday, June 14, 1996 in BOSTON, MA
Probable Cause Approval Date: 08/21/1997
Aircraft: Airbus Industrie A-320, registration: N347NW
Injuries: 106 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

The pilot reported that a two to three degree uncommanded pitch decrease occurred about 600 feet above ground level at the same time as a flight control system pitch warning fault message was received. The crew declared an emergency and landed without further incident. During troubleshooting, the maintenance technician reported the right half of the stabilizer trim control package felt hot to the touch. The left side was cool. The trimmable horizontal stabilizer (THS) actuator was replaced, and the airplane was returned to service without further incident. Examination of the airplane logbooks revealed flight control system, pitch axis, discrepancies, and maintenance action following the previous two flights. Both elevator aileron computers, the THS electronic module, the THS actuator position monitor, and the pitch trim motor were replaced. Examination of the flight data recorder revealed erratic stabilizer movement on the previous two flights. The THS actuator and the pitch trim actuator servo motor functioned normally when tested. A defective diode was discovered in the electronic module. An Airbus Industries engineer reported the pitch oscillations could not be attributed to the defective diode.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

an undetermined intermittent fault in the trimmable horizontal stabilizer actuator system.

Full narrative available

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