NTSB Identification: ATL91IA017.
The docket is stored on NTSB microfiche number 45538.
Scheduled 14 CFR operation of USAIR
Incident occurred Sunday, November 11, 1990 in ATLANTA, GA
Probable Cause Approval Date: 05/10/1993
Aircraft: FOKKER F28 MK0100, registration: N850US
Injuries: 75 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.

DURING DESCENT THE AFCAS STABILIZER TRIM & AUTOPILOT DISENGAGED. BOTH WERE RESET WITH A 2ND DISENGAGEMENT. NOSE DOWN CONTROL PRESSURES WERE ENCOUNTERED & THE FLT CREW ATTEMPTED TO USE MANUAL TRIM, FOLLOWED BY A JAMMED STABILIZER MASTER WARNING. AFCAS STABILIZER TRIM WAS LEFT OFF IAW ABNORMAL PROCEDURES. FDR DATA SHOWED THE STAB THEN CREEPED TO A FULL ACFT NOSE DOWN POSITION & HYD PRES TO THE STAB ACTUATOR WAS AUTOMATICALLY REMOVED, PER DESIGN. SUBSEQUENT TESTING OF THE HORIZONTAL STABILIZER CONTROL UNIT ASSEMBLY SHOWED THAT, AFTER SPRAYING THE SERVO VALVE WITH WATER AND COLD SOAKING, ICE WOULD FORM INSIDE THE END CAP AND A JAMMED SERVO VALVE CONDITION WOULD OCCUR. THE END CAP AND SPRING EXHIBITED SUBSTANTIAL RUST CORROSION. CREW MOVEMENT OF THE MANUAL TRIM RESULTED IN THE JAMMED STAB WARNING.

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

FROZEN MOISTURE IN THE STABILIZER ACTUATOR HYDRAULIC SERVO VALVE WHICH RESULTED IN REDUCED MOVEMENT OF THE SERVO VALVE AND LIMITED ITS ABILITY TO RESPOND TO FLIGHT CONTROL INPUTS. THE LACK OF RESPONSE FROM THE SERVO VALVE RESULTED IN THE JAMMED STABILIZER MESSAGE. A FACTOR WAS THE INADEQUATE DESIGN OF THE SERVO VALVE WHICH PRECLUDED MOISTURE DRAINAGE.

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