ASN Wikibase Occurrence # 365961
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| Date: | Sunday 11 November 1990 |
| Time: | 19:10 LT |
| Type: |  Fokker 100 |
| Owner/operator: | USAir |
| Registration: | N850US |
| MSN: | 11276 |
| Total airframe hrs: | 2674 hours |
| Engine model: | Rolls-Royce TAY650715 |
| Fatalities: | Fatalities: 0 / Occupants: 75 |
| Aircraft damage: | None |
| Category: | Serious incident |
| Location: | Atlanta, GA -
 United States of America
|
| Phase: | Unknown |
| Nature: | Unknown |
| Departure airport: | Philadelphia, PA (PHL |
| Atlanta, GA (KATL) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
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.
Probable Cause: 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.
Accident investigation:
 |
|
Sources:
NTSB ATL91IA017
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 20-Mar-2024 15:55 | ASN Update Bot | Added |
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