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NTSB Report
November 11, 1990 · Fokker F28 MK0100 · N850US
Atlanta, GA
Overview
Report Status
Probable Cause
Event Type
Incident
Date
November 11, 1990
Injury Outcomes
No injuries (75 uninjured)
Location
Atlanta, GA
Aircraft
Fokker F28 MK0100
Tail Number
N850US
Operator
Usair
Injuries
None
NTSB Number
ATL91IA017
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.
NTSB 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.
Full Narrative
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.
Flight
Event TypeIncident
Event Time18:10 EST
Nearest Airport0nm N
Runway0
Runway Length0
Runway Width0
Flight Plan FiledIFR
Aircraft FireNo
Aircraft ExplosionNo
Aircraft
AircraftFokker F28 MK0100
Tail NumberN850US
Aircraft CategoryAir
OperatorUsair
Operating RulePart 121
Engines2
Engine TypeTurbojet
Engine DetailsRolls-Royce · TAY650715 · Turbojet · 15000
Seats109
People & Injuries
Injury SummaryNo injuries (75 uninjured)
Fatal0
Serious0
Minor0
Uninjured75
Total75
Crew 1Age 41 · First Class
Conditions
ConditionsVMC
LightDusk
Wind300° @ 4 kt
Visibility20 sm
CeilingNone
Temperature / Dew Point61 F / 39 C
Altimeter30.00 inHg
Observation Time22:50
Weather SourceWFAC