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NTSB Report
April 5, 1991 · Embraer EMB-120RT · N270AS
Brunswick, GA
Overview
Report Status
Probable Cause
Event Type
Accident
Date
April 5, 1991
Injury Outcomes
23 injuries (23 fatalities)
Location
Brunswick, GA
Aircraft
Embraer EMB-120RT
Tail Number
N270AS
Operator
Atlantic Southeast Airlines
Injuries
Fatal
Aircraft Damage
Destroyed
NTSB Number
DCA91MA033
WITNESSES REPORTED THAT THE AIRPLANE SUDDENLY TURNED OR ROLLED LEFT UNTIL THE WINGS WERE PERPENDICULAR TO THE GROUND. THE AIRPLANE THEN FELL IN A NOSE-DOWN ATTITUDE. EXAM OF THE LEFT PROP COMPONENTS INDICATED A BLADE ANGLE OF ABOUT 3 DEG, WHILE THE LEFT PROP CONTROL UNIT (PCU) BALLSCREW POSITION WAS CONSISTENT WITH A COMMANDED BLADE ANGLE OF 79.2 DEG. EXTREME WEAR ON THE PCU QUILL SPLINE TEETH, WHICH NORMALLY ENGAGED THE TITANIUM-NITRIDED SPLINES OF THE PROP TRANSFER TUBE, WAS FOUND. THE TITANIUM-NITRIDED SURFACE WAS MUCH HARDER AND ROUGHER THAN THE NITRIDED SURFACE OF THE QUILL. THEREFORE, THE TRANSFER TUBE SPLINES ACTED LIKE A FILE AND CAUSED ABNORMAL WEAR OF THE GEAR TEETH ON THE QUILL. WEAR OF THE QUILL WAS NOT CONSIDERED DURING THE CERTIFICATION OF THE PROPELLER SYSTEM.
NTSB Probable Cause
THE LOSS OF CONTROL IN FLIGHT AS A RESULT OF A MALFUNCTION OF THE LEFT ENGINE PROPELLER CONTROL UNIT WHICH ALLOWED THE PROPELLER BLADE ANGLES TO GO BELOW THE FLIGHT IDLE POSITION. CONTRIBUTING TO THE ACCIDENT WAS THE DEFICIENT DESIGN OF THE PROPELLER CONTROL UNIT BY HAMILTON STANDARD AND THE APPROVAL OF THE DESIGN BY THE FEDERAL AVIATION ADMINISTRATION. THE DESIGN DID NOT CORRECTLY EVALUATE THE FAILURE MODE THAT OCCURRED DURING THIS FLIGHT, WHICH RESULTED IN AN UNCOMMANDED AND UNCORRECTABLE MOVEMENT OF THE BLADES OF THE AIRPLANE'S LEFT PROPELLER BELOW THE FLIGHT IDLE POSITION. (NTSB REPORT AAR-92/03)
Full Narrative
WITNESSES REPORTED THAT THE AIRPLANE SUDDENLY TURNED OR ROLLED LEFT UNTIL THE WINGS WERE PERPENDICULAR TO THE GROUND. THE AIRPLANE THEN FELL IN A NOSE-DOWN ATTITUDE. EXAM OF THE LEFT PROP COMPONENTS INDICATED A BLADE ANGLE OF ABOUT 3 DEG, WHILE THE LEFT PROP CONTROL UNIT (PCU) BALLSCREW POSITION WAS CONSISTENT WITH A COMMANDED BLADE ANGLE OF 79.2 DEG. EXTREME WEAR ON THE PCU QUILL SPLINE TEETH, WHICH NORMALLY ENGAGED THE TITANIUM-NITRIDED SPLINES OF THE PROP TRANSFER TUBE, WAS FOUND. THE TITANIUM-NITRIDED SURFACE WAS MUCH HARDER AND ROUGHER THAN THE NITRIDED SURFACE OF THE QUILL. THEREFORE, THE TRANSFER TUBE SPLINES ACTED LIKE A FILE AND CAUSED ABNORMAL WEAR OF THE GEAR TEETH ON THE QUILL. WEAR OF THE QUILL WAS NOT CONSIDERED DURING THE CERTIFICATION OF THE PROPELLER SYSTEM.
Flight
Event TypeAccident
Event Time14:51 EST
Nearest Airport0nm N
Runway0
Flight Plan FiledIFR
Aircraft FireGrd
Aircraft ExplosionNo
Aircraft
AircraftEmbraer EMB-120RT
Tail NumberN270AS
Aircraft CategoryAir
DamageDestroyed
OperatorAtlantic Southeast Airlines
Operating RulePart 135
Engines2
Engine TypeTurboprop
Engine DetailsP&W · PW-118 · Turboprop · 1800
Seats30
People & Injuries
Injury Summary23 injuries (23 fatalities)
Fatal23
Serious0
Minor0
Uninjured0
Total23
Crew 1Age 34 · First Class
Conditions
ConditionsVMC
LightDaylight
Wind160° @ 10 kt
Visibility7 sm
CeilingBroken clouds 10000 ft
Temperature / Dew Point70 F
Altimeter30.00 inHg
Observation Time19:50
Weather SourceWFAC