ALIQUIPPA, PA, USA
N513AU
BOEING B-737-300
The airplane crashed while maneuvering to land at Pittsburgh International Airport. The airplane entered an uncontrolled descent and impacted terrain about 6 miles east of the airport. The airplane struck the ground at an angle of descent of about 80 degrees, in a slight roll to the left, and the airspeed was about 260 knots at impact. The investigation revealed that during the accident sequence, the airplane rudder deflected rapidly to the left and reached its left aerodynamic blowdown limit shortly thereafter. Examination of the rudder system revealed that it is possible, in the main rudder power control unit (PCU) of the airplane (as a result of some combination of tight clearances within the servo valve, thermal effects, particulate matter in the hydraulic fluid, or other unknown factors), the servo valve secondary slide could jam to the servo valve housing at a position offset from its neutral position without leaving any obvious physical evidence and that, combined with rudder pedal input, could have caused the rudder to move opposite to the direction commanded by a rudder pedal input. This condition of the PCU was also consistent with analysis of the cockpit voice recorder, computer simulation, and human performance data, including operational factors
On September 8, 1994, USAir flight 427, a Boeing 737-300, N513AU, crashed near Aliquippa, PA at about 1903 eastern daylight time. The airplane was destroyed by impact and post crash fire. The 2 pilots, 3 flight attendants and 127 passengers were fatally injured. The flight was being conducted under 14 CFR Part 121. Visual meteorological conditions prevailed at the time of the accident and an IFR flight plan was filed. The flight crew was in contact with the local controller and was being maneuvered to enter a right downwind pattern for runway 28R at Pittsburgh International Airport, Pittsburgh, PA. The flight crew did not report any problems with the airplane and radar data indicates that the closest other traffic was about 4.5 miles and 1,500 feet vertically separated from flight 427 at the time of the accident. The airplane struck the ground at an angle of descent of about 80 degrees, in a slight roll to the left, and the airspeed was about 260 knots at impact. The airplane was severely fragmented by the impact. There was an intense postcrash fire. Both engines were producing power at impact and were running symmetrically. The thrust reversers were stowed at impact. The flaps were at a "Flap 1" setting. At this setting, the leading edge slats and Kreuger flaps are extended. The spoilers were retracted and the landing gear were retracted. These are the expected positions forthe airplane during the initial approach. The horizontal stabilizer was in an intermediate position, consistent with a speed of about 190 knots. The elevator power control unit was at 14 degrees nose up. The rudder was determined to be 2 degrees to the right (airplane nose right) at impact. Additional Persons (continued) Mr. Paul Mingler CFM International Flight Safety Office One Neumann Way MD J60 Cincinnati, OH 45215-6301 Mr. Paul M. Rehder AVIALL Technical Services Dallas Engine Services 9311 Reeves Street Dallas, Texas 75235-2095 Mr. William West National Air Traffic Controllers Association Suite 701 1150 17th Street, N.W. Washington, D.C. 20036 Mr. Juergen-Peter Schuetz Transportation Workers Union #545 Operations Coordination/Dispatch 2018 Broad Hill Farms Road Coraopolis, PA 15108 Mr. Terry Kleiser International Association of Machinists and Aerospace Workers 73 Auburn Street Saugus, MA 01906 Ms. Nancy Gilmer Association of Flight Attendants 2244 Commissary Circle Odenton, MD 21113 Mr. Wally Walz Parker Berta Aerospace Parker Hannifin Corporation 14300 Alton Parkway Irvine, CA 92718-1814 Mr. Frank P. Jakse Senior Research Specialist Monsanto 800 N. Lindbergh Boulevard St. Louis, MO 63167 Mr. Harvey Patrick PATS, Inc. 9570 Berger Road Columbia, MD 21046-1569 Mr. Jim Eichenlaub Manager Hopewell Township Municipal Building Clark Boulevard Aliquippa, PA 15001 LT James R. Nevill Station Commander Pennsylvania State Police 1400 Brighton Road Beaver, PA 15009 Mr. Joseph L. LaFleur Director Pennsylvania Emergency Management Agency Box 3321 Harrisburg, PA 17105-3321 Mr. Wayne N. Tatalovich Coroner Beaver County Courthouse Third Street Beaver, PA 15009
A loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.
Source: NTSB Aviation Accident Database
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