NTSB Identification: CHI89IA083.
The docket is stored on NTSB microfiche number 39509.
Scheduled 14 CFR AMERICAN AIRLINES
Incident occurred Tuesday, May 02, 1989 in MINNEAPOLIS, MN
Probable Cause Approval Date: 08/26/1992
Aircraft: MCDONNELL DOUGLAS DC-9-82, registration: N434AA
Injuries: 96 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.

THE FLIGHT CREW REPORTED THAT WHILE IN CRUISE FLIGHT AT 33,000 FEET, THEY EXPERIENCED A HYDRAULIC SYSTEM FAILURE. THEY MANUALLY EXTENDED THE LANDING GEAR FOR LANDING AND MADE AN UNEVENTFUL NO FLAPS/NO SLATS LANDING. POST-ACCIDENT INVESTIGATION REVEALED THAT THE LOSS OF HYDRAULIC PRESSURE IN BOTH SYSTEMS WAS DUE TO THE FAILURE OF PTU S/O VALVE BODY ATTACH SCREWS ON THE LEFT SYSTEM VALVE. THIS ALLOWED THE LEFT VALVE BODY TO SEPERATE FROM THE REMAINING ASSEMBLY, PORTING LEFT SYSTEM HYDRAULIC FLUID OVERBOARD, AND SUBSEQUENTLY DEPRESSURIZING THE RIGHT SYSTEM (ALTHOUGH THERE WAS NO LOSS OF RIGHT SYSTEM HYDRAULIC FLUID). THE MANUFACTURER HAD A RE-DESIGNED, STRONGER UNIT AVAILABLE TO OPERATORS; THIS IMPROVED UNIT HAD STRONGER VALVE BODY ATTACH SCREWS, WELDED VALVE BODIES, STEEL GEARS, AND A SHEAR PIN IN THE MOTOR-TO-VALVE GEAR TRAIN.

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

FAILURE OF THE HYDRAULIC POWER TRANSFER UNIT SHUT-OFF (PTU S/O) VALVE DUE TO FAILED (FATIGUED) VALVE BODY ATTACH SCREWS.

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