NTSB Identification: LAX07IA014.
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Scheduled 14 CFR Alaska Airlines, Inc.
Incident occurred Thursday, October 19, 2006 in Los Angeles, CA
Probable Cause Approval Date: 03/31/2008
Aircraft: Boeing 737-790, registration: N614AS
Injuries: 5 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.
During cruise flight, just after entry into foreign airspace, the flight crew was alerted to a System B hydraulic failure, and they decided to return to, and land in the United States (US). Upon re-entry to US airspace, the flight crew declared an emergency and requested to land at the departure airport. On final approach, about 5 minutes after selecting flaps to 15 and lowering the landing gear, the flight crew was alerted to the System A hydraulic quantity gage, that had dropped to 0 pounds per square inch (psi). At this point both System A LOW PRESSURE LIGHTS illuminated and the airplane went into the manual reversion mode. The flight crew indicated that the airplane was difficult to control; however, they landed uneventfully at the airport. Maintenance personnel for the airline, and the airplane manufacturer, inspected the hydraulic systems. They noted that the initiating event was a failed engine driven pump. The secondary event was a cracked hydraulic line that failed due to fatigue. A metallurgical examination revealed the fatigue fracture initiated at the toe of the welding root where they were able to locate a crack that had propagated through half the perimeter. The airframe manufacturer redesigned the affected tube, part number 272A4451-87, a titanium tube, with part number 272A4451-200, a stainless steel tube.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: A complete failure and depressurization of both hydraulic systems due to two separate events: 1) the failure of a engine driven pump on the 'A' system, and 2) the fatigue fracture and failure of a hydraulic line on the 'B' system. Full narrative available
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