NTSB Factual Report

HISTORY OF FLIGHT

On October 19, 2006, about 0100 Pacific daylight time, a Boeing 737-790, N614AS, returned to Los Angeles International Airport (LAX), Los Angeles, California, after experiencing a hydraulic system malfunction while en route to Licenciado Benito Juarez International Airport (MMMX [MEX]), Mexico City, Mexico. Alaska Airlines operated the commercial flight under the provisions of 14 Code of Federal Regulations (CFR) Part 121, as ASA flight 250. An instrument flight rules (IFR) flight plan had been filed. There were no injuries to the two flight crew, three cabin flight attendants, and the unknown number of passengers; the airplane was not damaged. The flight departed LAX about 2400.

According to Alaska Airlines flight safety personnel, after entering Mexican airspace at cruise altitude, the flight crew was alerted to a System B hydraulic failure. The flight crew opted to return to the United States (US). Upon reaching US airspace the flight crew declared an emergency with a return to LAX. While on a 5-mile final approach to LAX at 2,500 feet above ground level (agl), the flight crew selected flaps 15 and extended the landing gear. Five minutes later, about 800 feet agl, the System A Hydraulic Quantity gage dropped to 0 pounds per square inch (psi), which illuminated both System A LOW PRESSURE LIGHTS, and the airplane went to manual reversion. The flight crew noted that the pressure gage read 2,900 psi for about 1 minute after the hydraulic quantity went to 0 psi; the pressure gage also dropped to 0 psi. At that point, the airplane became difficult to control in the mechanical reversion mode; however, the flight crew was able to land the airplane without further incident at 0330.

TESTS AND RESEARCH

Hydraulics

Alaska Airlines maintenance personnel along with a Federal Aviation Administration (FAA) airworthiness inspector and personnel from Boeing examined the airplane, and noted that the hydraulic failure of System B (the initiating event) was attributed to a catastrophic failure of the engine driven pump. ASA maintenance personnel found a spoiler hydraulic line that had fractured and subsequently bled out the System A hydraulic (the second event), and led to the dual hydraulic failure event.

Maintenance personnel replaced System A and B case drain filters, as well as, the System B return filter; all of the filters contained chunks of metal. The engine driven pump (EDP), part number 66087, serial number 2040, for the number 2 engine was replaced. As maintenance personnel was performing the leak check for the EDP, hydraulic fluid was observed coming from the number 2 engine pylon area. While troubleshooting the leak, maintenance found the B-nut (part number 312A2110-2) in the hydraulic pressure tube assembly, unscrewed from its mating tube; it had migrated about 9 inches from the end of the tube. Maintenance personnel replaced the entire tube assembly as a precautionary measure. The EDP for the number 2 engine was sent to Parker Aerospace for additional inspection/teardown.

Maintenance personnel conducted a Hydraulic System Filter replacement through the remainder of the hydraulic system. The ground spoiler "UP" line was cracked and replaced as were other items as required by Alaska Airlines EA2920-01025 Hydraulic System Restoration.

System A Failure

The tube assembly (part number 272A4451) and case drain filter were sent to Boeing's laboratory for additional testing. Laboratory personnel reported that the tube had a fatigue facture with an initiation crack located at the toe of the welding root that grew to about half the perimeter of the tube. They also noted the tube end fitting had been damaged and most likely occurred after removal of the B-nut as there was no evidence of the B-nut having been seated against those surfaces. Laboratory results of the tube and fitting materials were consistent with the drawing requirements.

System B Failure

The Engine Driven Pump (part number 66087, serial number 2040) was examined by Parker Aerospace Engineering. During disassembly technicians noted steel particles in the pumps housing, a sheared and seized impeller drive shaft, erosion at the inlet port of the port plate and cap, and a heavy wear pattern of the port plate. They also noted cylinder barrel face wear with bronze transfer, damaged front housing stroking piston bore, the rotating group was stuck in the cylinder barrel bore with damage to bottom of barrel, the rotating group shoe gap was found to be excessive, and rotating group thrust washer was found to have worn with light galling. The drive shaft seal was found to be in fair condition with little damage, and all of the bearings were found to have ingested metal debris. Parker Aerospace was not able to determine the cause the of engine driven pump failure.

ADDITIONAL INFORMATION

According to Alaska Airlines, Boeing indicated that the failure of the Spoiler hydraulic line/tube (part number 272A4451) was a known event. Boeing has modified the tube, and Alaska Airlines reported that they will modify their fleet with the new hydraulic line/tube.

The Boeing Air Safety Investigation department confirmed that the loss of hydraulic for System B was due to a disconnect of tube part number 312A2110-2, from tube part number 272A3151-7; the B-nut that joins the two tubes together had unscrewed. While on final approach the System A tube, part number 272A4451-87 fractured and lead to the failure of System A. Examination of the B-nut for tube 312A2110-2 revealed no discrepancies with the threads of the B-nut. Boeing personnel further noted that tube part number 272A4451-87 was replaced in production at l/n2346 by tube 272A4451-200. The original tube 272A4451-87 was constructed out of titanium; the new tube, part number 272A4451-200 is constructed of stainless steel.

NTSB Probable Cause

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.