ASN Wikibase Occurrence # 314387
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| Date: | Friday 27 July 2012 |
| Time: | 13:14 LT |
| Type: |  Boeing 737-3H4 |
| Owner/operator: | Southwest Airlines |
| Registration: | N379SW |
| MSN: | 26586/2580 |
| Year of manufacture: | 1994 |
| Total airframe hrs: | 59940 hours |
| Engine model: | CFM INTL. CFM56 SERIES |
| Fatalities: | Fatalities: 0 / Occupants: 76 |
| Aircraft damage: | Minor |
| Category: | Serious incident |
| Location: | El Paso, Texas -
 United States of America
|
| Phase: | Approach |
| Nature: | Passenger - Scheduled |
| Departure airport: | Las Vegas-Harry Reid International Airport, NV (LAS/KLAS) |
| El Paso International Airport, TX (ELP/KELP) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The aircraft experienced a structural failure of the nose landing gear (NLG) actuator retract beam when the landing gear was selected down during approach. The flight crew heard a loud noise during gear actuation, performed a go-around, and then returned to land uneventfully. Examination of the aircraft revealed a fracture across the horizontal leg of the actuator retract beam that propagated into the radius of the vertical leg. The fracture surface features were consistent with overstress failure with no sign of fatigue. The NLG hydraulic line to the actuator was found ruptured with abrasion damage aligned coincident with the NLG tire.
Disassembly of the actuator revealed the restrictor fitting was installed in the head end of the actuator and the union fitting was installed on the rod end cap although engineering documentation requires the restrictor fitting to be installed in the rod end cap and the union fitting to be installed in the head end. The union and restrictor are normally placed in the actuator prior to installing the actuator on the airplane.
Hydraulic fluid leaking from the actuator through the ruptured line allowed air to fill the actuator cavity on the head end of the actuator while hydraulic system pressure remained on the rod end. When the landing gear was selected down, air in the actuator allowed the actuator piston to move rapidly toward the head end. Due to the union rather than the restrictor being installed on the piston end, higher than normal hydraulic flow was permitted in the rod end of the actuator increasing the rate of piston movement. When the piston reached the end of range of motion, the resulting force was transferred to the NLG retract actuator beam.
Probable Cause: the rupture of the nose landing gear hydraulic line that resulted from abrasions due to contact with the nose landing gear tire. Contributing to the incident was the incorrect installation of union and restrictor on the actuator.
Accident investigation:
 |
|
Sources:
NTSB DCA12IA113
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 02-Jun-2023 16:49 | ASN Update Bot | Added |
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