Pilot Debrief

BOEING 737-3H4 near El Paso, TX — 2012-07-27

Final reportDCA12IA113
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Date
2012-07-27
Location
El Paso, TX, USA
Airport
KELP
Aircraft
BOEING 737-3H4
Registration
N379SW
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Approach

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.

Contributing factors

Contributing to the incident was the incorrect installation of union and restrictor.

NTSB narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Inspection of the nose gear before landing could have prompted a go-around.

NTSB coding

  • Aircraft structural failure · Approach
  • Maintenance

Evidence available

  • FDR / data
  • 19 docket documents
View NTSB final reportView NTSB docket

Docket documents19

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