EMBRAER ERJ-145 near Houston, TX — 2013-03-29

Probable cause

The uncommanded left deflection of the airplane's nose wheel due to a malfunction of the electrohydraulic servo valve (EHSV) within the nose wheel steering manifold assembly. The reason for the malfunction could not be determined; however, a foreign substance was found between the armature and coil of the EHSV that once removed improved the performance of the unit. It is unknown how the foreign substance was introduced to this location.

NTSB narrative

The aircraft experienced several uncommanded swerve events during multiple landings between the dates of 3/19/2013 and 3/29/2013. In each case the crew was able to regain and maintain runway heading. Maintenance troubleshooting led to the removal of the nose wheel steering manifold assembly on 3/29/2013. There were no further reports of uncommanded swerving or pulling to the left after this component was removed. Component level testing found that there were anomalies with the performance of the electrohydraulic servo valve (EHSV) within the nose wheel steering manifold assembly after being cold soaked. A teardown and examination of the EHSV identified the presence of a clear gel-like substance between the armature and the coil. Pressure gain plots showed that hysteresis improved following the removal of the substance from the armature. Since no other anomalies were identified with the EHSV during the examination, and since the removal of the gel-like substance improved the performance of the EHSV during bench testing, it is likely that this substance, when cold soaked, influenced the movement of the armature such that the EHSV commanded a left turn.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Maintenance could have replaced the manifold assembly sooner to prevent swerving.

NTSB coding

• Loss of control on ground · Landing-landing roll

Evidence available

• FDR / data
• 8 docket documents

Docket documents8

• Flight Data Recorder - Attachment 1 (FDR tabular data for event flight)fdr
• Flight Data Recorder - Specialist's Factual Reportfdr
• EHSV Hysteresis Test Resultsother
• Safety Recommendation Transmittal Letterother
• Uncommanded Nosewheel Steering Anomalies During Landing in Embraer EMB-145 Regional Jetsother
• Hydraulic Fluid Analysis Reportreport
• Materials Lab Reportreport
• Systems 9 - Factual Report of Group Chairmanreport

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