Embraer ERJ-145 N12922 incident near Houston, Texas

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 analysis

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.

Text from the NTSB. Open the NTSB docket for ENG13IA020 for the full record.