Incident Overview
A flight experienced an inability to extend the nose landing gear, resulting in a landing with the nose gear retracted. Post-incident examination determined that the nose gear drag brace was locked in the up position.
Investigation Findings
Disassembly of the drag brace revealed that the link assemblies connecting the locking pawls to the locking actuator had failed. Metallurgical examination attributed the failure to corrosion cracking. The corrosion was caused by an electrolytic reaction between the metal in the bushing of the link and the metal of the link itself.
Component Condition
Further inspection of the bushings installed in the link showed an absence of cadmium plating and chromate coating. The bushings had been obtained by Eastern Airlines from a vendor who manufactured the part using plans supplied by Eastern. Those plans specifically called for cadmium plating and chromate coating.
Conclusion
The failure of the link assemblies due to corrosion cracking, resulting from the lack of protective coatings on the bushings, prevented the nose gear from extending.