What happened
Following an uneventful flight and landing, the Bombardier aircraft was parked at the gate with the parking brake set and nosewheel steering shut off. The crew turned off the seatbelt sign and connected a ground power unit. After the green "AVAIL" light illuminated on the overhead panel, the captain selected ground power and turned off the No. 2 generator. At that moment, the crew observed a momentary flash of the cabin and cockpit lights, followed immediately by the collapse of the nose landing gear.
The investigation
An examination of the nose landing gear selector valve revealed that the spanner end cap was backed off and there was slack in the lock wire on the spanner end cap. A test bench examination of the unit identified an internal hydraulic fluid leak, and further disassembly showed that glyd rings No. 1 and No. 4 were damaged.
Findings
Investigators determined that the condition of the lock wire, end cap, and glyd rings had been addressed in two previous service bulletins from both the valve manufacturer, Tactair, and the aircraft manufacturer, Bombardier. The bulletins noted that damaged glyd rings and associated hydraulic leaks in improperly assembled valves could cause the nose landing gear to collapse when accompanied by an interruption of electrical power. While the valve serial number (550) was within the range specified by the Tactair bulletin, the Bombardier service bulletin contained an error regarding its effectivity range. The subject aircraft, serial number 7192, fell outside the incorrectly listed range in the Bombardier bulletin, which stated the bulletin applied to serial numbers 7045 through 7067 and 7069 through 7189. The service bulletin is currently being revised.