What happened
On 20 April 2003, a Boeing 747-436, registration G-BNLC, was performing a passenger flight through the Riga FIR when the crew detected smoke and fumes within the cockpit. The incident began after a crewmember requested entry to the flight deck to begin a rest period. Although the crew initially denied access, they eventually used the electrical unlock function. The crewmember experienced difficulty opening the door, eventually forcing it open with a loud impact.
Following this, the crew attempted to relock the door, but the electrical mechanism failed to engage, triggering a 'LOCK FAIL' light on the centre pedestal. The crew followed established procedures to manually lock the door. Shortly after, the flight crew noticed a smell of electrical burning. While the commander initially suspected the galley ovens, the smell intensified, and cabin crew reported unusual odors in the first-class cabin.
As the fumes became more prominent, the crew donned oxygen masks and declared a MAYDAY. The aircraft diverted to Riga, Latvia. During the approach, the commander briefly removed his mask to improve visibility due to misting on the visor. The aircraft landed safely without further incident, though a medical examination of one crewmember revealed elevated carbon monoxide levels in his blood.
The investigation
Investigators examined the cockpit door lock striker assembly and identified that the overheating was localized to the solenoid. The investigation established that the solenoid had been receiving continuous full electrical current, which it was not designed to sustain.
It was determined that the solenoid shaft had failed to fully retract when power was removed. This prevented the locking pin from retracting, causing the spring-loaded striker to jam against the pin. This mechanical jam prevented the solenoid from extending fully when the crew attempted to relock the door, which also prevented the internal micro-switch from activating. Because this micro-switch failed to signal that the door was locked, the system did not reduce the electrical current, leading to the overheating and subsequent smoke.
Findings
- The overheating was caused by the incorrect installation of a spring clip during the manufacture of the solenoid unit.
- This improper installation allowed a hexagonal nut to rotate, preventing the solenoid shaft from retracting fully.
- The mechanical jamming of the striker assembly caused the 'LOCK FAIL' light to illuminate and prevented the electrical current from being reduced to a safe level.
- There were no specific flight crew procedures for addressing a continuous 'LOCK FAIL' indication, other than those found in the MEL and FCN.