Incident Overview
A flight crew conducting an international passenger flight under Title 14, CFR Part 121, detected smoke and a burning smell in the cockpit. The pilot declared an emergency and diverted to the nearest airport, where the aircraft landed without further incident. According to the crew, the smoke appeared shortly after a crew change that required the cockpit security door to be opened and closed.
Examination Findings
Maintenance personnel and a National Transportation Safety Board investigator examined the security door. They found an excess length of wiring, which supplied power to the electrically locking door solenoid, lying on top of the metal-encased, unshielded locking solenoid inside the doorframe. Several of the wires were encased in a plastic anti-chafe mesh. A portion of the mesh had melted and emitted a burnt plastic odor.
During discussions, a mechanic reported observing similar doors overheat when misalignment between the door and frame prevented full extension of the locking bolt.
Door Modification and Maintenance
The cockpit door had been modified under Supplemental Type Certificate (STC) ST01391LA to meet FAA reinforced cockpit door requirements. It used a "demand access" electrically operated door latching solenoid. The door manufacturer issued an addendum to the aircraft manufacturer's maintenance manual covering removal, installation, and system tests of the striker/solenoid unit. However, the installation section did not specifically address securing excess wiring located above the striker/solenoid within the doorframe.
The installation instructions included a caution noting that the security bolt must engage fully; failure to do so would cause the solenoid to remain energized and overheat. The striker/solenoid assembly was constructed so that the metal-encased electrical coil of the unshielded solenoid formed the uppermost portion within the doorframe.
A warning light was installed to illuminate if the solenoid failed to shut off. The crew did not report seeing this warning light during the flight. Subsequent ground tests confirmed the light and locking mechanism functioned properly, but in-flight testing was not conducted.
The door conversion was completed on March 21, 2003. At the time of the incident, the door had accumulated approximately 16,416 flight hours and 1,170 cycles.
Related Guidance
FAA Advisory Circular (AC) 65-15A, Chapter 11, provides general recommendations for aircraft electrical wiring installation and maintenance. It states that such practices are not intended to replace manufacturer's instructions. On page 441, it notes that "Single wires or wire bundles should not be installed with excessive slack." Page 442 adds that wires should be routed and installed to protect them from high temperatures.