What happened
On April 6, 2017, a privately owned Beechcraft B200, registration JA01EP, was conducting a training flight from Gifu Airfield toward Takamatsu Airport. While cruising at approximately 20,000 feet over Komatsu City, the crew began to notice an odor resembling burning electrical components. Shortly thereafter, white smoke emerged from the center pillar of the cockpit windshield, which then transitioned to black smoke accompanied by soot.
In response to the smoke, the crew implemented electrical smoke and fire checklists. The mechanic on board identified the source near the center pillar and advised the pilots to deactivate the windshield heater. Upon turning off the heater, the smoke subsided; however, the right windshield simultaneously developed a widespread, spider-web pattern of cracks. The pilot declared an emergency, and the aircraft returned to Gifu Airfield for an emergency landing.
The investigation
The JTSB investigation focused on the electrical system and the physical state of the windshield. Investigators examined the terminal block and the heating elements of the windshield system. They discovered that the right windshield terminal lug was loose, with a 0/05mm gap between the screws and the terminal lugs. The investigation also looked into the maintenance history of the aircraft, noting that the windshields had been replaced by an external company in January 2012.
Findings
- The investigation established that the smoke was generated by the overheating of the terminal block and its surrounding components.
- The overheating was caused by increased electrical resistance at the contact points, resulting from the loosened screws at the terminal block.
- It is believed the screws were not sufficiently tightened during the 2012 windshield replacement, allowing aircraft vibrations to gradually loosen the connection.
- The heat from the terminal block caused the glass at the lower corner of the windshield to expand, leading to the radial cracking pattern.
- While pilots had previously reported intermittent burning odors, these were treated as a known aircraft characteristic rather than a specific malfunction, and no specific maintenance was performed to address the underlying cause.