Incident Overview
On April 6, 2017, a privately owned Beechcraft B200, registration JA01EP, was conducting a training flight from Gifu Airfield to Takamatsu Airport. Around 10:10 Japan Standard Time, while at approximately 20,000 ft over Komatsu City, Ishikawa Prefecture, white smoke appeared from the right lower part of the center pillar of the windshield. The smoke changed to black, and soot became airborne. The entire right windshield subsequently cracked in a spider-web pattern. The crew executed checklist procedures, including turning off the windshield heater, which gradually reduced the smoke. They declared an emergency and returned to Gifu Airfield, landing at 10:41. No injuries were reported.
Flight History
The aircraft departed Gifu at 09:51 with an instructor captain in the right seat, a trainee pilot in the left seat, and a mechanic in the cabin. The flight was intended to obtain an Instrument Flight Certificate. Shortly after reaching FL150, the crew noticed an odor like electrical burning. At FL200, smoke emanated from the windshield center pillar. The mechanic observed the ammeter indicating normal and advised turning off the windshield heater. After the heater was switched off, smoke subsided, but the right windshield cracked completely. The captain declared an emergency and landed at Gifu.
Damage and Injuries
The right windshield sustained cracks across its entire surface. Burnt marks were found at and around the terminal block of the right windshield. No persons were injured.
Windshield Heater System
The aircraft had two independent windshield heater systems for the left and right windshields. Each system embedded heating elements in the windshield and was controlled by a switch in the cockpit. A windshield anti-ice controller regulated power to the elements based on temperature. The circuit was protected by a 50-amp circuit breaker. The right windshield terminal block had a loose terminal lug with a 0.05 mm gap between screw and lug. The controller and heating elements functioned normally when tested. The circuit breaker had not tripped and operated correctly.
Maintenance History
In January 2012, the windshields were replaced by an outsourcing company due to partial delamination. New screws and washers were installed. The maintenance manual did not specify a tightening torque for the terminal block screws; standard torque (20-25 in-lb) was used. Periodic inspections every 200 flight hours checked for cracks and sealant condition, but no inspection of the terminal block connection was required. Between the 2012 replacement and the incident, no maintenance actions addressed the terminal block. Multiple pilots had intermittently reported a burning odor, which was attributed to aircraft-specific smell and remained under observation. After the incident, the windshields were replaced in July 2017, and no further burning odors were reported.
Analysis
Smoke was most likely generated around the right windshield terminal block due to overheating from increased electrical resistance caused by loosened screws. The loosening likely resulted from insufficient tightening torque during the 2012 windshield replacement, exacerbated by aircraft vibration over time. The cracked windshield was a consequence of the thermal stress.