What happened
On July 2, 2006, a Boeing 737-500, registration OK-CGH, was operating flight CSA 866 from Prague-Ruzyně to Minsk (UMMS). Approximately 20 minutes before landing, while the aircraft was descending through the Minsk FIR, the crew detected an unpleasant odor accompanied by a streak of smoke in the cockpit. The smoke appeared to originate from the overhead panel (P5) near the map light reading lamp.
The flight crew immediately activated the "SEAT BELTS" warning lights, notified Minsk approach, and donned oxygen masks. The pilot in command (PIC) coordinated with the cabin crew regarding the cabin air quality and potential evacuation needs. The crew initiated the "Non-Normal Check List/Electrical Smoke" and performed an accelerated descent to FL150 at 4,500 ft/min. After approximately one minute, the smoke dissipated. Once the situation stabilized, the crew removed their masks and inspected various instrument panels (P1, P2, P3, P5, P8, and P9); all systems were found to be functioning normally. The crew proceeded with a standard landing at Minsk without declaring an emergency.
Upon arrival at the gate, the crew discovered that the thrust reversers could not be deployed. Subsequent inspection revealed that all four circuit breakers for the reverser power supply on panel P6-2 had been tripped. During the landing, the crew had to use manual braking as the reversers failed to extend.
The investigation
The ÚZPLN investigation focused on the source of the smoke and the malfunction of the thrust reverser controls. Technical personnel were dispatched to Minsk to inspect the wiring and equipment. While no thermal damage or smoke source was found in the cockpit wiring during the initial check, the investigation later identified thermal damage to the L4 window sunshade.
Further inspection at Prague-Ruzyně revealed a 2x2cm area of thermal damage on the sunshade at the point where it contacted the map light fixture. The investigation also examined the thrust reverser issue, noting that the breakers had likely tripped during the deployment attempt after landing. Subsequent maintenance tests confirmed the reversers were functional, and the issue was resolved after the breakers were reset and the system was tested according to the manufacturer's manual.
Findings
- The smoke and odor were caused by thermal damage to the L4 window sunshade resulting from heat generated by the map light bulb at its point of contact with the shade.
- The thrust reverser circuit breakers likely tripped during the activation of the reversers upon landing.
- The crew responded effectively to the smoke in accordance with emergency procedures.
- The aircraft was found to be airworthy and fully operational following the replacement of the damaged sunshade and the thermal sensor.