Incident Overview
On 26 March 2024, a Bombardier CL-600-2B19 aircraft, registration ZS-CMK, operated by CemAir (PTY) LTD as flight KEM404, was conducting a scheduled commercial flight under Part 121 from O.R. Tambo International Airport (FAOR) in Gauteng province to George Airport (FAGG) in the Western Cape province. The aircraft departed FAOR at 0810Z under instrument flight rules with two flight deck crew, one cabin crew, and 48 passengers on board.
At approximately 0920Z, while cruising at Flight Level 300 (FL300) and about 10 minutes before the top of descent, the starboard (right-side) cockpit windshield cracked. The first officer, who was the pilot flying, promptly donned the oxygen mask and declared an emergency to Cape Town air traffic control (ATC). ATC cleared the aircraft to descend to FL090. The aircraft descended safely and later executed an uneventful landing on Runway 11 at FAGG. The damage was confined to the starboard cockpit windshield; all occupants were not injured.
Windshield Examination
Inspection of the starboard windshield assembly revealed significant fractures on the outer glass layer. Extensive delamination was found between the outer and intermediate layers at the heating element interface. This delamination was attributed to a temperature gradient that increased stress on the glass. The primary cause of the failure was localised arcing where delamination had occurred, likely due to moisture ingress between the glass layers. The arcing increased localised stress, leading to the windshield's crack.
Probable Cause
Electrical arcing in the heating element, evidenced by the discoloured polymer interlayer, was a significant factor which caused the crack on the windshield. The arcing caused localised high temperatures and electrical stress. Extensive delamination between the outer glass layer and the interlayer on the heating element interface compromised the heating element’s effectiveness. The temperature gradient and stress of the malfunctioning heating element likely caused the delamination. The stress which exceeded the tempered glass layer’s limits led to the fracture and further delamination.