Flight Details
On 11 March 2018, a Boeing 737-400 aircraft, registration TC-TLC, operated by Tailwind Airlines, was conducting commercial charter flight XC1845 from Antalya Airport, Turkey, to Zurich Airport, Switzerland. The aircraft carried 89 passengers and 7 crew members. Departure from Antalya occurred at 08:34 UTC under day visual meteorological conditions with clear weather along the route.
Incident Description
The aircraft initially climbed to Flight Level (FL) 320 and maintained that altitude during cruise. After crossing the Bulgarian border at reporting point VADEN, the crew decided to climb to FL 340. During the climb, upon reaching FL 337, an audible cabin altitude warning activated, and the cabin altitude light illuminated. The captain observed the cabin differential pressure reaching the red limit line, and the cabin climb rate indicator showed a climb rate at the 3 o'clock position, indicating an increasing cabin altitude. The crew immediately donned oxygen masks and deployed passenger oxygen masks. They initiated an emergency descent and subsequently performed a forced landing at Sofia Airport, Bulgaria, at 10:08 UTC. The landing was executed without any issues for passengers, crew, or the aircraft. Passengers were later transferred to another aircraft to continue to Zurich.
Investigation Findings
The safety investigation determined that the serious incident was caused by failure of the heater of the outflow valve seal, possibly leading to icing of the valve and its inability to move. Additionally, the right safety relief valve remained in the open position after its activation during the depressurization event.
Occurrence Classification
The incident was classified as a serious incident by the Aircraft Accident Investigation Unit (AAIU) in accordance with Annex 13 to the Chicago Convention and EU Regulation 996/2010.
Injuries and Damage
No injuries to persons were reported. The aircraft sustained no damage from the event.