What happened
On July 1, 2013, an ATR 72-500 operated by TransAsia Airways, registration B-22806, departed Taipei Songshan Airport for Kaohsiung. During the initial climb, the flight crew noticed a significant rise in cockpit temperature, accompanied by a white mist appearing from the air conditioning vents and side panels. The crew attempted to manually adjust the temperature and reset the number 1 air conditioning pack and engine bleed air system, but the heat continued to intensify.
As the aircraft climbed through approximately 4,000 feet, an "OVERHEAT AIR" warning appeared on the instrument panel. Shortly thereafter, the crew received an "ELEC SMOKE" warning on the Crew Alerting Panel. Due to the intense heat and the presence of the mist, the captain declared a "Pan Pan" call to air traffic control, requesting an immediate return to Songshan Airport. During the descent, the crew activated the avionics vent exhaust overboard, which caused the mist and warning lights to dissipate. The aircraft landed safely at Songshan Airport with no injuries to the 72 passengers or 4 crew members.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined the aircraft's air conditioning and electrical systems. The investigation focused on the failure of the temperature sensors and the crew's response to the smoke warning. Investigators also reviewed the airline's training manuals and the manufacturer's smoke procedures to determine why the crew did not immediately implement standard smoke emergency protocols.