What happened
On September 17, 2015, an ATR 72-500, registration OK-GFS, operated by Czech Airlines, was performing a scheduled commercial flight from Prague/Ruzyně (LKPR) to Hamburg (EDDH). During the initial climb, the crew noted a malfunction in the number 2 engine bleed air system. While the crew attempted to reset the system during the climb, a significant temperature increase occurred in the air conditioning ducts.
At approximately 3,800 ft, the flight crew received warnings for high cabin temperature and air system faults. Shortly thereafter, at 4,140 ft, the cabin crew reported smoke in the passenger cabin. The commander immediately declared a "MAY DAY" and initiated an emergency return to Prague. During the descent, the crew experienced increased workload as the commander flew the aircraft manually. Although the crew followed emergency procedures for smoke, they failed to update the altitude selector on the autopilot, which led to the aircraft climbing to 4,420 ft before ATC instructed a descent back to 3,000 ft.
Despite the smoke, the aircraft landed safely on runway 24 at 16:15 local time. Following the landing, the aircraft remained on the runway while emergency services performed inspections. There were no injuries among the 4 crew members or 59 passengers.
The investigation
The ÚZPLN investigation focused on the source of the smoke and the temperature rise. Investigators examined the aircraft's flight data recorder (QAR), cockpit voice recorder (CVR), and radio communications. The investigation also reviewed the crew's adherence to standard operating procedures (SOPs) and the effectiveness of the emergency response by both the flight and cabin crews.
Findings
- The primary cause of the smoke was a leak of hot air (reaching temperatures up to 200°C) caused by damaged or worn rubber-textile sealing rings in a pneumatic compensator located under the passenger cabin floor. This hot air caused dust to become airborne, which was then distributed throughout the cabin by the recirculation fan.
- The high cabin temperature was caused by a leak in a rubber-textile compensation bellows connecting the air conditioning heat exchanger to the air conditioning ducting.
- The crew's workload was increased by manual flight during the emergency, and a failure to update the altitude selector on the autopilot led to an unintended climb.
- The cabin crew effectively managed the situation by using Personal Breath Equipment (PBE) and providing improvised respiratory protection to passengers using wet paper towels.
Safety action
As a preliminary measure, the operator issued Safety Bulletin 08/15 for the ATR 72 fleet, recommending the inspection and replacement of pneumatic compensator seals during the next available maintenance interval.