What happened
On 16 November 2003, an Embraer EMB-145LU, registration HB-JAU, operated by Swiss International Airlines, departed Zurich Airport for Hanover. During taxiing, an unusual noise was noted near the recirculation fans. Shortly after takeoff, a cabin attendant reported smoke spreading across the cabin floor between rows 4 and 10. This was accompanied by visible frost appearing in the area and a significant drop in cabin temperature.
In response to the developing situation, the pilot decided to execute an immediate return to Zurich, requesting a priority landing. During the descent, the smoke gradually dissipated. The aircraft landed safely at 19:59 UTC, and passengers were permitted to disembark normally at the gate.
The investigation
The investigation examined the aircraft's recent maintenance history, noting that similar issues regarding the air conditioning system had occurred on two previous flights within the preceding days. While a previous hose crack had been repaired, the aircraft had been released for this flight with the second environmental control system (ECS) pack listed as inoperative under the Minimum Equipment List (MEL).
Technical examinations of the aircraft revealed that a hose leading from the recirculation fan to the ECS pack had separated. Further inspection of the spray nozzle, which is responsible for directing water away from the water collector, uncovered a small piece of solder residue. This foreign body had blocked the nozzle, preventing water from being properly diverted.
Findings
- The primary cause of the incident was a foreign body in the spray nozzle of the left environmental control system.
- The blockage caused water to accumulate in the collector and reach the turbine, where it formed ice crystals.
- These crystals blocked the condenser, creating high pressure that caused the recirculation fan hose to fail.
- The failure allowed ice crystals and fumes to enter the passenger cabin, leading to the observed frost and temperature drop.
- The maintenance decision to release the aircraft with a deferred defect, despite a history of nearly identical symptoms, was deemed inappropriate.
Safety action
Following the investigation, the operator implemented two new periodic maintenance checks to prevent recurrence:
- A check every 500 flying hours to ensure spray nozzles remain clear.
- A cleaning procedure for the spray nozzles every 2000 flying hours.
These new procedures were developed in coordination with the manufacturer, Embraer.