What happened
On October 13, 2018, a Bombardier CRJ 900, registration ES-ACJ, was second in the landing sequence for runway 11 at Warsaw Chopin Airport (EPWA) when the crew reported smoke in the cockpit. At 11:51 LMT, the crew notified Warsaw Tower of a danger due to smoke, prompting a second-level alarm for airport emergency services.
The aircraft landed at 11:53 LMT under the assistance of the Airport Fire Service. Following touchdown, the crew requested an external inspection for smoke. While the fire service reported no smoke visible outside the aircraft, the crew noted that gray smoke had been emanating from the cockpit ventilation ducts shortly before landing, accompanied by a burning smell. This forced the crew to don oxygen masks. Simultaneously, cabin crew reported smoke appearing in the rear of the passenger cabin. The captain informed passengers that the smoke was caused by a malfunction in the air conditioning system. After the smoke dissipated, the crew noted a noticeable increase in cockpit temperature.
The investigation
Technical personnel inspected the aircraft and identified a failure in the left Pneumatic Air Cycle Kit (PACK). Subsequent detailed analysis was conducted by Lufthansa Technik (LHT) on the Air Conditioning Unit (ACU), specifically focusing on the Air Cycle Machine (ACM).
Before disassembly, investigators found that the ACM turbine was blocked. Upon dismantling the unit, the inspection revealed significant wear on the foil air bearing, spacers, and adjustment washers. Additionally, there were abrasions and scratches on the turbine rotor and shroud caused by contact during rotation, along with evidence of significant overheating of the bearing.
Findings
- The investigation determined that the operational wear of the foil air bearing was the probable cause of the incident.
- The foil air bearing operates by creating an air cushion between the shaft and the bearing surface; however, the Teflon coating degrades with each start/stop cycle of the ACM.
- It is believed that during a phase of flight involving a drop in engine-supplied air pressure, the air cushion failed to form, leading to increased friction and the eventual seizure of the turbine.
- The components involved had been in service for over 10,000 flight hours, and the observed damage patterns are consistent with known failure modes for this specific part number.