What happened
On 20 March 2002, a Cessna 750 Citation X, registration N999CX, was performing a ferry flight from Zurich Airport to Geneva. Shortly after takeoff at 07:34 UTC, the flight crew noticed smoke appearing in the rear of the cabin, which quickly spread into the cockpit. The pilots donned oxygen masks and initiated an immediate return to Zurich.
While the crew followed the established cockpit/cabin smoke and fire checklists, the smoke persisted throughout the descent. The aircraft landed on runway 16 at 0 and7:52 UTC, approximately 18 minutes after departure. Although the airport fire services were on standby, the aircraft was able to taxi to the maintenance hangar under its own power. There were no injuries to the two crew members and no damage to the aircraft.
The investigation
Investigators focused on the aircraft's recent maintenance history, noting that the plane had arrived in Zurich two days earlier following a previous instance of smoke. At that time, the crew had suspected a faulty display management unit (DMU). Although the DMU was replaced and ground tests were performed using engine idle thrust, the smoke issue was not detected during those static trials.
Post-incident inspections of the air conditioning machines (ACMs) revealed heavy contamination from both liquid and combusted oil. Subsequent testing confirmed that smoke only occurred when bleed air was drawn from the left-hand engine. A borescope inspection of the left-hand engine later identified that oil was leaking through the entire engine and onto hot components.
Findings
- The primary cause of the smoke was a defective fan carbon seal on the left-hand engine's fan bearing.
- This faulty seal allowed oil to escape and burn on hot engine parts, with residue entering the bleed air supply duct.
- The maintenance ground tests failed to replicate the smoke because they were conducted only at engine idle power, whereas the leak became prominent during the climb phase.
- The manufacturer, Rolls-Royce, had previously issued a Service Bulletin to address similar issues with this specific seal design.