What happened
On March 14, 2016, a Cessna 525 Citation CJ1+, registered as HB-VWM, departed Lugano-Agno (LSZA) for Frankfurt (EDDF) under Instrument Flight Rules. The flight was being conducted for commercial purposes with a crew of two and four passengers on board. Shortly after the landing gear was retracted during the initial climb, the pilot monitoring detected an unusual odor, which the pilot flying subsequently described as a strong, unpleasant smell resembling oil or engine fumes.
As the aircraft reached approximately 3,000 ft AMSL, a blue-tinted haze began to develop in the cockpit and spread into the passenger cabin. Due to the reduced visibility and the nature of the odor, the crew declared an emergency and immediately aborted the climb, staying below the cloud base to maintain visual conditions. The passengers requested oxygen masks, which the crew promptly provided. After reducing engine power to idle and turning off the air conditioning, the haze cleared significantly. The crew performed a visual approach and landed back at Lugano-Agno approximately four minutes after the initial takeoff.
The investigation
The investigation by SUST focused on identifying the source of the odor and the haze. Because the aircraft was not equipped with a Flight Data Recorder or Cockpit Voice Recorder, investigators relied on crew testimony and physical inspections.
Investigators examined recent maintenance performed at Altenrhein Aviation, specifically a compressor wash of both engines. While maintenance procedures require sealing off air ducts during such procedures to prevent contamination, investigators found that the air ducts contained traces of dried residue, possibly from compressor wash liquid. However, chemical analysis of the residue did not find traces of hydraulic or turbine oils, and the crew could not match the incident odor to known maintenance fluids.
Findings
- The exact cause of the odor and haze could not be definitively identified.
- Condensation of water vapor was considered a highly probable cause; the cooling of high-humidity air within the air conditioning system, combined with moisture added by the six occupants, could have created a visible mist.
- The crew's decision to abort the climb and return to the airport was assessed as a safety-conscious action.
- A failure to follow manufacturer instructions during engine compressor washing—specifically failing to cover air ducts—was noted as a general safety risk, although it was not proven to be the direct cause of this specific event.