What happened
On October 2, 2018, an Airbus A321-111, registration HB-IOC, operated by Swiss International Air Lines, was flying from Zurich to Malaga. During the initial engine start at Zurich, the flight crew noticed a smell of burnt rubber. Although the intensity decreased, the crew continued the flight. During the climb, the odor returned at approximately flight level 150.
As the aircraft reached flight level 276 near Geneva, the smell intensified, prompting the crew to call flight attendants into the cockpit. One attendant reported a burnt rubber smell, while another described a metallic odor similar to welding. The crew decided to divert back to Zurich. Due to the persistent odor, the crew opted for a direct approach and an overweight landing to avoid further exposure to potential fumes. The aircraft landed safely, and while no damage was found on the exterior, the crew noted that the power supply for the captain's Electronic Flight Bag (EFB) charger had been non-functional since the start of the flight.
The investigation
SUST examined the aircraft and found worn seals in the first air conditioning pack, though this was not the primary cause. A subsequent investigation into the electrical system revealed that wires supplying the captain's EFB charger had been damaged by a sharp metal edge on a cover. This damage caused intermittent short circuits that melted the plastic insulation.
Further investigation revealed a second, separate incident on October 6, 2018, where the crew experienced a "dirty socks" smell during a flight from Naples to Zurich. During this second event, the co-pilot experienced dizziness, headaches, and nausea after landing. Maintenance checks following this second event also focused on the engine, APU, and bleed air systems, but found no further issues beyond the identified electrical fault.
Findings
- The primary cause of the fumes was a short circuit in the EFB charger wiring caused by contact with a sharp metal edge.
- The flight crew did not implement the required Quick Reference Handbook (QRH) procedure for smoke or fumes, which would have involved attempting to isolate the faulty equipment.
- The crew's reaction to the initial odor was delayed, likely because they did not perceive the risk of fire as high.
- In the second incident, the crew did not use oxygen masks despite the presence of an identifiable odor, leading to physical symptoms in the co-pilot.
Safety action
- The investigation highlighted the importance of simulator training to ensure pilots correctly interpret the QRH "SMOKE/FUMES/AVNCS SMOKE" procedure, specifically recognizing that "fumes" are covered under the same protocols as visible smoke.
- The findings emphasize that if the source of an odor is not immediately identifiable or accessible, crews should prioritize the use of oxygen masks to prevent exposure to potentially toxic gases.