What happened
On June 3, 2015, an Avro 146-RJ100, registration HB-IYT, operated by Swiss Global Air Lines, departed Zurich for Bari. Approximately 25 minutes into the climb, the crew observed a "HI TEMP" warning for the green hydraulic system. Following established checklists, the crew deactivated the green system to prevent further issues. To avoid an aircraft on ground situation in Bari, the crew declared a PAN PAN and elected to return to Zurich.
During the downwind leg of the approach to runway 28, a second "HI TEMP" warning appeared, this time for the yellow hydraulic system. While the checklist allowed for the system to remain active for the approach, the crew proactively used the emergency gear lowering system to ensure the landing gear was down. The aircraft landed safely, though the loss of the green system resulted in the loss of nosewheel steering and reduced flap extension speed. The aircraft had to be towed to its stand, blocking the runway for approximately five minutes.
The investigation
SUST examined the hydraulic components and the operational environment. Post-flight inspections of the hydraulic reservoirs revealed they were warmer than usual. Maintenance testing identified that the engine-driven pump (EDP) for the green system was delivering an overpressure of 3300 PSI, exceeding the nominal 3100 PSI. Additionally, the heat exchanger cooling fan for the yellow system's AC pump was found to be non-functional.
An inspection of the replaced EDP revealed significant wear on the drive shaft. The cooling fan was found to have a defective motor stator and degraded electrical wiring. The investigation also reviewed the air traffic control procedures in place at Zurich, noting that an approach to runway 34 would have required a MAYDAY declaration due to active VFR traffic in the area.