The A350 is equipped with the most powerful auxiliary power unit (APU) ever built by Honeywell (Chalet 106), producing 200 shp more than the 331-600, the previous top-of-the-line model, but with virtually no additional weight.

A redesigned reduction gearbox accommodates the more-powerful 230-volt AC, 150-kVA starter generator. The latest digital electronic control unit (ECU) provides carefree handling, cold-soaked starts at up to 43,100 ft. at ISA-24C, health monitoring and exception logging. The ECU communicates with the aircraft by means of the AFDX Ethernet local-area network.

Similar to other members of the 331 APU family, the HGT1700 is a single-shaft design with a single load compressor, dual engine compressors and three turbine stages. Advanced computational design tools were used to refine the aerodynamics of both the load and engine compressors. The HGT1700 is the first APU to use the rich-quench-lean, low-emissions combustor technology incorporated in Honeywell’s latest business aviation propulsion engines. This results in a 10% reduction in NOX emissions.

This is also Honeywell’s first variable-speed APU, allowing the engine to run at 90-100% rpm depending upon electrical and pneumatic loads. The feature potentially yields a 10% improvement in fuel efficiency, along with proportionately lower emissions. Revised acoustical treatment of intake and exhaust ducting in the aircraft tailcone reduces airport noise.

Individual components and the entire APU underwent extensive ground and flight testing before the first units were installed on the A350. The APU is designed for easy maintenance access. Routine inspections may be performed while on the aircraft using about a dozen basic hand tools.