Bombardier has published a Service Bulletin for a Honeywell GTCP36-150 APU retrofit for Challenger 601s and 604s. The new APU replaces the original GTCP36-100 series and provides significant performance, operational and maintenance benefits over original equipment, the company said. The -150 APU is standard equipment on new Challenger 604s and the Challenger 605, scheduled to enter service in next year’s third quarter.