Bombardier has developed an improved Honeywell GTCP36-150 (CL) auxiliary power unit retrofit programme for the Challenger 604 and Challenger 601 aircraft. "The new unit has higher temperature margins, resulting in an increased hot section inspection interval of 2,400 hours, increased reliability and lower maintenance costs," the company says. Compared with the original GTCP36-100 the new APU offers 500-hour intervals, up from 400 hours, plus enhanced operational limitations for in-flight start capabilities, bleed air extraction and APU generator usage conditions. The new APU has been standard equipment on Challenger 604 aircraft since the third quarter of 2005.