Boeing has rolled out a permanent fix for a design problem in the 787-8's auxiliary power unit (APU) that flight test crews identified more than three years ago.

A new APU design has recently entered service that prevents the shaft from “bowing” under certain conditions and needing a replacement, Boeing says.

Some of the 787-8’s earliest operators have started receiving the new units of the Pratt & Whitney APS5000, confirms Japan Airlines airplane performance group director Eisuke Hama.

“In early September, all the APUs were changed or replaced with an improved one,” Hama says.

The APS5000, a single-shaft, variable speed gas turbine that develops 450kVA, serves a crucial role in the 787’s electrical system. On the ground, the APU is used to power the variable frequency starter generators, which, in turn, start the engines. In-flight, it can supply back-up power if the four engine-mounted electric generators fail at the same time.

During flight test, however, Boeing detected a design flaw with the system. After the APU is shut down, if the inlet door to the tail-mounted APU is closed, hot air could be trapped inside the unit. Over time, the extra heat could cause a shaft imbalance, or “bowing”. The imbalance wouldl later potentially prevent the APU from starting, leading to a condition called a “bowed rotor hung start”.

“We caught the bowed rotor issue on our flight line because we were doing successive starts that would not be typical in an airline operation,” says Mike Fleming, Boeing’s vice-president for 787 support and services.

To prevent a bowed rotor from developing, Boeing developed a procedure that proved unpopular with 787 flight crews. It requires them to leave the APU door open for at least 40min after it is shutdown. Since the APU is usually turned off as the 787 is leaving the gate, the 40min clock would often expire during a high workload time of flight as the aircraft ascends to cruising altitude through multiple air traffic control systems.

“This is a nuisance,” says Yohannes Heilemariam, Ethiopian Airlines’ chief pilot. “Sometimes we forget [to close the door].”

Check out our dedicated 787 in-service report page here

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