Boeing Tackles 787 APU Overheating Issue

By Guy Norris
Source: Aviation Week & Space Technology
May 27, 2013
Credit: Guy Norris/Aviation Week

As Boeing 787s reenter service on routes around the world following the aircraft's prolonged grounding to resolve battery problems, the company is already working to address other issues that were emerging before flying ceased in mid-January.

Most of these problems, such as a string of failures concerning power panels in the electrical system unrelated to the later lithium-ion battery dilemma, fell into the “teething trouble” category that Boeing uses to describe early service life. These issues impacted the early dispatch reliability of the aircraft, giving it a percentage level in the high 90s, roughly similar to the initial performance of the 777-200 shortly after its entry into service in mid-1995.

Although the challenge of addressing many of these issues pales by comparison with the engineering resources involved with solving the battery problem, at least one concern with the operation of the auxiliary power unit (APU) has prompted Boeing and the unit's manufacturer, Pratt & Whitney Aero Power, to undertake a design revision. Operators have discovered that after the APS5000 APU is shut down with the inlet door closed after landing, heat continues to build up in the tail compartment. After 20 min., this causes the rotor shaft to bow, and the shaft takes up to 2 hr. to straighten again.

As many 787 operators, including All Nippon Airways (ANA), have flown the aircraft on shorter routes with reduced turnaround times, this has resulted in restrictions on when the APU can be restarted. An advisory bulletin from United Airlines says if an APU restart is attempted 20-120 min. after shutdown with the inlet door closed, the “bowed rotor shaft can cause turbine rub and significant damage.” If this occurs, an advisory message on the engine-indicating and crew-alerting system shows the APU failed to start and requires the unit to be inspected with a borescope.

Boeing acknowledges that “heat conditions have been found to sometimes influence 787 APU starting performance. As a result, operators have been provided with a revised operating procedure that has eliminated this finding. An improvement to the APU is being introduced in the next few months to remove the operating procedure.”

The revised operating procedure calls for the APU selector switch to be put in the “on” position during a shutdown, which will allow the inlet door to open and the the unit to cool down. The door must remain open for 40 min. before being closed, to enable the APU to be restarted. The notice indicates that the APU could be restarted without causing damage if reactivated within 20 min. of shutdown, or after 120 min. have elapsed.

The revised procedure also has connections with the operation of one of the 787's two lithium-ion batteries. The notice includes a precautionary note that advises against using the APU battery power to keep the door open as this will “only have about 15 minutes before being discharged.” It adds that ground power must be used to keep the APU door open for 40 min. and warns that, if this is disconnected, the door will close even with the APU switch on the flight deck remaining in the “open” position. Boeing declines to specify the design changes that will be made to improve cooling and ventilation of the APU compartment.

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