Boeing Tackles 787 APU Overheating Issue

By Guy Norris

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.

In the meantime, Boeing continues to modify the battery system on the last of the original 50 aircraft delivered up to the grounding in January. As of May 21, the company had completed work on 45, of which 40 had flown. It has also delivered the first two new 787s with the revised battery systems installed while still at the Everett, Wash., production site.

United resumed 787 flights on May 20 with a service between Houston and Chicago. The airline, which has six 787s, expects to bring its complete fleet back into service within days and launch international flights with the aircraft on June 10 with a service from Denver to Tokyo. Other long-haul flights, from Los Angeles to Shanghai and Houston to London, will be added in the summer months.

ANA will begin to resume services with its 18-strong 787 fleet on June 1, and plans to introduce the type on additional international routes later this summer including Tokyo Narita to Beijing and Shanghai, and Tokyo Haneda to Taipei. The increase will bring the number of overseas destinations served by ANA's 787s to five. Japan Airlines, which operates seven 787s, is also expected to resume flights on June 1, while Ethiopian Airlines became the first to operate the 787 on a commercial service since the grounding when it flew between Addis Ababa and Nairobi, Kenya, on April 27. Polish flag carrier LOT is scheduled to return its aircraft to service on June 5 with a flight from Warsaw to Chicago, while Qatar Airways resumed 787 services with a flight from Doha to Dubai on May 2.

This aircraft will remain in the public scanner for a long time to come due to wide publicity about the LI battery. A minor incident is bound to get blown up out of proportion. If there is a flight diversion, the first question that will popup will be "Is it that battery again ?". Howsoever difficult, it will be better for Boeing to get rid of the LI battery from this aircraft. Saving 50lb weight and then adding 150lb to protect it makes no sense any way.
The APU shaft bowing problem was experienced on previous aircraft types also. It was solved or worked around till fixed. But this is B787; all problems will be magnified many times for the near future.

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