Boeing Tackles 787 APU Heating Issue

By Guy Norris
Source: AWIN First
May 22, 2013
Credit: Boeing

As Boeing 787s re-enter service on routes around the world following the aircraft’s prolonged grounding for battery problems, the company is already busy resolving other issues that were emerging before they ceased flying 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 problem, fell into the ‘teething trouble’ category which Boeing uses to describe the steep learning curve of early service life.

These issues impacted the early dispatch reliability of the aircraft, giving it a reliability level in the high 90% levels, roughly similar to the initial performance of the 777-200 shortly after its entry into service in mid-1995.

Although fixing many of these issues pale 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 Hamilton Sundstrand into a design revision.

Operators have discovered that after the APS5000 APU is shutdown with the inlet door closed after landing, heat continues to build up in the tail compartment. After some 20 min. this causes the rotor shaft to bend or ‘bow’, and the shaft takes up to two hours to straighten back out.

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 that if an APU start is attempted 20-120 min. after shutdown with the inlet door closed, the “bowed rotor shaft can cause turbine rub and significant damage.” If such an event 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 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 on the last shutdown which will open the inlet door, allowing the unit to cool down. The door must remain open for 40 min. before closing it in order to enable the APU to be restarted.

The notice indicates that the APU could be re-started without causing damage if it was re-activated within 20 min. of shutdown, or after 120 min. had elapsed.

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