GCUs Can Shut Down In Flight If They Are Not Cycled Periodically

The FAA has issued an AD (2015-09-07) after being advised by Boeing of an issue identified during laboratory testing. The software counter internal to the generator control units (GCUs) will overflow after 248 days of continuous power, causing that GCU to go into failsafe mode.

If the four main GCUs (associated with the engine mounted generators) were powered up at the same time, after 248 days of continuous power, all four GCUs will go into failsafe mode at the same time, resulting in a loss of all AC electrical power regardless of flight phase.

The FAA issued the AD after evaluating all the relevant information, and determining the unsafe condition described previously is likely to exist or develop in other products of the same type design.

The FAA says the AD is an interim action. The manufacturer is currently developing a GCU software upgrade that will address the unsafe condition identified in this AD. Once this software is developed, approved, and available, the agency might consider additional rulemaking.

The AD applies to all Dreamliners. The agency estimates the cost of compliance to be $2,380 per deactivation cycle.

The AD is effective immediately, but the FAA will accept comments on the AD until June 15.

(Image from file)

FMI: AD