Peekay4 said
No, it is perfectly clear.
Each GCU is independent. The counter is internal to each GCU. Failure of one GCU only affects that particular GCU, and will not affect any others.
yet the FAA doc first page says very clearly
https://s3.amazonaws.com/public-insp...2015-10066.pdf
This AD was prompted by the
determination that a Model 787 airplane that has been powered continuously for 248 days
can lose all alternating current (AC) electrical power due to the generator control units
(GCUs) simultaneously going into failsafe mode. This condition is caused by a software
counter internal to the GCUs that will overflow after 248 days of continuous power. We
are issuing this AD to prevent loss of all AC electrical power, which could result in loss
of control of the airplane
Loss of ALL AC power includes APU
But further on says
If the four main GCUs 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.
Now granted the 248 day is a stretch- but its still a good bet that both engines ( 4 gen total ) are started within minutes of each other, and absent some special maintence on one engine, the time count would be within minutes.
The role of the APU in that case is not well defined.
And is there a common GCU counter for all in the system ?
Can you start via windmill any engine with a ' bricked ' GEN ? as in the case where both engines drop off within a few minutes
Or can the APU cross feed to the engine ignition system ??
Anyone KNOW for sure ?