The U.S. Federal Aviation Administration (FAA) has released a new airworthiness directive (AD) for all Boeing 787 airplanes. This AD requires a repetitive maintenance task for electrical power deactivation on Model 787 airplanes. This AD was prompted by the determination that a Model 787 airplane that has been powered continuously for 248 days can lose all 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. We have been 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 commented in a statement.
This AD was issued after the FAA all the relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the same type design. “This AD requires a repetitive maintenance task for electrical power deactivation. We consider this AD 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, we might consider additional rulemaking,†the FAA continued
“An unsafe condition exists that requires the immediate adoption of this AD. The FAA has found that the risk to the flying public justifies waiving notice and comment prior to adoption of this rule. 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. Loss of all ac electrical power can result in loss of control of the airplane. Therefore, we find that notice and opportunity for prior public comment are impracticable and that good cause exists for making this amendment effective in less than 30 days,†the announcement concluded.
