The FAA has disclosed that B787s could lose all AC electrical power after being continuously powered for 248 days. In some ways, the bug resembles the Y2K “Millennium Bug” that had some people in a panic in 1999. While the Millennium Bug never came to anything, this time the FAA considers the threat is real.
If left unchecked, it could leave an aircrew unable to control the plane.
The FAA issued a directive on Friday ordering “a repetitive maintenance task” for B787s due to the power supply problem.
Extracts from the FAA order:
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.
We consider this AD [airworthiness directive] 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.
FAA’s Justification and Determination of the Effective Date
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.
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.
Each power system thus has to be shut down and rebooted within each 248 day cycle. It’s the latest odd glitch to hit the radical, fuel-efficient Boeing aircraft. Previous problems have included several instances of batteries overheating.
The resemblance to the “Millennium Bug” is that both are linked to software counters. The bug, or Y2K, concerned a digital rollover from 1999 to 2000. The B787 rollover is linked to a cycle counter – 248 days of continuous power, with 248 being a seemingly arbitrary number corresponding to a few days over 35 weeks or five months.
However it works, they had better get it right.
Written by Peter Needham