In a new problem for the Boeing 787, the US aviation regulator has ordered repairs to correct a software bug that could cause the aircraft to suddenly lose all power.
The Federal Aviation Administration issued a directive dated Friday warning that after a 787's generators have run continuously for 284 days, they could abruptly shut down, leading to a loss of aircraft control.
The FAA said Boeing itself identified the problem: an internal software counter in the generator control units (GCU) will overflow after 248 days of straight use.
In that case, the FAA said, "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," it said.
The FAA said the generators on aircraft need to be completely shut down to reduce the immediate danger while manufacturers of the equipment involved develop software upgrades to eliminate the problem.
Until then, it advised the operators to shut off the generators every 120 days to avoid danger in the future.
Boeing said it had already advised 787 operators two weeks ago of the potential problem, and that all had completed the steps to turn off and restart generators.
"It is important to note this issue was observed in the lab only (after 8 months of continuous power which would be highly unusual)," said spokesman Doug Alder in an email.
"No airplane in the fleet experienced that condition."
The 787 Dreamliner was grounded globally in 2013 over a separate electrical problem.
Early that year several planes experienced problems with batteries overheating that caused a fire on one aircraft. The FAA forced the global fleet of 50 787s out of service for three months while the problem was investigated and changes made to prevent its recurrence.