More trouble for Dreamliner as Federal Aviation Administration warns glitch in control unit causes generators to shut down if left powered on for 248 days

The US air safety authority has issued a warning and maintenance order over a software bug that causes a complete electric shutdown of Boeing’s 787 and potentially “loss of control” of the aircraft.

In the latest of a long line of problems plaguing Boeing’s 787 Dreamliner, which saw the company’s fleet grounded over battery issues and concerns raised over possible hacking vulnerabilities, the new software bug was found in plane’s generator-control units.

The plane’s electrical generators fall into a failsafe mode if kept continuously powered on for 248 days. The 787 has four such main generator-control units that, if powered on at the same time, could fail simultaneously and cause a complete electrical shutdown.

“We are issuing this AD [airworthiness directive] to prevent loss of all AC electrical power, which could result in loss of control of the aeroplane,” said the Federal Aviation Administration directive. “If the four main generator control units (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.”

Should the electrical shutdown happen at a critical phase in flight such as take-off or landing, or while manoeuvring in the air, the loss of control could be catastrophic.

The FAA considered the situation critical and issued the new rule without allowing time for comment. Boeing is working on a software upgrade for the control units that should rectify the bug.

“The airworthiness directive action addresses a condition that only occurred in the lab. Simulated testing determined that this condition is possible in cases where an aeroplane’s power is left on for more than eight continuous months. No airplane in the fleet experienced that condition,” a Boeing spokesman told the Guardian.

According to Boeing’s records, all of the 787s currently in service have been turned off and turned on again as part of maintenance. The FAA’s directive mandates action recommended by Boeing on 19 April.

“If there is a definitive record of a powercycle within the last 120 days, no operator action is immediately required. Operators will perform periodic power cycling at scheduled intervals until incorporating a software update. Boeing will issue in the fourth quarter of 2015,” said the Boeing spokesman.

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