Federal regulators will order operators of Boeing 787 Dreamliners to shut down the plane’s electrical power periodically after Boeing discovered a software error that could result in a total loss of power.
The Federal Aviation Administration said that Boeing found during laboratory testing that the plane’s power control units could shut down power generators if they were powered without interruption for 248 days, or about eight months. The findings were published in an airworthiness directive.
Boeing said the problem had occurred only in lab simulation and no airplane had experienced it. Boeing said that powering the airplane down would eliminate the risk that all power generators would shut down at the same time.
The company said it was working on a software update that should be ready by the fourth quarter this year.
Boeing has so far delivered 264 787s to carriers including Japan Airlines, Air India and Ethiopian Airlines. It alerted operators about the potential problem two weeks ago.
The plane maker said that power was shut down in all airplanes in service in the course of the regular maintenance schedule, and that it would be rare for a plane to remain with power on without interruption for eight months.
No immediate action is required from operators whose planes had a power cycle in the past 120 days, according to Boeing.
The Boeing 787 was built with a lighter carbon composite structure, and many mechanical components have been replaced with electrical ones to save weight and provide more economical operation. As a result, it is also much more reliant on electrical power than previous generations of airplanes.
The 787 has six electrical generators. Two 250-kilovolt-ampere units are mounted on each of the two engines, and two 225-kilovolt-ampere units are used as backup generators. The generators provide power for a variety of functions on the aircraft, including running the plane’s avionics, pressurizing the cabin and de-icing wing parts.
Each generator is linked to a control unit. Boeing found that if the four engine generators were left on continuously for about eight months, a software internal counter would overflow and cause the control units to enter a fail-safe mode. The FAA warned that this could result in a loss of all electrical power, regardless of whether the plane was in flight.
In the event that the main power generators all failed, however, the plane’s main lithium-ion battery could provide power to the flight deck for six seconds until a ram air turbine, or RAT, deploys. This fan turbine provides a small amount of emergency power that would allow pilots to turn the engines’ power back on or fly the plane to an airport.
The 787’s electrical systems have experienced a series of problems since the plane entered service in 2011. The entire 787 fleet was grounded for more than three months in early 2013 after two incidents involving lithium-ion batteries, including a fire on a parked 787, which eventually required Boeing to redesign the battery casing as well as the internal fuel cells.
Qatar Airways and other operators have also reported failures with the plane’s main electrical panel, which led to the grounding or delay of flights.
Last year, Boeing received a waiver from the FAA that allowed it to deliver its first 787-9, a stretched version of the Dreamliner, even after two components failed to meet airworthiness regulations.
One of those components was linked with the ram air turbine and needed to be redesigned to improve its reliability after it failed a noncertification flight test. The redesign was not be ready until early 2015, more than six months after the first 787-9 entered service in mid-2014.
When Boeing sought the exemption, it argued that the chances of a dual engine failure and total loss of power were extremely remote.