All Boeing 787 operators will be required to periodically deactivate the electrical system to avoid a problem with a newly-discovered software bug that could cause the aircraft to lose alternating current (AC) power, the US Federal Aviation Administration says in a new airworthiness directive.

The agency adopted the final rule after Boeing reported the results of a laboratory test showing a total loss of power is possible if the generator control units run continuously for eight months, says the FAA’s 30 April notice in the Federal Register.

The binding airworthiness directive is being published less than two weeks after Boeing privately alerted operators about the problem, the company says in a statement to Flightglobal.

It is rare for a commercial aircraft to remain powered on for eight months with no interruptions.

So far, no 787 operator has experienced the software problem that causes four onboard generators to stop working at the same time, Boeing says.

Boeing is working on a software update to fix the problem that should be ready in the fourth quarter, the company says.

The 787 relies on electric power more than previous commercial aircraft. In addition to powering onboard avionics, the 787’s electrical generators also are used to pressurise the aircraft cabin and de-ice the leading edge of the wing.

Two 250kVA generators are installed on each of the 787’s pair of turbofan engines. Another two back-up generators each rated at 225kVA are connected to the auxiliary power unit. If all six generators fail at the same time, a lithium-ion main battery keeps power running to the flightdeck systems for about 6s until a ram air turbine can deploy and begin generating enough power to help the pilots navigate while attempting to restart the engines or glide to an unpowered – or “dead-stick” – landing.

All six power generating systems are managed by a corresponding generator control unit (GCU). Boeing’s laboratory testing discovered that an internal software counter in the GCU overflows after running continuously for 248 days, according to the FAA. The overflow causes all four GCUs on the engine-mounted generators to enter failsafe mode at the same time.

The software overflow problem is the latest in a string of reliability bugs to surface in the 787’s electrical system. The FAA grounded the 787 fleet for four months in 2013 after two batteries overheated, leading to a redesign of the battery system installation. Last June, the FAA approved an exemption to allow the 787-9 to enter service on schedule despite a substandard reliability record on the GCU for the RAT. The agency approved the exemption because it was deemed extremely improbable that all six power generators on board could fail at the same time.

A redesigned RAT was cut into the 787-9 production line on schedule in February and Boeing is continuing to retrofit previously delivered aircraft, a company spokesman says.

Source: Cirium Dashboard