Based on test data, the FAA will allow Boeing 787 operators to fly with inoperative “pressure burst discs” and burst disc indicators on main and auxiliary power unit (APU) lithium-ion battery enclosures for as many as 34 flights before fixing the safety equipment.

The extension is included in an update to the 787 master minimum equipment list (MMEL) that the FAA says increases dispatch flexibility for the 787.

The burst disc safety equipment was included as part of the 787 main and APU lithium-ion battery redesigns that Boeing developed after batteries failed on two different aircraft in January, grounding the fleet. As of early June, all 787 operators had begun flying again with the modified aircraft.

Along with internal changes to the battery, the fix includes a 1/8th-in.-thick stainless steel battery enclosure and titanium vent duct routed to the exterior of the aircraft for each battery. The burst disc is mounted between the battery enclosure and the vent duct to isolate the batteries—located within the pressurized sections of the aircraft—from the outside ambient air pressure.

“Should a battery failure occur, and generate significant heat, pressure, and gasses, a metallic frangible disc (also referred to as a vent burst disc) at the interface of the enclosure and vent duct will open and allow the heat, pressure, and gasses to safely vent overboard through the [duct],” the FAA explains in its April 26 airworthiness directive requiring the battery fixes. “This will prevent the introduction of any heat, pressure, or gasses in the electronics equipment bays or any occupied area of the airplane.”

The burst disc indicator is located within the battery enclosure, requiring some disassembly of the box and an internal inspection to determine its status. Operators are required to perform an inspection of the indicator every 14 flight cycles.

That check is independent from the changes to the master minimum equipment list, which defines the on-board equipment that must be operational in order to dispatch the aircraft.

The FAA tells Aviation Week that while Boeing originally assumed that the battery would “need to be in a pressurized compartment to operate safely,” testing of the modified battery system by Boeing, and witnessed by the FAA, “has demonstrated in a test environment there is no damage to the battery after taking the battery to maximum altitude, unpressurized, for 35 flight cycles.”

In the new minimum equipment list, the FAA allows operators who find a failure of the burst disc indicator during the 14-cycle inspection to continue dispatching the aircraft for an additional 21 flights before fixing the equipment.

“FAA arrived at a 21-flight-cycle MMEL relief with the following logic: In a worst-case scenario, the indicator fails on the first flight after the mandatory maintenance 14-cycle inspection has been completed,” the FAA tells Aviation Week. “After inspection, 13 flight cycles later, it is discovered the indicator has failed. If the Fault Isolation Manual process determines it is a faulty indicator, the 21 flight cycles Boeing requested for their MMEL relief falls within the 35 flight cycles Boeing has already tested.”