FAA set to review 787 test data as Boeing gears up to restart production flights
The tension is mounting for and its 787 operators as the company prepares to hand over certification data to the from exhaustive tests of the redesigned lithium-ion battery system.
The timing of Boeing's recovery plan from the 787 grounding—now entering its 10th week—and the resumption of commercial services in particular, all hinges on FAA approval of these certification tests. Boeing, which fast-tracked the certification process by conducting around-the-clock ground tests in its extensive Seattle laboratory complex, hopes it will be wrapping up the final reports this week. Testing is also scheduled to include flights of the revised battery system on one aircraft, provisionally set for March 23-24, and ground evaluation in another.
Yet the under-the-gun manufacturer and its customers could seeback in the air sooner than most industry observers expect. Assuming the FAA agrees the redesign complies with the special conditions set out in its January airworthiness directive, airline sources say Boeing could be granted approval to resume production flight tests as early as the end of this month. The move would help clear the logjam of completed aircraft building up at Everett, Wash., and Charleston, S.C., and mark a major step toward resuming normal operations.
The aircraft, including some that were undergoing work at a paint facility in Fort Worth when the grounding occurred, will only fly if they have been modified with the battery enclosure and venting system. Airline sources say the batteries on each aircraft also must be either original “901” units that have passed Boeing's new inspection system or be of the updated design agreed on with lithium-ion battery manufacturer GS Yuasa.
In addition, a number of other aircraft stranded at airports around the world are also expected to be cleared for one-off ferry flights to modification centers by the end of the month. Aircraft thought to be cleared for these flights, which will be permitted without the need for the battery enclosure, include' 787s in Chicago and Warsaw and a 787 in Los Angeles; all will be ferried to United's maintenance facility in Houston. Several aircraft are also expected to make ferry flights to Doha International Airport by March 31, including one airplane stored at and four others at various sites around Qatar. In addition to the battery tests and checks required by the FAA for a one-off ferry flight approved in February, the batteries in each aircraft will be required to pass Boeing's new inspection procedure before flying.
However, despite these encouraging signs for operators, it could still take approximately two more months before services resume. Even assuming swift FAA approval, airlines indicate the earliest commercial flight could resume is not until late May.(ANA), the launch airline for the 787 and—with 17 of the type in its fleet—the carrier that has been impacted the most by the grounding, believes that even when regulators agree that the redesign meets the revised special conditions applied to certification, the retrofit kit will take approximately a month to be installed in its aircraft. Similar holdups face those awaiting delivery of new aircraft. Norwegian Air, which was due in April to accept the first of three 787s scheduled for 2013, has leased two —one for two months and the other for three—to provide capacity in the interim.
Final ground tests of the revised battery system are meanwhile being conducted in the high bay of Boeing's Mechanical Systems Laboratory and the nearby Electrical Components Lab. Evaluations also include tests of the complete battery system on two aircraft. Line No. 86, the aircraft destined for LOT, was slated to undergo wrap-up flight tests late last week. ZA005, one of Boeing's original developmental aircraft, will also be used for ground tests in which the battery will be deliberately failed.
Boeing is confident it can complete its test work plan in short order, mainly because of the engineering development work already completed behind the scenes, says 787-8 Engineering Vice President Ron Hinderberger. Following the FAA's March 12 decision to accept Boeing's redesign proposal, these tests have begun accumulating credit toward certification. Hinderberger says Boeing anticipates “completion should be done within the next week or two.”
New details of the redesigned battery system also reveal why Boeing has remained confident of FAA approval for the fix. The battery enclosure, which is designed to prevent a fire erupting rather than simply containing it, is made of 0.125-in.-thick stainless steel. A 1-in.-dia. titanium vent pipe connects the back of the enclosure to the outer skin of the aircraft where new exit holes—one for each battery—will be cut through the composite skin. The vent pipe is designed to evacuate vaporized electrolytes from the battery should any, or all, of the eight cells in the unit fail. In the event of a cell failure, a small pressure port in the rear of the enclosure is designed to rupture under pressure from the building vapor. The vapor will then exit the aircraft via the vent pipe.
The ability of the design to accommodate the simultaneous venting of all eight cells was one of the key additions to the revised certification requirements proposed by the RTCA, adds Hinderberger.
Boeing says the resulting pressure gradient is sufficient to evacuate the pipe and the enclosure without the need for additional forcing, both in the air and on the ground. Testing at Boeing indicates that even if vaporized electrolytes continue to accumulate inside the enclosure, it cannot be ignited. Only briefly did the mixture deflagrate for milliseconds when fed directly with oxygen. The vent pipe “does not create any new certification requirements as such,” says Hinderberger, who adds that both outlets for the forward and aft electrical and electronic equipment bays are positioned so they do not lead to re-ingestion of the vapor.
The redesign of the battery itself includes the addition of higher-temperature-resistant phenolic glass insulation between individual cells as well as between cells and the battery walls. “We are removing material inside that was only good to 150C [degrees centigrade] and replacing it with phenolic glass laminate, which can go to far in excess of 500C. We've put that between the cells and sidewalls and also installed them on bottom and top, so no matter which way [the battery cells] would move it will come into contact with the phenolic glass, eliminating a possible path to short circuit,” he says.
The battery will also sit on a redesigned frame containing drain holes to allow moisture to escape. In its testing, Vice President and 787 Chief Project Engineer Mike Sinnett says, Boeing found that moisture paths can lead to short circuits in cells which could lead to “stress in the cell,” or the buildup of heat and venting of vaporized electrolytes.
The redesign also addresses the risk of overcharging, which Boeing's original design work indicated was the only probable cause of a serious battery fire. “At an airplane level, what we are concerned about is a whole battery event that is so energetic it puts the aircraft at risk through heat and flame. The only known ability to create what I just described is overcharging,” says Sinnett. A revised charger will reduce the maximum charging levels to prevent overcharging and increase the minimum charging level to prevent “deep” discharging. The revised system will also “soften” the wave form in the charging sequence.
Sinnett says that during the two test flights in February that followed the FAA's initial clearance to fly ZA005 for data collection, “we saw a charging signature which made us ask questions regarding whether the battery could be seeing stress as a result of that signature. We decided to change the circuitry in the charger so we didn't see that signature anymore, in case it was a contributor. We don't know if it was a contributor, but we addressed it anyway.”
Boeing also detailed the beefed-up new inspection procedure that all batteries will undergo before installation. “We put the battery through tests of individual cells, then—once we have an assembled battery—we test it as a complete battery. At a cell level, we do more extensive voltage checks over several days and chart its voltage over time. We look for the cell to discharge or create a fingerprint, so to speak, that's 'out of family.' We are going to only allow [the batteries] to discharge at a certain rate that is less than we did before, so we expect to reject a higher number of parts than we did before,” Hinderberger says.
The voltage and temperature of each cell in the fully assembled battery will be subjected to repeated load tests of 18 kw over periods of 40-45 sec. “For every battery in service we have a manufacturing record, so we know how each one of the cells behaved when they were tested. We know today which cells will not pass the screening tests and when they return from the fleet we will reject those cells. In addition, they will still be tested when they are refurbished,” says Hinderbeger. He adds that 115 batteries are currently in the field.