Boeing has deployed some 300 personnel across 10 teams to start modification work on the global 787 fleet after US regulators approved a containment system to defend against battery failure.
But the airframer has yet to assess the impact on new deliveries, stating that it is still looking into the details of a revised schedule.
Approval from the US FAA clears Boeing to begin modifying the aircraft grounded in mid-January after two separate battery failures on board Japanese 787s, and to ship kits and new batteries to affected operators.
Boeing's 787 programme vice-president and general manager Larry Loftis, during a briefing in London, expressed his confidence in the containment system and the changes made to the original architecture - including about a dozen to the battery itself - despite the failure, so far, to establish the cause of the failures.
"In my mind it's a more robust solution than if we'd gone after a single root cause," Loftis says.
"We did extensive testing the first time round. We didn't expect to have a battery failure to take place. We stepped back and re-questioned every assumption we made."
He says: "We've learned a lot on how to test batteries, we're much better off for it."
US and Japanese investigations into the failures are continuing but Loftis acknowledges: "It's possible we'll never know the specific root cause."
Boeing has come up with a three-layer defence: minimise the probability of a failure, minimise the possibility of propagation in the event of failure, and prevent any impact on the aircraft in either case.
This has led to design enhancements for the battery and charger, says Loftis, to produce a "better quality supply of electricity" during the charging process and filter out "transient signals". The manufacturer has also tightened the voltage range in which the battery operates and lowered its maximum charge limit.
But the most notable part of the defence is the creation of a stainless steel containment shield, some 3mm thick, to isolate a failing battery from the rest of the aircraft.
Boeing has also developed a dedicated line through which to vent any gas to the atmosphere through a port in the lower fuselage.
Loftis says the airframer has implemented more robust acceptance tests, down to individual battery cell level, and narrowed tolerances in the testing process.
"There are compelling reasons to stay with the lithium-ion battery," he says, despite rival Airbus's switch to conventional nickel-cadmium for its A350. Loftis also defends Boeing's testing and development process with the new technology. "I don't think it was flawed," he says. "I think we've learned."
As part of the work to address the problem Boeing deliberately failed an original battery during laboratory testing, recording temperatures of up to 300°C on the surface of the containment box, which persisted for around 30min.
"It was quite an energetic failure mode," says Loftis. "That's a lot more energy than we'd normally expect to see."
But with the modified battery the severity of the failure was "much lower", he says, with temperatures reaching only 70-80°C before quickly cooling. "It helped to verify that we had a robust solution," he adds.
The testing programme has also involved trialling the containment system on prototype airframe ZA005 while on the ground, although with the aircraft systems - including its engines and pressurisation - operating as if it was in flight.
"We tricked the aircraft into thinking it was flying," says Loftis.
During the test all eight cells in the auxiliary power unit battery failed. But video of the deliberate failure test showed no visual evidence of battery problems outside the containment system, while external cameras showed the venting system operating as designed.
"The quality of the power being generated was exceptional," says Loftis, adding that the APU did not lose electrical power until the fifth cell had vented. "We were very, very pleased with that."
Fifty 787s, located in nine countries, are affected by the grounding. Boeing's teams will take about five days to modify each aircraft, and work will be carried out in the same sequence they were delivered. The modification kit includes the steel containment box, vent line assembly components, battery charger and wire bundles.
Loftis says the change adds around 70kg (150lb) to the aircraft's weight, and that the "fairly small" cost of modification is covered under warranty.
Boeing has maintained its production rate of five aircraft per month through the three-month grounding. It is ramping production to seven aircraft per month, the first of which is already going through the final assembly line, and Loftis says he does not see any "collateral impact" on the ramp-up schedule.
While the airframer does not have specific delivery dates, he says: "We're starting to have detailed conversations with our customers on delivery timing."
But he expects all aircraft scheduled for delivery in 2013 to be delivered by the end of the year.
Loftis does not expect any impact on the development schedule for the larger 787-9, which will enter the final assembly line in May. The corresponding containment system for its battery has already been designed.
Source: Air Transport Intelligence news
