Boeing took a significant step toward returning the 787 to service on Friday, when it flew Dreamliner Line Number 86 on a one-hour, 49-minute mission to demonstrate conformity of its battery system modification to U.S. certification authorities. Painted in LOT Polish Airlines livery, LN 86 took off from Paine Field in Everett, Washington, at 10:39 a.m. local time, traveled west, then south down the Washington State and northwest Oregon coastlines before turning around over the Pacific Ocean and virtually tracing its original flight path back to Everett, where it landed at 12:28 p.m.
Boeing expected the flight to mark the end of certification testing for the 787’s new battery system following more than a month of ground trials. Now, the timing of service entry depends solely on the speed with which the U.S. Federal Aviation Administration completes its evaluation of the testing data.
The so-called fix includes a modification to the charger and battery monitoring unit designed to narrow the acceptable level of charge–in essence, lowering the maximum charge allowed and raising the minimum level of discharge allowed. Engineers have also devised a way to “soften” the charging cycle in a way to put less stress on the battery.
Within the battery itself, Boeing engineers wrapped an electrical insulator between each of the eight cells to isolate them from each other and prevent propagation. It also installed electrical and thermal insulation above, below and between each cell to prevent heat migration. Meanwhile, small holes drilled into the bottom of the case that encloses the cells and battery management unit allows moisture to drain away from the battery. Larger holes in the sides of the case allow a failed battery to vent more efficiently, thereby lessening the possibility of damage to other parts of the power pack.
Finally, the new design uses a stainless-steel enclosure meant to isolate the unit from the rest of the equipment in the electronic equipment bays.