cockpitvisit
Did Boeing ever test the propagation in a battery installed in an actual airplane?
I'm not sure that is available. Working backward, FAA accepted a single cell would not cause propagation (further runaway). So the assumption is BOEING designed, built, and tested a platform that demonstrated this to the satisfaction of BOEING. And to FAA.
A cell only contains a very fixed amount of chemical energy in it, so it's difficult to imagine that "something happened" and emitted way more energy than expected.
But that is exactly what happened. "Propagation". Clearly, the fire, flares and heat spread to additional cells?
Volume
So maybe here as well the aircraft was desgned in a way that the effects of a thermal runaway were correctly recognized, but the likelihood was underestimated.
So, NTSB have narrowed it down. 1) Battery Design? should the batteries be Prismatic? Cylindrical? Should the batteries be in nonflammable liquid coolant? Active refrigeration or passive heat sink? Should the Enclosure be enlarged, mounted on a stand off platform? 2) Battery construction? High enough standards in layering the Cathode, Anode, Separator stack? Is the environment filtered? Are methods demanding enough as to paste application, folding, wiring, etc.? 3) Monitoring and Management.....
If the controls system caused, (allowed) the thermal runaway, then there is a huge problem. If the battery self ignited, there is a huge problem. If the two cannot be isolated one from the other, (via investigation), the problem is (may be) impossible.
But the third, and most important leg of this three legged stool is the FAA.
Believe it or not, if BOEING can still prove propagation is not possible, the System and Battery don't need to work any better than they did. All Boeing has to prove is one cell will not propagate into multi cell runaway.
The likelihood was recognized and assessed. What was unexpected was the misplaced confidence in the effects of thermal runaway being mitigable. (No Propagation to other cells).
It is this failure in design that is the problem.
If propagation to other cells had not occurred, there would have been no grounding, imo. There would have been smoke, perhaps leakage of electrolyte, and some singeing of the close in structures. But that is allowable, under the regs. (imo).
So it is the regs that offend, because they created the potential for this bizarre argument.
Allowing that single cell Thermal Runaway was acceptable at all turns out to be preposterous.
Which brings up the final turkey. #6 is not a cell, it is a BATTERY. One of eight.
The APU/MAIN BATTERY is (each of them) a group of eight batteries in a stainless steel case.
But FAA says it is allowable to call a BATTERY a CELL.
A ROSE..... by any other name? A lot of what will happen with the Regulations may hinge on nomenclature.....imo.