Category Archives: Sample Size
“Boeing’s fix includes more insulation between each of the eight cells in the batteries. The batteries will also be encased in a new steel box designed to contain any fire and vent possible smoke or hazardous gases out of the planes.
“…both the F.A.A. administrator, Michael P. Huerta, and Transportation Secretary Ray LaHood said they were are satisfied that the proposed changes would eliminate concerns that the plane’s two lithium-ion batteries could erupt in smoke or fire.”
-“F.A.A. Endorses Boeing Remedy for 787 Battery” by C. Drew and J. Mouawad, 19 April 2013 New York Times
Conspicuously absent from this pronouncement is a definitive identification of the root cause of the lithium battery fires. Therefore Boeing, the FAA, and the Department of Transportation are all guessing that the stated modifications will fix the problem. I hope they are correct. But if they are it will be a matter of luck, not engineering diligence. The dissembling of the FAA and Department of Transportation are clearly evident in their own words: they say that they are “…satisfied that the proposed changes would eliminate concerns that the plane’s two lithium-ion batteries could erupt in smoke or fire.” If they are so satisfied, then why is it necessary to have a steel box to contain a fire? If they are so satisfied, then why did they not provide the supporting evidence to support their conclusions?
Also, Boeing and these government agencies have touted a few test flights as being of particular significance in proving the safety of the batteries. This is nonsense. The battery fires are low probability events, occurring only once for thousands of hours of operation. This implies that there are subtle variables in the battery construction, chemistry, and/or operation, which when combined worst case will cause the batteries to overheat. This combination may only occur for a small number of manufactured batteries, and fires may occur only when those particular batteries are exposed to a worst case combination of stresses (temperature, charge currents, etc.).
Therefore a handful of test flights, of a few dozen hours or so total, are not nearly sufficient to empirically identify a low-probability event. The identification of such an event would require hundred or even thousands of test flights, which is obviously not practical. Therefore the only alternative is an investigation that drills down and positively identifies the true underlying failure mechanism (as recommended here: “Flying the Flaming Skies: Should You Trust the Boeing Dreamliner?“). It is my opinion that this has not been done, because if it had, this knowledge would be trumpeted by Boeing.
I’m not flying the Boeing Dreamliner until I see the evidence that supports the optimistic conclusions of Boeing, the FAA, and the Department of Transportation.