There is a design problem in the 787 which won't go away - an APU battery failure makes the APU shutdown, so the APU is only as reliable as the battery - these two holes in the swiss cheese are aligned by design. Worse, deep discharge puts the battery at risk, but such discharge would likely occur exactly when starting the APU.
IN OTHER WORDS, THE FIREBOX PROTECTION OF THE APU BATTERY MAY PREVENT A THERMAL RISK, BUT BATTERY FAILURE DUE TO DEEP DISCHARGE DUE TO IN-FLIGHT STARTING OF THE APU WILL ANYWAY TAKE DOWN THE APU EXACTLY WHEN IT IS NEEDED.
The risk is that in case of a double engine malfunction the APU start process would deep discharge the battery, causing battery failure, and then the battery would be taken offline by its safety mechanisms which would take the APU offline and leave the plane reliant on the RAT, and possibly incapable of an engine restart.
I don't think the FAA should have certified this design logic, nor do I think they should certify it now. And yes, I may be an idiot but I'm an idiot with an electronic engineering degree and a PhdD.
Last edited by edmundronald; 31st Mar 2013 at 20:48.