In the next few days a photo will likely surface of a modification to the 787 ZA004's tail cone and the world will wonder what it's intended to do. It's a small change to the Korean Air Aerospace Division (KAL-ASD) section 48 Aft, but it's an important one. The photo above shows ZA001's tail cone before the modification.
Natural fuel leaks from the auxiliary power unit (APU) fueling line were supposed to drain harmlessly out the bottom of the structure as designed, though rather than do that, the fuel was pooling into the APU exhaust cone.
The result, when the APU was started back up again, this page is told, the fuel torched out of the back of the tailcone. Naturally, this was a phenomena Boeing and the FAA wanted to avoid. Sources say APU drainage evaluations have been a particular focus of flight test over the past several months.
Boeing made very few modifications to the outer mold line of the 787 up until now, adding vortex generators on the tail and minor changes to the rigging of the wing slats. This new "flare", as it is known, will be a noticeable shelf and extends about three-quarters of the way along the exhaust cone and past the end, allowing for the fuel to wick off the end into the air, rather than pooling in the bottom of the cone.
Program sources say that the modification has already been installed on ZA004 and is being put into production this week. Those same sources add that a similar system was put in place for early 757s and designed out in later airframes.