While looking at the photos from this question I noticed a small protrusion from the side of the plane. You can see it in the photo:
787 photo

and in the diagram from l@aeroalias answer:

787 apu diagram

In the diagram it appears to have something to do with the apu or is fire protection system. There's a shaded line running to it.

What is this object?

Edit: It does look like an antenna. I found a diagram of 777 antenna placement here, but it doesn't show anything in that area.

777 antenna placement

  • $\begingroup$ Some of those lines don't seem to point to what they're labeled. APU Compartment Firewall, Inlet Door, etc. $\endgroup$
    – egid
    Commented Jan 18, 2016 at 21:03
  • 1
    $\begingroup$ @egid Yeah. Aeroalias noted that on the other question. $\endgroup$
    – TomMcW
    Commented Jan 18, 2016 at 21:06
  • $\begingroup$ Since its possibly part of the fire suppression system, maybe an air intake for the AFFF system, although I don't know why it would be shaped like that, maybe because of the boundary layer problem in flight wouldn't allow it to suck in enough air to generate foam... $\endgroup$
    – Ron Beyer
    Commented Jan 18, 2016 at 22:25
  • 4
    $\begingroup$ It looks an awful lot like a standard blade antenna, maybe for TCAS? Not sure if it has a twin on the other side. $\endgroup$
    – egid
    Commented Jan 19, 2016 at 0:20
  • 3
    $\begingroup$ A drain port for the APU? $\endgroup$ Commented Jan 19, 2016 at 5:12

1 Answer 1


As you have already imagined, it's the APU fuel line shroud drain, as visible here:

enter image description here


enter image description here

The fin gives an aerodynamic profile to the drain. The reason this drain exists seems to be:

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.


  • 1
    $\begingroup$ Seems to be a lot of drag causing surface area for a drain line. I'm surprised that the fin extends so far from the surface and is so "deep" (fore to aft). $\endgroup$
    – FreeMan
    Commented Dec 21, 2017 at 12:09
  • 2
    $\begingroup$ @FreeMan If the drain is too close to the surface, leaked fuel and oil will flow along the fuselage with the airflow, eventually reaching the hot exhaust and ignite. $\endgroup$
    – user71659
    Commented Dec 21, 2017 at 19:35
  • $\begingroup$ @user71659 ah, that makes sense! $\endgroup$
    – FreeMan
    Commented Dec 21, 2017 at 19:41
  • $\begingroup$ What do they mean by "natural fuel leaks"? $\endgroup$ Commented Aug 20, 2021 at 17:10
  • $\begingroup$ @user71659, Indeed. If the fuel leakage took place within the boundary layer, it would "fall" back and stick to the fuselage, hence the need for such lengthy fin. $\endgroup$ Commented Feb 16, 2023 at 15:37

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