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
    Jan 18 '16 at 21:03
  • 1
    $\begingroup$ @egid Yeah. Aeroalias noted that on the other question. $\endgroup$
    – TomMcW
    Jan 18 '16 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
    Jan 18 '16 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
    Jan 19 '16 at 0:20
  • 3
    $\begingroup$ A drain port for the APU? $\endgroup$ Jan 19 '16 at 5:12

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
    Dec 21 '17 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
    Dec 21 '17 at 19:35
  • $\begingroup$ @user71659 ah, that makes sense! $\endgroup$
    – FreeMan
    Dec 21 '17 at 19:41
  • $\begingroup$ What do they mean by "natural fuel leaks"? $\endgroup$ Aug 20 at 17:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.