What is this device protruding from the rear of this 787?

Question

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

Some of those lines don't seem to point to what they're labeled. APU Compartment Firewall, Inlet Door, etc. — egid, Jan 18 '16 at 21:03
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... — Ron Beyer, Jan 18 '16 at 22:25
It looks an awful lot like a standard blade antenna, maybe for TCAS? Not sure if it has a twin on the other side. — egid, Jan 19 '16 at 00:20
I agree it looks like a VHF or similar antenna too, though I haven't been able to find a diagram of antenna placements for the 787. — Zach Lipton, Jan 19 '16 at 13:28
@Porcupine911 Hadn't thought of that. Good suggestion, except I've found several sites that indicate that the 787 doesn't have a drain mast. They opted for a grey water tank instead. — TomMcW, Jan 24 '16 at 00:17
@TomMcW, I was thinking of a drain from something else. The 787 still has drains to remove any pooled fuel, oil, water, de-ice fluid, etc. — Porcupine911, Jan 24 '16 at 11:54

score 10 · Accepted Answer · answered Feb 04 '16 at 15:54

10

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

^(Source)

Detail:

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.

^(Source)

answered Feb 04 '16 at 15:54

mins

72,728
27
308
441

1

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). – FreeMan Dec 21 '17 at 12:09
2

@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. – user71659 Dec 21 '17 at 19:35
@user71659 ah, that makes sense! – FreeMan Dec 21 '17 at 19:41
What do they mean by "natural fuel leaks"? – End Anti-Semitic Hate Aug 20 '21 at 17:10
@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. – Paulo Carvalho Feb 16 '23 at 15:37

What is this device protruding from the rear of this 787?

1 Answers1

Linked