747-400 Liquid from Tail?

Old Jun 18, 16, 12:14 pm
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747-400 Liquid from Tail?

Moments ago I watched what I believe was UA838 (NRT->SFO) a 747-400 at ~6,200 ft directly overhead (per flightradar24) on approach to SFO. From the tail, was streaming some sort of liquid. It was not a contrail.

Any ideas what this was?
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Old Jun 18, 16, 3:54 pm
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sorry, from 6200 feet you saw "a trail of liquid"? You're going to have to describe that a bit more...
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Old Jun 18, 16, 5:03 pm
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Originally Posted by TA View Post
sorry, from 6200 feet you saw "a trail of liquid"? You're going to have to describe that a bit more...
I was on the ground. At 280 feet. I live on the approach for 28L/R for planes coming from the Pacific and sometimes the Pacific NW. Four engine plane directly overhead. Noticed a stream of liquid coming out the bottom, quickly dissipating. During the 90 seconds or so I watched, the liquid always dissipated within two plane lengths. Immediately checked flightradar24 to see what plane it was to try to determine what might be going on.

Initially I thought it might have been fuel dump for an emergency landing, but upon determining the flight was likely NRT->SFO (and not something that had just taken off) thought unlikely the plane would be overweight. Further, I believe dump is from the wings, not the tail.
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Old Jun 18, 16, 5:16 pm
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So you mean you saw a cloud coming from behind the tail?
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Old Jun 18, 16, 5:27 pm
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Originally Posted by TA View Post
So you mean you saw a cloud coming from behind the tail?
I'd describe it more like what water looks like when you throw it from a bucket.

This was quickly falling before dissipating, while vapor wouldn't noticeably fall in the ~2 plane lengths.
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Old Jun 18, 16, 5:37 pm
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Originally Posted by cardinal98 View Post

Initially I thought it might have been fuel dump for an emergency landing, but upon determining the flight was likely NRT->SFO (and not something that had just taken off) thought unlikely the plane would be overweight. Further, I believe dump is from the wings, not the tail.
FWIW, a fuel dump on the 47 will emerge very near the wingtips. A 777 about a third of the way inbound from the tips.
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Old Jun 18, 16, 6:35 pm
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I'm curious how at over a mile vertical distance (plus any addl X/Y distance) you could see liquid dripping, especially if it took well under a second to disapate.
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Old Jun 18, 16, 7:00 pm
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Originally Posted by fastair View Post
I'm curious how at over a mile vertical distance (plus any addl X/Y distance) you could see liquid dripping, especially if it took well under a second to disapate.
This was not "dripping" It was a torrent. X/Y distance < 0.25 mile.
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Old Jun 18, 16, 7:01 pm
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I too am curious about what you saw.

It's just hard to imagine how, if it was not a cloud of vapor, a real liquid stream would be visible that far away -- and moreover, how a plane could be putting out so much liquid to make it visible that far away? If only we could have a picture or a picture similar to what you were seeing? One stream, you say?

Is it possible that a reflection or refraction of the sun at a certain angle into a vapor trail made it look like liquid?
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Old Jun 18, 16, 7:18 pm
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Originally Posted by TA View Post
I too am curious about what you saw.

It's just hard to imagine how, if it was not a cloud of vapor, a real liquid stream would be visible that far away -- and moreover, how a plane could be putting out so much liquid to make it visible that far away? If only we could have a picture or a picture similar to what you were seeing? One stream, you say?

Is it possible that a reflection or refraction of the sun at a certain angle into a vapor trail made it look like liquid?
I've watched thousands of flights on approach and haven't seen this before, which is why I immediately turned to FT. It's certainly possible that it had something to do with the angle of the sun, but the planes are close enough to be obvious it wasn't coming from the product of combustion (not the engines), but the tail. I have, and will continue scouring Image search for something similar. Have also emailed a relative who is retired 757/67 pilot.
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Old Jun 18, 16, 7:28 pm
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Do you live in San Mateo hills or something like that, and were looking at it as it came down the coast to the west and made the usual turn north into the bay?
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Old Jun 18, 16, 7:36 pm
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I'm further south on 280 near Sand Hill / Alpine. Hawaii flights start the turn directly overhead, Asia slightly earlier and Pac NW earlier still. 789 from HND just passed straight up (x/y=0) @ 6,200 ft (net 6,000 ft).
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Old Jun 18, 16, 9:09 pm
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Since landing on the 28's is a short taxi, possibly could have been the exhaust from the APU being started.
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Old Jun 19, 16, 1:31 am
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Originally Posted by clubord View Post
Since landing on the 28's is a short taxi, possibly could have been the exhaust from the APU being started.
I think that's exactly it. Just as you sometimes have dripping from a car exhaust pipe, you can get it from the APU exhaust. This is a natural part of the combustion process.


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Old Jun 19, 16, 9:11 am
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gray water (sinks and galley drain) comes out the the plane towards the aft

could have been that?
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