Vortex or Vapor

[a1]

As I was walkin gto my next class in school I looked up and saw a 747. I thought oh well it was like anyother landing at KLAX but then it had a stright white wisp of something right on the tail section. I have no idea what it was. I was thinking of turbulance or a vortex but I have never seen one on the tail section of a 747 before.


The school is about 32miles from KLAX runway 24R. All the planes that fly over the school are on approach. There weren't any gear or flaps down I assume.

So what was it? :-?

[DaveSims]

Coming from the tail and not the wings or engines, I would guess a fuel dump.  I'm not sure where the 747's dump is.

[a1]

[quote]Coming from the tail and not the wings or engines, I would guess a fuel dump.

[Mobius]

Sounds like a vortex.  Any lifting surface generating lift generates a tip vortex, so if the conditions are just right, the high velocity vortex core is visible.  Although, it could be either that or fuel dump, but that's more of a steady stream and only really used in emergencies if I'm not mistaken (why dump fuel when you can just take less in the first place?).

[beaky]

If a vortex is visible, it's because of vapor.
So the answer is: both.

It's unlikely it was fuel being dumped, because those valves are in the wings, not the tail.

[beaky]

Although, it could be either that or fuel dump, but that's more of a steady stream and only really used in emergencies if I'm not mistaken (why dump fuel when you can just take less in the first place?).

A heavy might need to dump fuel before landing- sometimes they just have more left over than they'd planned, and many heavies like the 747 have max takeoff weights which are higher than their max landing weights. Fuel is the easiest thing to toss overboard, so...out it goes.

I don't think this is normally done during the descent, though (30 miles out)...

[Boss_BlueAngels]

The vortice could be coming from a portion of a deployed flap as well.  They don't only come from the tips of the wings.

[C]

Although, it could be either that or fuel dump, but that's more of a steady stream and only really used in emergencies if I'm not mistaken (why dump fuel when you can just take less in the first place?).

A heavy might need to dump fuel before landing- sometimes they just have more left over than they'd planned, and many heavies like the 747 have max takeoff weights which are higher than their max landing weights. Fuel is the easiest thing to toss overboard, so...out it goes.

I don't think this is normally done during the descent, though (30 miles out)...

I doubt it would be a fuel dump, as if it was particularly necessary then it would occur over open country or more preferably, the sea.

As for fuel itself, I suspect even on the heaviest jet, there would be enough of a margin between max zero fuel weight and max landing weight to ensure that overloading would not occur (unless ATC were very very helpful and shaved an hour or three off the journey time with some helpful directs!

[Slotback]

Impossible for it to be a fuel dump.

[expat]

If the aircraft was 30 miles out what altitude would it be? Way to early for flaps and gear. That just means drag and extra fuel burn. Also unlikely to be fuel. The jettison valves are on the wings and when an aircraft jettisons fuel, you will know about it. Normally happens a lot lower than contrail temperature hight. It will be one of the last thing to be done, it is after all you lifeline and you would look rather silly having dumped fuel first only to be told that your airport is closed for what ever reason and you have to divert. Dumping fuel is like warm beer on a hot day, the last thing you do and only if you must.

Matt

[C]

If the aircraft was 30 miles out what altitude would it be?

Well under 10,000ft. 6 or 7,000ish.

[expat]

If the aircraft was 30 miles out what altitude would it be?

Well under 10,000ft. 6 or 7,000ish.

Still a little high for gear, but a flap or two?

Matt

[C]

If the aircraft was 30 miles out what altitude would it be?

Well under 10,000ft. 6 or 7,000ish.

Still a little high for gear, but a flap or two?

Matt

Depends how quickly it was trying to get down. A larger airport like LAX's ATC would also probably try and keep you relatively fast til quite a late stage of the approach.

[elite marksman]

Well, <10,000 in the US means <250 KIAS as well. And, IIRC 747s usually drop a degree or two of flaps at 230-240 KIAS, so it could be  a trail off of a flap, or any other lifting surface, for that matter.

[Jayhawk Jake]

It was probably just a normal vortex.  It's unusual for it to come off the horizontal stab but theres no reason why it can't.