Hybrid Contrails Separating From Exhaust Contrails

Trailspotter

Senior Member.
Today I have observed an unusual type of contrail evolution that I have not encountered before. There [seemed to be] an asymmetric contrail behind a four-engine jet: three trails has merged together, whereas one stayed apart:

P1170293.JPG

The triple trail has dissipated quickly via Crow instability, whereas the single trail has persisted and spread a bit (in a limited area of the sky):

P1170294.JPG

P1170295.JPG

P1170296.JPG

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Actually, I've observed this type of contrail evolution twice on the two consecutive planes flying by the same route with a few minutes interval, but recorded only the second one. The planes left two parallel segments of persistent trails ending at about the same distance:

P1170300.JPG

In contrast, another four-engine jet some twenty minutes later has left a symmetric contrail that quickly dissipated, again via Crow instability:

P1170316.JPG

P1170317.JPG

P1170319.JPG

I have seen asymmetric trails behind four-engine planes before, but they either dissipated, or persisted all together. It is the first time I see the trail from one of the engines persisting, while the trails from the other engines dissipating.

What would be the likely origins of such asymmetric trails? I can hazard a guess, but would like to hear the expert opinion first.
 
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George B

Extinct but not forgotten Staff Member
Interesting . . . engine upgrade. . . ? The one engine is newer and more efficient??
 

FreiZeitGeist

Senior Member.
A german Chemtrail-Blog catched an A380 with 4 Engines, where only two of them made a contrail...

Well, it was painted in airbus-Colors and flied over a region commonly used for testfligths an training purpose (Final assembly is done in Hamburg Germany) - it seemed they tested/trained how to fly with only two operating engines.

Could be something simular, perphaps there was one Engine unused/defect or running with less power...
 

Trailspotter

Senior Member.
Could be something simular, perphaps there was one Engine unused/defect or running with less power...

No, it did not look like this. You can zoom on the top (and any other) of my photos above, there are trails coming from all four engines. The three of them have converged, transformed into a Crow instability pattern and 'kill" each other within a minute, whereas the fourth trail stayed apart and happily persisted for at least half an hour. And this has happened not with one but two similar planes flying one after the other through the same patch of the sky within a few minutes.
 

George B

Extinct but not forgotten Staff Member
No, it did not look like this. You can zoom on the top (and any other) of my photos above, there are trails coming from all four engines. The three of them have converged, transformed into a Crow instability pattern and 'kill" each other within a minute, whereas the fourth trail stayed apart and happily persisted for at least half an hour. And this has happened not with one but two similar planes flying one after the other through the same patch of the sky within a few minutes.
Seems the engine was operating and producing a persistent contrail . . . so logically it is doing something differently from the other engines . . . which is most likely cooler exhaust temperature and/or higher relative humidity of the plume. One other possibility is the exhaust particulate size is more optimal and thus is a more efficient nuclei for condensation of the water vapor which then freezes into ice crystals . . .
 

Mick West

Administrator
Staff member
I think the Crow trails are the aerodynamic contrails (of maybe some kind of hybrid trail) that are formed in the low pressure centers of the wake vortices. The big puffy trail is the broader contrails from the four engines, which are all leaving the same trails.

Initially they will be coincident, but the different size of ice crystals cause the exhaust contrail to fall faster than the aerodynamic contrail, so they seem to separate.

[Edit] actually it's the skinny hybrid trails that fall faster.
 

George B

Extinct but not forgotten Staff Member
I think the Crow trails are the aerodynamic contrails (of maybe some kind of hybrid trail) that are formed in the low pressure centers of the wake vortices. The big puffy trail is the broader contrails from the four engines, which are all leaving the same trails.

Initially they will be coincident, but the different size of ice crystals cause the exhaust contrail to fall faster than the aerodynamic contrail, so they seem to separate.
So aerodynamic contrails can be persistent in air optimal for formation but not exhaust contrails??
 

TWCobra

Senior Member.
In the close up you can see the two trails on the left wing under the influence of the cross span flow, which is what induces the vortex at the wing tip. The other side doesn't appear to do that. That may be just the camera angle though. One possibility is that the aircraft was flying in a strong jetstream at 90 degrees to the wind direction. In those circumstances I have seen over 20 degrees difference between the aircraft heading and the aircraft track which is the drift caused by a strong crosswind. If so it may mean that one side is being affected by aerodynamic turbulence behind the fuselage that it would not necessarily encounter without the crosswind. Just speculation though.
 

George B

Extinct but not forgotten Staff Member
In the close up you can see the two trails on the left wing under the influence of the cross span flow, which is what induces the vortex at the wing tip. The other side doesn't appear to do that. That may be just the camera angle though. One possibility is that the aircraft was flying in a strong jetstream at 90 degrees to the wind direction. In those circumstances I have seen over 20 degrees difference between the aircraft heading and the aircraft track which is the drift caused by a strong crosswind. If so it may mean that one side is being affected by aerodynamic turbulence behind the fuselage that it would not necessarily encounter without the crosswind. Just speculation though.
Makes sense . . . since we have been talking about jet streams . . . LoL!!
 

Mick West

Administrator
Staff member
Here's a similar thing from 1972, here it's not really an aerodynamic contrail, it's the contrails persisting longer in the vortex core (The "hybrid" I mentioned, an exhaust contrail modified by aerodynamics - but not initially formed as an aerodynamic contrail).



There's quite a good discussion of the contribution of the vortices to the contrail shape there:
https://plus.google.com/photos/1073...62113705530081/5363662378527662578?banner=pwa
 
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Mick West

Administrator
Staff member
I think the key thing with these is marginal conditions - you don't see these type of skinny contrails that break up as crow instabilities very often. Usually you just get the trails combining into one large puffy chaotic trail.

The conditions need to be just right for the wake vortices to make a difference.

Aha, this video provides a perfect illustration of the formation


Notice how the hybrid contrails (which is what I'm going to call them) separate out from the main combined exhaust contrail. It looks like the hybrid contrails are actually heavier, larger crystals, so they fall away, and last a little longer.
 

Trailspotter

Senior Member.
One possibility is that the aircraft was flying in a strong jetstream at 90 degrees to the wind direction. In those circumstances I have seen over 20 degrees difference between the aircraft heading and the aircraft track which is the drift caused by a strong crosswind. If so it may mean that one side is being affected by aerodynamic turbulence behind the fuselage that it would not necessarily encounter without the crosswind. Just speculation though.

Actually, it is very close to what I have seen. The aircraft was flying at 90 degrees to the wind direction. I have added one more photo the initial post zooming out from the aircraft to show more of the trail, as well as its position relative the first trail a few minutes older.
In the close up you can see the Crow instability pattern developing at some distance after the plane:

P1170294crop.jpg
 

Trailspotter

Senior Member.
I think the key thing with these is marginal conditions - you don't see these type of skinny contrails that break up as crow instabilities very often. Usually you just get the trails combining into one large puffy chaotic trail.

The conditions need to be just right for the wake vortices to make a difference.

Aha, this video provides a perfect illustration of the formation

I have seen this video before posting this thread. It is not exactly what I have seen. The Crow instability affected only three of the four engine trails and make them dissipate quicker. The fourth survived and thrived.
 

Mick West

Administrator
Staff member
I have seen this video before posting this thread. It is not exactly what I have seen. The Crow instability affected only three of the four engine trails and make them dissipate quicker. The fourth survived and thrived.

I suspect that might be a bit of an illusion, to do with the vortex trails being more visually lined up with one side of the combined exhaust contrail. You really need to a photo in between the first two photos (now the first and third) to show how things develop.

I think the end result it like this:

 
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Mick West

Administrator
Staff member
Actually your new photo shows the whole development, which I think is basically like as seen in the video:

 
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Trailspotter

Senior Member.
I suspect that might be a bit of an illusion, to do with the vortex trails being more visually lined up with one side of the combined exhaust contrail. You really need to a photo in between the first two photos (now the first and third) to show how things develop.

Actually your new photo shows the whole development, which I think is basically like as seen in the video

Well, it might be a bit of an illusion in both ways. The fourth trail might have combined with the rest, or it might have just expanded so that the gap between them was not visible any more at this view angle. Regardless of any of these possibilities, the evolution of total contrail seemed very unusual to me. It was different from the cases of Crow instability that I saw many times before.

The most typical observation was the two parallel "vortex" trails formed shortly after the aircraft, then at the same distance they transformed into loops and rings, which then faded quickly leaving no trace.

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In more persistent contrails Crow instability manifests itself in their peculiar shapes:

P1010788.JPG

In contrast, in my yesterday observation, the "vortex" trails separated very neatly from the rest and took with them about three quarters of initial contrail density. After these trails degraded, the remaining narrow trail was linear and smooth. Also, it persisted and spread afterwards.

I hypothesise that an unusual evolution of this contrail was due to its initial asymmetry. There is an intrinsic reason for contrail asymmetry. The wingtip vortices, rotating in opposite directions, interact with the contrails from the engines, spinning in the same direction. This will result in the trails on one side of a four-engine aircraft deviating differently than the trails on the other side. This asymmetry was probably amplified by a strong crosswind. At least, this is hypothesis that can be tested by further observations.
 
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