White, Grey, and Rainbow Colored CRJ-700 Contrail [Likely Intermixed Engine Configuration]

Mick West

Administrator
Staff member
Here's an interesting example of dark and light that I just took. It took me a while to figure out exactly what was going on, initially thinking it was one trail shadowing the other, but the sun is too high, and the difference far too stark for that. The explanation ended up being that the jet had two different engines.

Photos taken around 11AM PDT, 1800 UTC, 12 Noon PST. Mar 9 2014
Original photos can be found here: http://www.flickr.com/photos/metabunk/sets/72157642135454733/ (Camera time is 2m 20s fast, so subtract 2m 20s to get correct time)


The plane flew almost directly overhead, over the Santa Monica VOR, which is about a mile away. It was heading south, to San Diego.

This is one of the first photos I took, with the contrast enhanced. Here the difference is slight, but noticeable:


The denser trail on the pilot's right seems to be shadowing the trail on the left. A wider angle shows the change even out further back, but it's still there.



In the next shot, from my point of view it looks like the sun is to the left of the trail, yet the right side is much brighter and has a coloration more typical of an aerodynamic contrail.


Wider angle:


The Jet as it passed overhead. FAA data, so about five minutes behind.
http://www.flightradar24.com/2014-03-09/18:00/12x/SKW3484/2db6b8c



In the last photo I took of the plane, the trail on the left is almost invisible.



On that day, March 9th 2014, I took many photos as conditions were great for contrail formation. I did not notice the two-tone colors until later, when I was able to view them on the computer.

There were many clouds, both high cirrus, and some fractus cumulus.

I have uploaded all the photos I took to Flickr in two sets:

http://www.flickr.com/photos/metabunk/sets/72157642297148355/
http://www.flickr.com/photos/metabunk/sets/72157642135454733/

The second set just shows this CRJ700 jet with intermixed engines. The first set is all the other photos, before and after, from that day.

Of note from the CRJ set:
  • A variety of clouds are shown
  • The left trail gets progressively more faint as the plane descends (it's descending towards San Diego Airport, started the descent at the time of the first photo)
  • In 0703 it is descending through the cloud layer. This is the last photo I took that shows the plane.
  • in 0706 the trail shows a transition from distrail to contrail, indicating where it went below the clouds, but still kept on conning.
In the Venice Set:
  • A nice sundog and cirrus fallstreaks in the first few photos
  • The first plane (0662 to 0687) exhibits no coloration, just normal white exhaust contrails
  • In 0673/4 you see distinctive "hybrid" contrails cause by the wake vortices, indicating marginal conditions (i.e. not ice supersaturated)
  • 0709 to 0715 are just low altitude planes on approach to LAX.
  • 0716 to 0734 all show (I think) the same plane
  • 0730 to 0734 show the colors in both sides of a trail
  • 0735 is a wide angle shot I took to gauge the position of the sun
  • 0744 is a calibration shot, showing the camera time 2m20s fast.
Here's 0730:


The distinctive color spectrum is quite noticeable. This would also be a plane flying south to San Diego, and likely descending. Notice the two trails are identical (the small third trail is probably a galley dump - water or beverages from the kitchen)

My theory is:
  1. The air is marginal for contrail growth, around 100% ice supersaturated, so around 60-70% relative humidity with respect to water.
  2. The colored trails indicate a very short period of water condensation, followed by much slower ice crystal size increase by deposition, this progresses though the color spectrum as it increase in size..
  3. The white trails have a longer (but still very short) period in the the liquid phase, and so when it freezes, the ice crystal is already larger than size that creates colors
This implies a difference in the mixing curve as the exhaust goes from its initial temperature and humidity to the ambient. The difference might be from either the start or the end conditions of the mixing, as seen by a white and colored trail visible on A) different planes in different conditions, and B) the same plane with two different engines.

With the CRJ two-tone contrail, the newer engine has cooler exhaust, and so enters the rapid "condensing" portion of the mixing curve earlier than the older hotter engine, and so condenses larger droplet before transitioning through "freezing" to "deposition" and the slow ice crystal growth.

[This post has been updated with material from the thread below]

[Update] here's the most likely explanation
 
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Hama Neggs

Senior Member.
[Admin note: the following discussion was conducted prior to determining the intermixed engines were responsible]

Must be one trail shading the other.
 
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deirdre

Senior Member.
I cant even figure out why its headed to the right in the first photo and left in the third. o_O
 

Mick West

Administrator
Staff member
The sun being on the left is a bit of an optical illusion


The above is my view, and the plane's track from flightaware.com, the sun seems to be on the left of the plane, but the bright part is on the right.



However, if you look directly along the track the sun is actually just slightly to the RIGHT of the plane, and the angle is so steep that the shadow the right contrail casts on the left is very dense.
 

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solrey

Senior Member.
Reminds me of some iridescence I captured in ocean spray on a trip to the coast about 6 weeks ago. The darker colors were on the sunward side. Taking the pic below I was on the coast of Oregon facing WNW around 10:00 a.m., the sun is behind and to the left of me. As the angle of the sun changed, so too did the position of the iridescent band. Looks like Mick might have captured a similar effect with himself, the sun and contrail all at the correct relative angles?


DSCN5928.jpg
 

Mick West

Administrator
Staff member
Reminds me of some iridescence I captured in ocean spray on a trip to the coast about 6 weeks ago. The darker colors were on the sunward side. Taking the pic below I was on the coast of Oregon facing WNW around 10:00 a.m., the sun is behind and to the left of me. As the angle of the sun changed, so too did the position of the iridescent band. Looks like Mick might have captured a similar effect with himself, the sun and contrail all at the correct relative angles?


View attachment 6420

No, it's the right trail casting a shadow on the left. [EDIT: Not so sure now, see below]

The plane is flying basically towards the sun, which is in the south.

The plane is slightly angled to the the left of the sun, meaning the contrail on the right is fully in the sun, but partially shadows the contrail on the left [Edit: but because the sun is so high, and the side angle so small, the amount of shadowing is negligible]

The plane is in level flight, the sun is 45 degrees above the horizon.

It's VERY hard to visualize what is going on, let alone explain it. However it does appear to all work out.
 
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MikeC

Closed Account
To check if I understand what you are saying - the sun is above the contrail, so the shadow would only be on the bottom of the left exhaust - if you were above the jet then both contrails would be brightly lit?
 

Mick West

Administrator
Staff member
The other aspect of this is the coloring. This photo has the levels adjusted, but the colors are unaltered:


So we've got one rainbow contrail, and one bright white contrail.

I think that what is happening here is that the left contrail is receiving less direct light, so a greater proportion of the color comes from scattered light, which is more affected by particle size and Mie scattering has slower and smaller particle growth. The spectrum of colors here indicates the particle growth rate in the contrail. As per:
http://contrailscience.com/files/Gierens_Aerodynamic_poster_060625.pdf




The colors are still there in the right trail, there's just less total scattering, so less coloration, and the brighter light washes out any colors. In another shot taken the same day (or a different plane) , the light is more balanced, and the colors more apparent on both sides:


There's a related discussion here:
https://www.metabunk.org/threads/chemtrails-rainbow-cloud-vortex-iridescent-clouds.2636/
 
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WeedWhacker

Senior Member
Could the one engine be burning rich and expelling a lot of hydrocarbons?

Reasonable question, but no....modern High Bypass TurboFan engines do not "burn rich".

In the early days of turbojets, we saw more black in the exhaust, due to incomplete and inefficient combustion. Back then, the goal was sheer power and thrust, and less on economy or efficiency, since fuel was relatively very inexpensive.

Of course, now fuel is costlier, and there are increased U.S. EPA (and international equivalent) restrictions imposed, to lower emissions (by promoting more complete combustion) and also noise standards that have been implemented.

The strive to lower the noise "footprint" of turbine engines actually went hand-in-hand with the efficiency improvements. A win-win.
 

Mick West

Administrator
Staff member
To check if I understand what you are saying - the sun is above the contrail, so the shadow would only be on the bottom of the left exhaust - if you were above the jet then both contrails would be brightly lit?

Mostly, but not exactly.

Contrails don't reflect light. They scatter it via refraction and internal and external reflections of the billions of water drops and/or ice crystals. If you are above the contrail with the sun, then the light that you see is light that has gone into the contrail and come out again. If you below it, then the light you see is (mostly) light that came in the top, bounced around, then come out the bottom.

The amount of light coming out of a contrail is proportional to the amount coming in. In this case [if] the contrail on the left has less light coming in, and so a less coming out, in all directions, but mostly in the directions where light has to travel the farthest to get through. So it will also be (slightly) darker on the top.

[Diagram removed, as it was wrong]

This said, I'm still not convinced myself. I was earlier, but having thought about it more I still can't figure out how two identical contrails at these angles can create what we see. I'm starting to wonder if the engines might be on different power settings. The plane would probably be descending at that time.
 
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Mick West

Administrator
Staff member
I'm retracting my assertion that this is simply one contrail shadowing the other. I've uploaded ALL the photos I took of the plane and contrail here. Note these are all the same plane and same contail as it flew overhead (just one mile to the east of me), and then continued south.

http://www.flickr.com/photos/metabunk/sets/72157642135454733/

There seems to be a difference in size and color very early on:


Then the difference is so striking, that I really can't see it being just from shadow. This is at noon. The sun is shining down at 45 degrees. And the the plane is heading almost straight at the sun. Both sides should be lit the same.




Any CRJ-700 pilots out there? Asymmetric thrust settings?

I shall sleep on it.
 
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Could it be fuel contamination? Partial engine malfunction? To me even different throttle settings would produce white contrails of different size not a darker one.
 

Mick West

Administrator
Staff member
Could it be fuel contamination? Partial engine malfunction? To me even different throttle settings would produce white contrails of different size not a darker one.

I doubt it. I suspect if the trail actually are different, then is because one engine falls just outside the curve, and a lot of the water in it is not freezing. The colors might be a clue there, as it might a liquid water related spectrum.

The point at which the difference in the trails started is exactly when it started descending (over Venice/LAX):
 

Mick West

Administrator
Staff member
@justanairlinepilot, you fly CRJs, does it sound plausible that there might be an asymmetric thrust situation (while descending) that could lead to the exhaust (and hence the contrail) being slightly different between the two engines?

Since the engines are so close to the body, then asymmetry would be less of a problem, so maybe it's more common with CRJ type aircraft?
 

deirdre

Senior Member.
whats this third 'line' coming out of the back of the plane?
 
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justanairlinepilot

Senior Member.
I saw your picture and assumed it was a shadow of the other engine exhaust. The CRJ has a very good FADEC system. Synchronicity of the "fan" portion of the engine is controlled electronicly, FADEC will keep both engines synced within .1%. If both engines were producing different thrust then I would assume the contrail length would be different.

To answer your question, no, I highly doubt the picture represents asymmetric thrust.

@justanairlinepilot, you fly CRJs, does it sound plausible that there might be an asymmetric thrust situation (while descending) that could lead to the exhaust (and hence the contrail) being slightly different between the two engines?

Since the engines are so close to the body, then asymmetry would be less of a problem, so maybe it's more common with CRJ type aircraft?
 

Mick West

Administrator
Staff member
So you think there is an old and a new engine on the same plane here?

That would make sense as while descending there would be a period of time where conditions are marginal. So a small engine difference would make a big contrail difference.
 
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Ross Marsden

Senior Member.
@justanairlinepilot, could you give a short tutorial about how engine thrust is measured? Is it sensed directly by elements on the engine pylons, or is it deduced from engine performance measurements like temperatures and pressures from intake to exhaust?
 
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Mick West

Administrator
Staff member
The plane is N223AG, listed as being 12.3 years old, and having two GE CF34-8C1 engines. New to SkyWest in July 2013, so possibly refitted on engine at that time?

http://www.airfleets.net/ficheapp/plane-crj-10010.htm

Although here the engines are listed as GE CF34-8C5B1
http://flightaware.com/resources/registration/N223AG

The 8C1 engine is from the original CRJ700, 8C5B1 is for the CRJ700 NextGen,

And according to this:
http://www.sunairlines.net/bombardier-crj700/engine-ge-cf34-8c1-ge-cf34-8c5b1
So mixed engines are quite possible. That spec comparison did not cover efficiency.
 

justanairlinepilot

Senior Member.
I can't speak for every jet airliner but our jet is measured by percent N1, which is the output of the bypass fan blades. The other significant measurements are ITT (internal turbine temp) and N2, combustion chamber fans.

Ross, sensors are placed in the engine inlet. One significant sensor is called the T2 sensor. It's a heated probe similar to the pitot static probe.

Some of the older jet engines are EPR, Exhaust gas pressure ratio. We don't do that anymore as we have FADEC (Full Authority Digital Engine Control). Everything is very automated and calculated for us, allowing us to do our jobs much more efficiently, we don't have to worry about monitoring several engine parameters watching for over-temps or hung/hot starts.

@justanairlinepilot, could you give a short tutorial about how engine thrust is measured? Is it sensed directly by elements on the engine pylons, or is it deduced from engine performance measurements like temperatures and pressures from intake to exhaust?
 

justanairlinepilot

Senior Member.
It's common to have a new/rebuilt engine placed on a hull for various reasons. They have a predetermined life but as all machines go, they can fail early.


The plane is N223AG, listed as being 12.3 years old, and having two GE CF34-8C1 engines. New to SkyWest in July 2013, so possibly refitted on engine at that time?

http://www.airfleets.net/ficheapp/plane-crj-10010.htm

Although here the engines are listed as GE CF34-8C5B1
http://flightaware.com/resources/registration/N223AG

The 8C1 engine is from the original CRJ700, 8C5B1 is for the CRJ700 NextGen,

And according to this:
http://www.sunairlines.net/bombardier-crj700/engine-ge-cf34-8c1-ge-cf34-8c5b1
So mixed engines are quite possible. That spec comparison did not cover efficiency.
 

justanairlinepilot

Senior Member.
CRJ-900 in Austin TX. Quite an amazing jet for fuel/passenger efficiency.
 

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Mick West

Administrator
Staff member
More info on the engine difference:
http://www.geaviationservicesolutions.com/archives/2006/v06i01.pdf
So it's seeming very probable that what the photos show is a CRJ-700 with one older CF34-8C1, on the pilot's left, leaving the "darker" trail, and one CF34-8C5B1, which has higher efficiency, and hence runs cooler, leaving the white trail.
 

justanairlinepilot

Senior Member.
Way more info than I care to know an I fly the damn planes...good job Mick.


More info on the engine difference:
http://www.geaviationservicesolutions.com/archives/2006/v06i01.pdf
So it's seeming very probable that what the photos show is a CRJ-700 with one older CF34-8C1, on the pilot's left, leaving the "darker" trail, and one CF34-8C5B1, which has higher efficiency, and hence runs cooler, leaving the white trail.
 
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