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Today I noticed overflying aircraft (over Sydney at FL400) which appeared to have three contrails. When I checked Flightradar24 app, I saw that this overflying aircraft is B787. What would be the reason for three contrails with only two engines? Could the third be caused by APU, or just by a wake turbulence, or anything else? I tried to take a photo using binoculars (sorry for the quality, I tried my best). And also a screenshot of the A/C from Flightradar24 app. photo enter image description here

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    $\begingroup$ Clearly it's the auxiliary chemtrail engine :P $\endgroup$ – 0xdd Apr 11 '18 at 14:12
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    $\begingroup$ I'm giving you a +1 just for the cell-phone pic through binoculars effort! Nice to know that works out pretty well! $\endgroup$ – FreeMan Apr 11 '18 at 14:52
  • $\begingroup$ Related. $\endgroup$ – Vladimir Cravero Apr 11 '18 at 16:15
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    $\begingroup$ Just FYI, although the "wave interference" explanations here are likely correct, there are such things as "aerodynamic contrails". These don't originate in engine exhaust, but rather due to rapid compression and expansion of air as it's disturbed by the rapid passage of some part of the aircraft. Almost every day if you're under the approach at SAN you can see these in wingtip vortices. No engines there. On one landing when I happened to be behind the wing I saw a contrail coming off of one of the hydraulic actuator fairings. $\endgroup$ – Jamie Hanrahan Apr 12 '18 at 16:30
  • $\begingroup$ Looks like a diffraction pattern to me. $\endgroup$ – aroth Apr 15 '18 at 14:55
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Contrails are created due to variances in the pressure of the air, coupled with the amount of moisture in that air. When the 2 engines of the B787 blast their exhaust out, the high pressure of the exhaust temporarily causes the moisture to condense. Two contrails are visible.

However, as the pressure waves from the 2 exhaust trails propagate through the open atmosphere, they interfere with each other. This interference can also affect the pressure of the moist air between the 2 "primary" contrails, sometimes resulting in another, "secondary" contrail. In fact, because of this, if there were enough moisture in the air, and the engines were powerful enough, there wouldn't actually be just 3 contrails. Rather, the contrails would present themselves as the crests of a propagating wave much like the crests of water waves as a duck swims by. In effect, we would see the 3 contrails separating, and between each of the 3 contrails, 2 more "tertiary" contrails would be visible.

Here on Earth though, the pressure variances and the density/moisture content of the air normalize far before any of that exotic pressure wave stuff could occur. Except we DO get that one in the middle!

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    $\begingroup$ It's not condensation but freezing what causes contrails. And much of it is caused by tiny particles (like soot) and not pressure fluctuations. $\endgroup$ – Peter Kämpf Apr 11 '18 at 18:25
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    $\begingroup$ @Peter -- yes, that's true. However, the rest of what he wrote is still correct (the part about interference patterns) and applies independent of the cause of the trail's formation. $\endgroup$ – DrFriedParts Apr 12 '18 at 1:51
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    $\begingroup$ To say that I am skeptical is an understatement, even putting aside having flown alongside contrails and seen that they are nothing like interference fringes, but let's start with one straightforward issue: this theory depends on the engines being coherent acoustical sources, which they are not. $\endgroup$ – sdenham Apr 12 '18 at 18:34
  • $\begingroup$ @sdenham -- The theory depends on the engine exhaust creating spatially coherent concentration gradients (which they do). i.stack.imgur.com/IGnc9.png $\endgroup$ – DrFriedParts Apr 13 '18 at 20:38
  • $\begingroup$ @DrFriedParts That is a completely different claim, with 'spatially coherent' gratuitously added to make it look relevant. Tim's answer specifically claims the effect is due to interference between pressure waves, and interference between waves is a specific phenomenon of physics that is not relevant here. In general usage, 'interference' may loosely mean something like 'interaction', and the interaction of the two vortices may well be the cause of this phenomenon, but that overlapping usage does not in the slightest justify attributing this phenomenon to the interference of pressure waves. $\endgroup$ – sdenham Apr 14 '18 at 12:33
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Fascinating question. Not being an expert in contrail aerodynamics I'm forced to speculate, but I'm guessing the apparent three trails is caused by the spreading of the trails by wingtip vortexes.

I find similar examples called hybrid contrails. Here's one animation of a four-engine A340 from that article:

enter image description here

I think if you ignore the trails from the outer engines, this looks similar to your photograph. Here, the inner trails are twisted and spread by the wingtip vortexes into a pair of semi-transparent "tubes" of condensation trailing the aircraft. The apparent darkness of the tubes is thickest at the edges, since at these points we are looking through a greater thickness of vapor. As the tubes expand they touch, leaving three areas at the red arrows where the trail appears darkest from below.

enter image description here

A similar formation can be seen in the first few seconds of this video. It happens quickly, but paused at the right moment there appear to be three trails:

enter image description here

Within the next second the formation has spread enough it becomes an indistinguishable single trail again, but under conditions less favorable for persistent trails such as in your photograph, the trail may have dissipated by this time.

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Those three contrails that formed are diverging, while maintaining some coherence. It's hard to be sure, but the third one seems to appear when the diverging fan of the initial two reach the centre line of the airliner. My guess is that they mark the vortices (vortexes) in the air shed by the tips of the flight surfaces. There's a big one coming off the end of each wing, and a smaller one coming off the end of each horizontal stabilizer. These latter are too close together and probably too much generally affected by turbulence generated by the fuselage to be seen as distinct.

So initally, two: one source of ice crystals emerging from each engine. But these then get sucked into the vortices, and two transitions to what appears to be three.

The trailing vortices of a big airliner are huge. They can cause serious turbulence for another airliner crossing the "wake" of a previous flight, and can cause damage or loss of control for a small aircraft. They are one of the good reasons for a mandatory minimum separation between aircraft approaching an airport. (ISTR, three miles).

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I expect the earlier explanations are correct, but it also strikes me that there are more that two jet type engines on a 787 - the APU is right there in the middle and while its exhaust isn't nearly as strong as the main engines I can imagine that it might be strong enough to perturb the flow at the point where the main jet exhaust meets and have a visible effect on the main engine exhaust flow.

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    $\begingroup$ Nope. There are restrictions on the operation of the APU. Varies by brand, but it may not light-up above 15'000 ft, operation above 25'000 ft is not allowed, or operations are not recommended. No major airline has a standard operating procedure that includes APU use on cruise. $\endgroup$ – DrFriedParts Apr 12 '18 at 1:49
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    $\begingroup$ There certainly are APUs capable of starting/running at all altitudes. I believe this is an ETOPS requirement, or ETOPS has other requirements which have that same effect. I expect the reason is at least some planes need the APU to cover some ETOPS contingencies -- descending to 15,000 is out of the question as it would worsen their fuel economy so they could not meet their ETOPS range requirements. $\endgroup$ – Harper - Reinstate Monica Apr 12 '18 at 15:57
  • $\begingroup$ @DrFriedParts: Is that a worldwide restriction? Or specific to your personal country of origin? $\endgroup$ – Lightness Races with Monica Apr 13 '18 at 11:40
  • $\begingroup$ @LightnessRacesinOrbit DrFriedParts seems to be saying that it's basically part of the APU "owner's manual", not a legal restriction. $\endgroup$ – David Richerby Apr 13 '18 at 14:24
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    $\begingroup$ @Harper -- ETOPS APU's are required to be able to start in flight reliably from cold-soak or remain on the entire flight (no manufacturer/operator chose this latter option for obvious reasons). However, my original statement is still correct. No major carrier suggests using it on cruise (except in certain emergency scenarios) and many models of APU deployed today do not run above 25k'ft. So either it can't or you shouldn't. $\endgroup$ – DrFriedParts Apr 13 '18 at 20:31

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