Strange contrail south of London today

[Seloco]

Did anyone else notice a strange vapour trail just south of London UK today around 1150Z? Going from east to west, it formed a continuous sinusoidal shape. Since there were several other trails in the same area of sky at the same time which retained a straight shape, and since the trail itself retained integrity within the wavy shape, its difficult to see how air currents could have been forming its shape. It looked as if it was being made by a single or closely coupled twin engined aircraft that was following a sinusoidal track with a "wavelength" I'd estimate at a mile or so. Unfortunately by the time I noticed it, the aircraft was too far to the west to see.

I've never seen this phenomenon before; anyone any ideas?

[Final 3 Greens]

Aaaaaaaagh, the chemtrailers are back :}

[Seloco]

No, F3G - just a serious question on an unusual observation.

[Superpilot]

I'm not the only intrigued one! I saw it too.

[HEATHROW DIRECTOR]

A normal civil jet flying at that sort of level would not normally be able to change direction at the sort of rate suggested and it's most unusual to have military fighters in that area.

Hope someone comes up with the goods..

[Cornish Jack]

Hmmmm
Not all contrails are the same, visually - B52s, for instance are very individualistic - sort of broken figure eights. Presumably vortex effects etc will modify the way they form....... and, of course, the amount of behaviour modifying chemicals they are dispersing at the time!! :E

[pax britanica]

Interesting-I also saw this on a day that was absolutely crsytal clear and presumeably very very very cold up at FL300 +

In fact my wife pointed it out because by the time it passed us-near Blackbushe it paralleled a 'normal' contrail and looked so odd.

It definately had the flat sine wave shape and from directly beneath it had a number of puffy circular 'blobs' along its length as tho every so often it expelled extra exhaust. Can jet engines have uneven combustion like that I wouldnt really have thought so without it being very evident?

I was interested to read the post about the B52 trails because that is an unusual aircraft having so many engines and perhaps that explains it but do B52s mix in with the westbound commercial herd or flock heading for the N A tlantic at mid day?

Unfortunately I didnt see this trail until the plane had passed well away to the west because with binocularls it would have ben easy to identify as it was so clear today
PB

[wingman863]

Thats a pretty weird contrail. It has the sort of blob feature mentioned above but without the wavy pattern along its length.

Blob Contrail (http://images.google.com/imgres?imgurl=http://www.projectstrat.com/contrails/003.jpg&imgrefurl=http://www.projectstrat.com/contrails/contrails.htm&h=576&w=768&sz=106&tbnid=jBruMDXaxA4J:&tbnh=105&tbnw=141&hl=en&start=15&prev=/images%3Fq%3Dcontrails%26svnum%3D10%26hl%3Den%26lr%3D%26rls% 3DGGLD,GGLD:2004-05,GGLD:en%26sa%3DN)

[captain cumulonimbus]

One of my training captains once told me that "blob contrails" as you so eloquently describe them:O are caused as a result of the speed governor mode of the autopilot.If you encounter a varying wind,with a specific speed set on the speed selection mode,the engines will automatically change throttle settings to maintain the selected airspeed as the winds are encountered.thus causing unequal exhaust gas temps as the engine settings are reduced and increased.

i'm not 100% convinced of this explanation,but if anyone has a more authoritative answer please,i'd love to know.I've also seen contrails that seem to have 'knots' in them at regular intervals,but only on one of the individual trails,the other one (in a twin eg) will be straight like normal.

Cheers,Cb

[Seloco]

OK, so several of us noticed the same unusual trail yesterday but no one has come up with an explanation. I'd say that there are only two explanations:

EITHER it was a normal straight trail that was immediately distorted into the sinewave shape by air currents,

OR it was a trail made by an aircraft at high level that was flying a continuous sinusoidal course.

The first of these seems unlikely, since the trail itself retained its thin coherent shape until it evaporated in the normal way. Incidentally its thinness would seem to rule out multi-engine aircraft such as the B52 previously mentioned; it had to be either a single or very closely coupled twin to make such a very precise single trail. It is also difficult to imagine an air current scenario that would distort a trail so rapidly and precisely into that sinusoidal shape, yet leave all other trails nearby completely unaffected.

So, if the second explanation is the more feasible, what reason could there be for an aircraft to fly such a strange course for so long? I'm wondering whether. for instance, a reconnaisance aircraft with side-facing sensors would need to deploy alternating banking in order to direct those sensors towards the ground. Or could it simply be some sort of control test? Either way, I can't help feeling it must have been a rather uncomfortable manoeuvre to undertake for any length of time.

Any test pilots out there able to comment?

[OLNEY2d]

I always assumed the shape of a given contrail was goverened by the wake-vortex pattern of the specific a/c type and its interaction with local atmospheric conditions.

Interestingly, yesterday was an exceptional day for contrails; some were visible from here stretching completely unbroken from a/c entering over Clacton routeing to HEMEL and routeing north and visible to overhead Pole Hill ! - Not sure of the track miles involved - but it must have been very cold and still up there.

[derekl]

Three of us saw that contrail from West Wycombe, right beside Wycombe air centre (Booker). (We were shooting, not flying!) There were other contrails at all sorts of altitudes, but nothing as weird as that one. I have to admit my mind strayed to stories of the legendary Lockheed "Aurora" aircraft . . . :rolleyes:

[Hilico]

Looking at the blob contrails link, that's exactly what I saw near CLN last week. Weather conditions, no doubt about it. I also reckon the engines have little to do with it, as the vortices come off the wingtips.

[r1830]

Is it possible the aircraft was tracking a navigational ground station (VOR) while on autopilot? It might fly a sinusoidal course if the right circumstances exist.

I can think of four reasons this might occur. There could be more.
1. The ground station is transmitting a poor or weak signal.
2. The signal from the ground station is being distorted. (Terrain, weather, mechanical)
3. The aircraft is not receiving the signal well.
4. The autopilot is weak or malfunctioning.

For example:
I fly out of Anchorage, Alaska. On some days when we are to the East or South of the VOR tracking a radial to or from the Anchorage VOR we get distorted signals. There is a large range of mountains 15 miles (25 kilometers) east of Anchorage that run NNE to SSW. I don’t believe it is a problem with the aircraft or autopilot because it has happened on different airplanes. Also, when we use a different VOR like Kodiak or Johnstone on the same day, the aircraft tracks straight.

When we use the VOR mode of the Autopilot it tracks the radial (course) selected by the pilot. When flying back to Anchorage from Cordova, we are westbound flying on the 084 degree radial of the VOR (HSI needle is set to 264 degrees To the station). The autopilot will keep the aircraft centered on the course. One moment we will be centered, and the next it will show a deflection to the right of about ½ to 1 dot. The autopilot will correct the aircrafts heading to get itself back on course. The needle will center and then not much later after that it will deflect to the left the same amount. The autopilot will correct again and turn to the left to intercept the course. The result is we end up flying a zigzag path unless we do something to correct it. Besides being a little uncomfortable to the passengers and annoying to us pilots, it looks unprofessional.

This doesn’t occur all the time, but most of us chose not to use the VOR mode. We instead will use the Heading mode of the autopilot. Instead of tracking the radial the pilot has selected it flies a heading the pilot has selected. We will select a heading and allow the needle to swing back and forth. If the needle stays to one side or the other for an extended period of time, we will then adjust our heading to get back on course. This method results in a lot less banking and or zigzagging and I think is more professional. It is a lazy pilot that allows it to zigzag back and forth on VOR mode in this case.

This may or may not have been the cause of the sinusoidal contrail.

Anyway, hope this helps.
:D

[HEATHROW DIRECTOR]

Given the area in which this event occurred the aircraft would have been under radar control in busy airspace and it's unthinkable that such deviations from track (if that's what caused the phenomenon) would not have been noticed. If the pilot reported problems with navigation ATC would have put him on a radar heading. But would it have been tracking a VOR radial or would it have been on INS?

[r1830]

These deviations occur well within the width af an airway. The airways are 8 miles wide, 4 miles either side of centerline. I am talking about deviations that are approximately 1/8 mile to 1/2 mile either side of center. As long as the aircraft is correcting to course, the controllers do not say anything. It usually takes a couple of miles off course and diverging to have the controllers say anything. The autopilot will still fly better than a pilot hand flying most of the time.

Plus there is separation provided of a few miles in front, behind and to the side of the aircraft. Where this seperation cannot be maintained, different altitudes are flown by the aircraft.

We do not have GPS or INS in our aircraft. We navigate with VORs and NDBs. It is embarrasing when the controller clears us direct to an intersection. We have to reply, "unable." We are vector equipped though.:D

Forgive me as sometimes I forget, there are aircaft equipped with better navigational equipment. I have no idea if this was the cause of the unique contrail seen. But, it is a posible explanation or scenario. So far no one has come up with the type of aircraft used.

Could have been an older corporate jet with out INS or GPS.
Could have been a pilot hand flying and not holding course well.
Could of been a pilot wanting to mess with us spotters and make us post a couple of pages of speculation as to what caused that anomaly.:rolleyes:

[Superpilot]

Is it possible the aircraft was tracking a navigational ground station (VOR) while on autopilot? It might fly a sinusoidal course if the right circumstances exist

No way, the amplitude of the sine wave was too high and the turns way to sudden for them to resemble an a/c tracking a VOR.

[r1830]

Ok, I wasn't sure. I didn't see it.:O

[pax britanica]

Well this did generate a lot of interest but then it was unusual . I am sure the explantion about the engines changing power settings may be right -I wish now I had photographed it because I cannot remember if the 'blobs' were evenly spaced or at any particiular point on the sine wave.

From pretty much underneath the amplitude of the wave wasnt very great and this would have all taken place easily within the confines of the airway as well as I am able to judge from the ground.

Looking back the trail doesnt seem to have been wide enough for a B52 and would a B52 be trucking along with all the westbound civvies anyway. If it was a single or tail engined biz jet I wondered if it could have been a yaw problem with the a/c slipping a little from side to side -I am not sure of the aerodynamics of this and perhaps yaw is divergent which this path wasnt. I am also not sure this wasnt VOR tracking as that can be seen quite often overhead of me as a/c turn of the east west tracks to ehad south for Midhurst and its quite common to see the turn go ona little too long and swing back .
Anyways generated some interest thats for sure
PB

[Seloco]

Assuming that this was a trail made by an aircraft flying at 30,000 feet or above (ie contrail level), then my estimate was that the wavelength (ie "crest peak" to "crest peak") of its sinusoidal course was 2-3 miles, and the amplitude (ie "crest" to "trough") about a mile. However it might of course have been considerably higher, particularly if military, in which case those estimates could double. From where I was observing (Chertsey) I would estimate that the sinusoidal track had been carried out for at least 30 miles. I can't believe that this could be a result of normal autopilot deviations, albeit within airway constraints - the pilots would have become aware of it rather early on, methinks.

What a shame that we have no posts from anyone who saw the aircraft making it, and none of us had cameras handy either!

Incidentally the "blobs" mentioned by several posters seem to me to be a frequent effect during the breakup of normal contrails as well. Although there were some apparent on this one, I was struck by how tight most of the wavy trail remained until it finally evaporated.

[cwatters]

Could this be the result of wave lift as used by glider pilots? I'm told that wave lift is very smooth and can sometimes be found in the east of england. I didn't see the trails mentioned.

[HEATHROW DIRECTOR]

<<Plus there is separation provided of a few miles in front, behind and to the side of the aircraft. Where this seperation cannot be maintained, different altitudes are flown by the aircraft.>>

I think most of us are aware of that but my point is that no controller at London Control is going to watch an aircraft weave about in that manner without doing something. Aeroplanes simply do not weave about the sky in a random fashion in busy airspace. They will either fly recognised tracks on standard routes or they will be on radar headings. In either case they will fly virtually straight lines. Even the slightest deviation would be observed on radar and queried. An aircraft flying the sort of pattern described here in the current security climate would raise considerable concern.

[Seloco]

There's a possible but perhaps far-fetched answer to my original question here:

http://vehicle.me.berkeley.edu/Publications/AVC/strategies_ains2003.pdf

Very thin trail, course roughly aligned with the M3....

Time to wait for the men in black to arrive!

[pax britanica]

HD-with reference to your comment I am begining to think it is the contrail and not the aircraftproducing this phenomenum(?). Although from where I saw it the aircraft had passed me I dont recall the actual head of the trail diverging, and the apparent angles to either side of track were I am sure large enought o have been noticed on radar.As yousay that was a busy time and the controller wouldnt let one aircraft in a crowded sky zig zag along like it appeared to.

One idea that comes to mind that might fit the picture is say a two or three engine biz jet -tail engines and thus from the ground only one contrail.

A previous poster poster described how the engines could perhaps 'hunt' a bit trying to maintain Mach number -perhaps it was very high and it was unsually cold I am sure -and that produced the blobby -uneven trail effect .

Would it be possible for the occasional extra jet efflux from say one engine to deflect the contrails away to the side slightly where after a while the tip vortices spin them back inwards so that the actual trail is a sort of spiral-the sircraft of course runs pretty much straight and the spirals from the ground looks like a low amplitude sine wave.
Anyway thats my little hypothesis based upon whats been posted by others- any takers?
PB

[Caudillo]

How about instead of poor navigational performance it could be the opposite? Given the accuracy of modern equipment, were it that calm and still aloft, it's not impossible a 2nd aircraft may have made good the same track and height I imagine - would it in that case be possible that it's the result of couple of contrails interacting in this interesting manner?

[Pontious]

Hello people,
Was out walking on the marshes of the Ribble estuary in Lancashire on saturday afternoon and noticed the most number of contrails I've ever seen in a single day. They were almost all SE-NW in orientation with the bulk of traffic consisting of the usual 'Trannies' heading for the NAT Track system but 2 were distinctive and I'm not talking 'Donuts on a rope'.
They were both 'U' shaped, about 20 miles apart and untouched by any other trails. One looked as though it was originally southbound along UA25 before executing a '180', the other was exactly the opposite on airway UA1.
I've often seen unusually shaped 'trails off the nearby coast due to the Irish Sea Air Refuelling Area and stuff on test from BAe Warton but never anything like this. It resembled a holding pattern cut in half.
Any info. gratefully received.

Yours faithfully

Confused of Preston.


:ok:

[derekl]

I'm told that aircraft sometimes 'tack' like this to lose a little time when approaching a busy hold. Could it be as simple as that?

[PaperTiger]

Meteor ? Leonid not Gloster.

[Seloco]

I don't think so, Paper Tiger, although there have certainly been reports of meteors following erratic courses. What I saw was too regular, both in sinuosity, speed and height to have been caused by the Leonid variety careering into the atmosphere.

Oh, and I'm afraid the very precise single trail ruled out the Gloster version, although I suppose with one engine out.......

[HEATHROW DIRECTOR]

derekl.. I don't think so - once they get close to holding areas they're under pretty tight radar control. In any event, this aircraft was very high and if it needed to lose time a reduction in speed would have been the technique. In extreme cases it would simply enter a holding pattern.

If we're not going to get a definite answer here's another one for the pot - couple of years ago I saw a satellite image of the North Sea which clearly showed a long, spiral vapour trail - just as if a fast jet had flown some distance doing barrel rolls. Never had that fully explained but, of course, "Aurora" featured in suggestions!

[WHBM]

Have just seen a shape like that originally discussed; I've seen them before and it was just a matter of time, but so nice and promptly ...... !

Must have been directly overhead Heathrow, at crusing altitude, headed southwest, about 1230 local today. And as before when I have noticed this, as it was laid out from the aircraft it was perfectly normal and straight, but then got distorted into the discussed sinusoidal shape. It then vapourised quite quickly after doing that, breaking up into separate sections like bananas in line astern first.

So it seems it is not a feature of the aircraft, but of the air currents afterwards.

[derekl]

Here's another theory to add to the pot: perhaps it was an AWACS (except I would have expected more obvious multiple contrails and what I saw looked like a single trail).

[Inverted81]

http://trancemaster.nkiteam.de/cp2/albums/userpics/10042/contrai%20closeupkl.jpg

Sounds very much like Contrail seperation as a result of the vortex wake of the aircraft meeting the trail. Vortex wake in my basic understanding will drop behind an aircraft. So perhaps the current conditions will allow this interaction to occur...

any use?
81

[Jetstream Rider]

It sounds very much like vortex interaction. I think this navigational and hunting stuff are red herrings.

I have seen contrails break up into sinusoidal patterns before - usually not very long lived. If you fly up close to a contrail you see two distinct sections. The first is a tight spiral and the second a diffuse apparently structureless haze. They often separate, with the distinct spiral dropping away below the haze. Two engines produce two spirals and if these interact you can get some very interesting shapes formed (like the pic above).

I doubt if it is to do with the cold either - more likely the airflow up high being smooth and slow.

Look carefully into the water behind an oar, or a vertical structure stood in water. At certain flow rates, vortices are shed from the tip of the blade and interact downstream in a vortex street. These often form sinusoidal patterns which then break up further downstream. Higher velocities give more random shapes and beyond that it all blends into turbulence. Very slow velocities will provide a more stable downstream flow.

Contrails are a bit like the oar above, but in three dimensions (water in the above example is 2 dimensional because of the surface). The difference in viscosity of the fluid (air) makes the velocities very different for the same effects in water.

[Seloco]

I've had the chance to study a few more trails above SW London today and, whilst several of them exhibited distortion that might be considered wave-like, it was relatively random, short-lived and accompanied by a general diffucion of the trail itself. As such it is easy to see how these could have been caused by wake vortex or air current effects.

However none of these observations, or indeed those proposed by others above, coincide with the essential characteristics of what I and others observed on Saturday, namely:

* a single trail that remained extremely thin and well defined prior to evaporation
* a regular sine pattern that remained stable and consistent behind the aircraft for perhaps 20-30 miles
* a consistent wavelength and amplitude of the sinusoidal curve
* a pattern that was not replicated by any of the many other trails in the same area and at the same time that day, which generally decayed through slow and stable diffusion prior to evaporation.

I don't think we yet have the answer!