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-   -   Strange contrail south of London today (https://www.pprune.org/spectators-balcony-spotters-corner/199072-strange-contrail-south-london-today.html)

Seloco 19th Nov 2005 13:15

Strange contrail south of London today
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 19th Nov 2005 13:43

Aaaaaaaagh, the chemtrailers are back :}

Seloco 19th Nov 2005 14:22

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

Superpilot 19th Nov 2005 15:40

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

HEATHROW DIRECTOR 19th Nov 2005 16:42

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 19th Nov 2005 17:37

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 19th Nov 2005 18:09

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

wingman863 19th Nov 2005 18:39

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

Blob Contrail

captain cumulonimbus 19th Nov 2005 20:31

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.


Seloco 20th Nov 2005 12:09

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 20th Nov 2005 14:02

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 20th Nov 2005 14:11

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 20th Nov 2005 15:03

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 20th Nov 2005 15:20

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 donít 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 Ĺ 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 doesnít 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.

HEATHROW DIRECTOR 20th Nov 2005 15:44

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 20th Nov 2005 16:02

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 20th Nov 2005 16:21


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 20th Nov 2005 16:25

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

pax britanica 20th Nov 2005 16:51

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

Seloco 20th Nov 2005 17:56

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.

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