Plane with three of four contrails?

[flight cat]

Hi all,
I've been reading these forums for about 3 years now, thought I'd sign up because I saw something odd (or maybe not!) in the sky on Saturday...

I was in the Gravesend area of the UK, and after lunch in the pub, I saw a plane fly directly overhead, possibly in a vaguely south western direction, and it looked to be pretty close to cruising altitude. The wierd thing was that the plane only had 3 vapour trails behind it, 2 close together on one wing, then a big gap, and a single trail on the other. I guess that means one of the engines was shut down?

I couldn't see anything on the rumours forum, does anyone have any idea what plane it was or what was wrong?

thanks
fc

[EI-MICK]

no maybe it was an MD11 or B727,both are 3 engined aircraft but the vapour trail difference sounds a bit strange.

[clifftop]

How much "lunch" did you have in the pub?:p:p

[Maude Charlee]

Quite common to see it, even from another aircraft cruising not too far away, and it is all just an illusion. Depending upon angles, it can be very difficult to spot the 4th contrail.

[The SSK]

Well there's a thread on R'n'N about a 3-engined A340 landing at LHR on Saturday, but it was at six in the morning.

[Rainboe]

It is very common to see 3 contrails only. The reason is the vortex that trails from the wingtip. As the air slides behind the aeroplane as it moves forward, the vortex makes the rapidly mixing exhaust twist around towards the wingtip as the airflow rotates in a widening cone towards the wingti below the wing and the centreline above the wing. It changes the viewing angle of the pair of contrails on each side, and from a particular viewpoint, one side will appear to combine, or just count as one. The further back they rapidly mix to appear to be just two contrails.
We had it graphically illustrated this very morning at 0525 as we followed a BA 777 into LGW 08R and we watched it gently descend into a low layer of wet stratus. It carved a long furrow into the cloud and we could clearly see the rotors. Quite a magical sight.

[tilewood]

This effect is most marked when watching a B747. At certain angles
it is fairly common to see just three trails.

[BOAC]

I reckon we've sorted this one out.:)

Thread now closed.