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-   -   Visual effect (https://www.pprune.org/tech-log/387018-visual-effect.html)

ChrisVJ 30th Aug 2009 02:11

Visual effect
Coming back from CDG to Seattle a couple of days ago.

Aircraft was an Air France A330 (Nice ride and half decent food.)

Altitude 40,000 ft, OAT -62C, GS 898Km/Hr (Can't get airspeed or Mach no from the SLF TV screens.) Somewhere over Edmonton AL.

Looked out of the window and saw what looked like the edge of a piece of glass jumping around on the wing. Displacement of the sight of the black front edge of the aileron and extending down to front edge of the middle of the spoilers. Movement of my eyeline in the window showed it was vertical, slightly curved (bowed aft)

Surprisingly I managed to get a picture on my point and shoot and I have cropped the middle bit. You can't see the less clear lower part where it is lost in the white of the wing.


The effect stayed around for a few minutes intermittently, disappeared for a while and then reappeared for a further few minutes.

It is not a shadow,
It is not a mark on the window
It is not a reflection

From moving the sightline it clearly appeared like the vertical edge of a piece of glass standing on the wing.

Was it a shock wave distorting the view?

I am curious and wonder if someone knowledgeable can enlighten. I really don't need the comments from ignoramouses telling me "Not to worry, the wing tip isn't falling off."

john_tullamarine 30th Aug 2009 02:23

Looks to be a shock made visible by the relative position of the sun and aided by refraction of the black background colouring. Have look at this link which talks about Schlieren photography .. the normal technique used to visualise shocks.

Characteristic normal shock where locally supersonic flow reverts abruptly to subsonic. The dancing about is quite characteristic of the unsteady flow nature of the shocks in transonic flow regimes.

Appears the boys may have been trying to make up a little on the schedule ? .. we've all been there, done that ...

ChrisVJ 30th Aug 2009 04:09

My impression was we had a headwind (That would be normal East to West,) but we arrived on time to the minute.

john_tullamarine 30th Aug 2009 04:33

Indeed ... so one pushes up the MN a bit to increase the TAS and GS.

Costs a bit more in fuel but there may have been other considerations .. curfews and connections in particular.

criticalmass 30th Aug 2009 05:16

I have observed a similar phenomenon a few times whilst paxing in 737s with the winglets; a visual "discontinuity" about one third up the leading-edge of the winglet at cruising altitude. On descent it moves away (can't recall which direction) and becomes invisible.

I assumed this to be a change in refractive index of the air due to shock-wave/compressibility issues, and after we landed I mentioned it to the aircrew as I disembarked via the for'ard door. Although they hadn't actually seen it themselves they said they'd try to observe it next time they deadheaded in a company aircraft.

Nice to see you actually managed to photograph it. It seems the phenomenon is only visible under certain conditions of sun's angle relative to the aircraft, plus speed and altitude etc.

Bullethead 30th Aug 2009 05:25

G’day Chris,

Your image is indeed the shock wave on the surface of the wing causing an optical distortion. It is usually more clearly visible on the wing that is into the sun and as you were flying on a westerly heading it makes sense that you could see it on the right hand wing. In your image there are two parallel distortions, the thicker more rearward one is the main shockwave and the thinner one just in front of it is a smaller shockwave called the lambda foot.

Here’s a previous discussion on the subject.


I first saw the shockwave on the wing of a B707 years ago when I was in the Aussie Airforce.


leewan 31st Aug 2009 08:18

When an a/c is flying at transonic speeds, airflow over certain areas of the wing would actually be supersonic. Depends on the aircraft, of course. I think that is what he managed to capture.

captjns 31st Aug 2009 10:14

Shock wave. Use to see this on the 727 and DC8. Very interesting to observe.

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