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Can anyone advise what the liquid seen on the wing of a 737-800 on approach as highlighted on the attached images is? The liquid was seen "pulsating" from the bases marked by the arrows. As the plane came to land, the intensity of the flow decreased until it eventually stopped altogether.

Image 1

Image 2

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    $\begingroup$ Lubricant, rain, melting ice.... Any liquid will vibrate when blasted with high winds. $\endgroup$ – Carey Gregory Aug 13 at 4:51
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    $\begingroup$ You said it was pulsating. I meant the same by saying "vibrate." I simply meant that any liquid is going to pulsate/throb/vibrate in a strong wind. My guess would be it's lubricant but who knows? Small amounts of water can get inside a wing easily. $\endgroup$ – Carey Gregory Aug 13 at 5:08
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    $\begingroup$ @VH-NZZ the conditions were not dry and hot when the plane was at altitude. Even in hot weather, air at cruising altitudes is extremely cold. $\endgroup$ – barbecue Aug 13 at 19:21
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    $\begingroup$ @VH-NZZ The aircraft was previously at cruising altitude, which is when the fuel and wings became cold. 2C per thousand feet makes it around -30C to -40C at cruising altitudes. $\endgroup$ – David Richerby Aug 14 at 18:40
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    $\begingroup$ @VH-NZZ Unless you obtained a sample of the liquid and conducted chemical analysis, you can't claim it was not water. $\endgroup$ – barbecue Aug 15 at 13:16
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The pictures show water condensation on the wing upper surface due to very cold fuel in main tank number 2.

The fuel is cold because the air temperature at cruise altitude is significantly colder than the air temperature encountered during the descent.

The condensation appears to be oozing from the wing but it is in fact flowing slowly over the surface, filling and overflowing every seam and imperfection.

Since you have provided photos and not a movie, I can't tell you what the "pulsating" really was, but it probably was either a slight boundary layer turbulence, or changes in the seam geometry due to wing flexing.


EDIT: There seems to be some confusion regarding the "pulsating" nature of the observed flow, with at least one correspondent1 insisting that the uneven flow proves that the fluid originates from inside the wing.

There are many mechanisms that could account for a pulsing or uneven flow of fluid over the wing, without resorting to the theory of a bleeding airplane.

The most likely reason depends on the fact that the wing surface is not a single sheet of metal, but contains discontinuous pieces such as spoilers, flaps, access panels, ailerons, etc. Because the wing flexes, the volumes of the spaces between these pieces, and the voids under some of them, continually change in flight.

Water flowing over the wing will fill some of these spaces and voids. Once a void is full, changes in volume will alternately squeeze out some water and suck some in. This could certainly look like a pulsating leak to someone who has not seen it before.

Another effect that can lead to pulsating does not require changing volume, but just a trough formed by the butting of two panels, roughly perpendicular to the airflow, filled by the trickling condensation. The air does not always flow smoothly over the wing2, and minor turbulence can press down on one section of the trough, causing water to spurt out of another section.

Seconds later the turbulent flow shifts, and the water hurries along the seam to pulsate in a different place.

The question can be completely resolved only by someone who can consider these theories while observing the effect.


  1. The original poster, actually.

  2. In spite of the large arrows painted on the wing surface.

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    $\begingroup$ @VH-NZZ There can be litres of water, why not? $\endgroup$ – Vladimir F Aug 14 at 6:40
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    $\begingroup$ @VH-NZZ A lot of air comes across an aircraft's wing in flight, and air can hold a lot of moisture. Have you done any back-of-the-envelope estimate of how much water could condense out of air on a cold wing of an aircraft at a given speed? Otherwise I have no idea why you would believe your intuition enough to say "it definitely wasn't water", given that our intuition evolved for scenarios that are completely different from what you're looking at here. Any number of things can result in pulsating behaviour; air is not homogenous. Heck, you usually get "pulses" of water in ordinary rain. $\endgroup$ – Luaan Aug 14 at 9:02
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    $\begingroup$ @VH-NZZ "Litres of water" are far from unexpected when flying at a low altitude over water. Evaporation fills the air with a lot of moisture and the wing surface after descending from cruise altitude is certainly way below the dew point of the ambient air. The wing surface comes in contact with dozens of cubic meters of air every second, with 10-20 grams of water in each cubic meter. $\endgroup$ – TooTea Aug 14 at 10:00
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    $\begingroup$ A contrail is the extreme example of condensation due to the passage of an aircraft (though not the exact mechanism as the one seen here). It's quite a large amount of water. $\endgroup$ – jcaron Aug 14 at 10:51
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    $\begingroup$ @VH-NZZ The only liquids it could plausibly be are fuel, hydraulic fluid and condensed water. Losing litres of either of the first two would be a huge problem; the fact that your plane didn't fall out of the air suggests quite strongly that it wasn't anything other than water. $\endgroup$ – David Richerby Aug 14 at 13:45

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