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Watching a video of F/A-18 fighter-jets taking off, I noticed some sort of gas blowing out behind the canopy shortly before the start of the takeoff roll.

Here's a screenshot:

Swiss two-ship F/A-18

In some videos the smoke is more dense than in others. Has this something to do with cockpit pressurization?

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    $\begingroup$ @RonBeyer The Swiss Air Force really needs to stop doing that chemtrail dispenser test as part of the take off checklist. It's supposed to be an airborn check. ;-) $\endgroup$ May 12 '17 at 13:38
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    $\begingroup$ Testing a tank vent during the checklist, perhaps...? $\endgroup$
    – xxavier
    May 12 '17 at 13:57
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    $\begingroup$ Slightly, OK... extremely OT, but are those green stripes on J5019 the proximity lighting that pilots use to find each other in close formation, radio/radar-silence, night flight? $\endgroup$
    – FreeMan
    May 12 '17 at 14:42
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    $\begingroup$ @FreeMan Yes. $\endgroup$
    – fooot
    May 12 '17 at 14:49
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    $\begingroup$ Maybe the pilots had burritos before the flight $\endgroup$
    – TomMcW
    May 12 '17 at 20:11
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It is not smoke. It is bleed air being blown behind the heat exchanger which forces cool air across the front of the heat exchanger, this is only happens on the ground. The left side of the aircraft has a secondary heat exchanger that is also cooled by a water spray nozzle. The water is created by the ECS taking bleed air and cooling it. The water is blown out by the bleed air so you can see it more on one side than the other. In the picture, it seems to be raining so the air being blown out is more visible because of the rainwater.

-6287 (Fixed-Wing Aircraft Safety Equipment Mechanic, F/A-18)

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The two round grills on the left leading edge extension sit over the top of the RADAR liquid cooling system components like the reservoir and pump.

The gas you see escaping upwards is water vapour from the primary and secondary heat exchangers used in the environmental control system (ECS). These ECS components are part of the more specific air cycle air conditioning subsystem and sit just downstream of bleed air subsystem. Air is forced over the heat exchanger to make it more efficient for cooling. The vapour is more noticeable when it is wet or humid outside, and when the pilot throttles up.

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Engine bleed air leaving the turbine compressor travels through a condenser/reheater then into a water extractor, this water is sprayed from a jet ejector in front of the secondary heat exchanger in the LH diverter channel. What you are seeing is simply water vapour.

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