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I was seeing this image. Always we can see an abnormal flame spit out of the engine when the pilot goes for afterburner. Why?

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    $\begingroup$ It's totally different from F-111's fuel dump flame. Probably the afterburner is simply too cold and the fuel doesn't burn very well. $\endgroup$ Aug 23, 2023 at 10:20
  • $\begingroup$ @user3528438: I wasn't also that sure. Can you elaborate an answer from your comment? $\endgroup$
    – sophit
    Aug 23, 2023 at 11:18
  • $\begingroup$ Do you mean always in the case of MiG-31, or always regardless of make/model? $\endgroup$
    – Jpe61
    Aug 23, 2023 at 15:49
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    $\begingroup$ @Jpe61 I just have seen in the case of MiG-31. $\endgroup$
    – Roh
    Aug 23, 2023 at 17:53
  • $\begingroup$ @user3528438 Ok but, why does it happen on the below portion of the flame? $\endgroup$
    – Roh
    Aug 23, 2023 at 18:07

1 Answer 1

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The thumbnail of this video is a little misleading (it isn't from the same video), but show the same phenomenon. The video has a different angle from a little further behind (also at a completely different time and place I think since the thumbnail has snow), but it looks like there's just some kind of unstable transients as the afterburners engage.

flames visible in the bottom of both engines of a MiG-31 as the afterburners engage for a takeoff

It's possible that the fuel flow rate as the afterburner engages is simply too high for the airflow for a couple seconds, so there's fuel pooling and combusting as a pool of flame at the bottom of the afterburner section of the nozzle instead of getting properly mixed and burning in the exhaust stream.

This other video also seems to show the same thing, but only visible in the right engine, and it's harder to see. I think that supports it being some kind of transient that doesn't always occur. Watching it frame-by-frame it might also be that the various nozzles of the afterburner (which are typically a ring of fuel nozzles around the afterburner duct) just aren't quite in-balance; I thought initially that the left engine had a flame front starting from the center (i.e. in the exhaust stream) but it's off to the left side.

a different MiG-31 takeoff showing unbalanced flame during the afterburner ignition

I haven't been able to find a video looking up the nozzle of a MiG-31 igniting its afterburner in flight; I think that would be a helpful test of my guess.

(My bona fides, since there's some speculation in this answer: an aerospace engineering degree and a pilot's license, though I've never worked directly on jets.)

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    $\begingroup$ You can embed YouTube videos, I believe. $\endgroup$ Aug 24, 2023 at 13:50
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    $\begingroup$ @RodrigodeAzevedo the embed won't show a specific frame, though; and, as I say, the first video has a misleading thumbnail, which is what would embed. ...also, I always forget that you can do that, but this time I stand by not embedding them. $\endgroup$
    – Erin Anne
    Aug 24, 2023 at 15:44

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