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The Republic Day celebrations at New Delhi, India had a series of flypasts by the Indian Air Force.

What struck me the most was that in a formation with a C-17 in middle and two Su-30MKI flanking it, there were contrails visible from the Su-30MKI but not from the C-17:

C-17, Su-30MKI Formation on Republic Day 2018 C-17, Su-30MKI Formation on Republic Day 2018

Both the C-17 and Su-30MKI have turbofan engines. For C-17, Pratt & Whitney F117-PW-100 and for Su-30MKI Lyulka AL-31.

As per 'How does contrail formation differ from turbofan to turbojet?', contrails depend on the bypass ratio of the turbofan engine:

(...) higher bypass engines available today increase the contrail occurrence.

As per available sources, the F117 has a bypass ratio of 5.9:1 and the AL-31 has a bypass ratio of 0.59:1. So, the F117 has higher bypass compared to the AL-31 (or maybe I'm wrong).

What would be the reason that in the first two photographs, the C-17 engines do not form contrail whereas the Su-30MKI seem to?

One possibility is that, this difference is due to possible different power settings on the two aircraft engines for the same speed (considering the different intended role).

Another thing was that such contrails were not visible on any other fighter aircraft (e.g., Tejas/LCA). Or even another pair of Su-30MKI along with a AEW&CS aircraft:

Tejas/LCA Formation on Republic Day 2018 Su-30MKI along with a AEW&CS aircraft

Notes:

  1. Sorry for the poor quality of photos due to the poor visibility in Delhi (winter fog/smog).

  2. Some photos are here:

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    $\begingroup$ Are you sure that they weren't using some sort of smoke generation system? Those don't look like contrails to me. $\endgroup$
    – Ron Beyer
    Commented Jan 26, 2018 at 18:41
  • $\begingroup$ @RonBeyer not sure/aware about smoke generation... But now that you mentioned it, I also doubt these are contrails... $\endgroup$
    – Kanchu
    Commented Jan 26, 2018 at 18:46
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    $\begingroup$ Absolutely not contrails. Smoke generators for sure. Actual contrails, condensation from the motors, won't form near sa level in warm air. $\endgroup$
    – Ralph J ♦
    Commented Jan 26, 2018 at 18:50
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    $\begingroup$ And amazingly, in that second pic the Sukhoi's can apparently produce a giant bird in their trails! :D $\endgroup$
    – TomMcW
    Commented Jan 26, 2018 at 19:10
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    $\begingroup$ They must be chemtrails $\endgroup$
    – MCMastery
    Commented Jan 26, 2018 at 23:34

2 Answers 2

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This 'party trick' is covered nicely on metabunk.org.

[It's the Su-30's] ability to inject unburnt fuel into the exhaust plume via the afterburner system. The super hot air of the exhaust instantly evaporates the volatile fuel, and then when it mixes with the cold air behind the plane the fuel reforms as a fine mist, essentially a fuel smoke cloud.

enter image description here(YouTube)

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    $\begingroup$ Ha, we actually came up with the same source moments apart. $\endgroup$
    – Jamiec ♦
    Commented Jan 26, 2018 at 18:53
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    $\begingroup$ Seconds apart! :D $\endgroup$
    – user14897
    Commented Jan 26, 2018 at 18:54
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    $\begingroup$ 'party trick' :) Interesting read! +1 $\endgroup$
    – Kanchu
    Commented Jan 26, 2018 at 18:56
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    $\begingroup$ And a +1 for the both of ya's (@Jamiec) for the "great minds think alike" effort. $\endgroup$
    – FreeMan
    Commented Jan 26, 2018 at 21:18
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    $\begingroup$ Is there any practical reason to do this? Or is it just for cool points? $\endgroup$
    – BruceWayne
    Commented Jan 27, 2018 at 3:15
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From: Forum post - "Trails from the back of Fighter Jets", Metabunk.org

Jets practicing for air-shows (or actually in air-shows) sometimes use smoke to create trails. This is made by injecting something like paraffin into the hot exhaust where it vaporizes and condenses as smoke.

And the accompanying image

Looks a lot like the image in your question.

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