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According to the BBC:

A feed from a camera fitted under the fuselage of the surveillance drone shows a Russian SU-27 making two extremely close passes while releasing, what appears to be, fuel as it approaches

Looking at the video it seems that the fighters are indeed releasing some liquid when passing close to the drone. It looks to me like with the speed of the drone any fuel dropped into the fuselage will quickly evaporate.

Would the drone be in risk of catching fire when getting in contact with the fuel? Would that be a plausible reason for releasing fuel into it?

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    – Ralph J ♦
    Mar 17 at 19:44
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    $\begingroup$ Maybe they thought that the drone was running low of fuel and were trying to be helpful $\endgroup$
    – Richard
    Mar 18 at 18:53
  • $\begingroup$ See this and other answers to a related question -- aviation.stackexchange.com/a/47997/34686 $\endgroup$
    – quiet flyer
    Apr 4 at 19:55

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I read about three possibilities which the SU-27 pilots wanted to enforce:

a) Covering the drone in fuel and then igniting it with the afterburners which should have lead to a fire and subsequent malfunction of the drone

b) Injecting jet-fuel into the piston engine of the reaper, leading to a catastrophic failure of the engine due to misfire. From this source:

With its extremely high flash point, jet fuel, in essence, creates a detonation that will cause a gas-based engine to misfire and eventually fail.

However, it cannot be ruled out that the engine simply would starve due to the fuel-rich air being ingested. Edit after the very good comment of 757toga: The reaper features a turboprop engine. Therefore it would not explode from ingesting fuel-rich air, but it could still starve from fuel-rich air.

c) The SU-27 pilots wanted to blind the optics of the drone by applying jet-A1 fuel.

In the end, the SU-27 pilots chose option d), bending the motor shaft and propeller of the drone by virtue of a direct collision. I do not believe that the fuel of the SU-27 is enough to bend exactly one propeller, therefore I believe a collision occured.

It should be noticed that the exact intentions of the pilots is of course unknown, I also do not believe that they will tell anyone, but they succeded.

Edit 2: The US-military claims that fuel was dumped over the drone, discussion below pointed towards the unignited afterburners could have been used for this purpose. Another theory proposed by JPE61 in his (now deleted) answer, is that they did not dump fuel at all, but merely added power resulting in the sudden onset of a contrail which only looks similar to a "fuel dump". In this scenario the intention of the SU-27 pilots would have been to generate a strong enough wing vortex near the drone to cause either structural damage directly or induce large roll angles leading also to a crash. This is a well known effect (see point 7-4-3. b. in this source) and has led to crashes in the past of smaller aircrafts flying behind larger ones. However, this contrail theory was ultimately rejected...

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    – Ralph J ♦
    Mar 17 at 19:42
  • $\begingroup$ @757toga corrected it, thought I already had... $\endgroup$
    – U_flow
    Mar 17 at 22:07
  • $\begingroup$ See this and other answers to a related question -- aviation.stackexchange.com/a/47997/34686 $\endgroup$
    – quiet flyer
    Apr 4 at 19:54
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I don't know about catching fire, although they might get lucky, but I'd imagine that dumping a hefty quantity of liquid into the engine intake might well upset the engine enough to turn the drone into a glider, before it has a chance to evaporate. Possibly it would also damage or impair the sensor package.

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  • $\begingroup$ Yes, but as it clears out the engine should restart--long before it hits the sea. $\endgroup$
    – Loren Pechtel
    Mar 16 at 23:37
  • $\begingroup$ @LorenPechtel doesn't have to. If the MQ-9 spins out of control, aerodynamic forces may well rip it apart during its fall for example. And if it's pretty low (quite possible, MQ-9 isn't Global Hawk but a tactical asset) it may simply have run out of altitude before it could restart. $\endgroup$
    – jwenting
    Mar 17 at 12:35
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I think the "fuel dump" is a complete secondary issue.

Whether it was a normal fuel dump, or unlit fuel poured into the unlit afterburner stream, or AB, or wake turbulence....

It was a purposeful move on the part of the Su-27 pilot to harass and disrupt the flight of the MQ-9.

The first pass did not give him the desired result. On the second pass, he misjudged and went a meter or so too close. Resulting in impact with the prop and probably tailplane.

And it can't be said that the Reaper just magically appeared in front of him with no time to maneuver, due to the vastly different speeds. He came around and did it again. And any competent Air Force would know the existence of the Reaper (and all the other aircraft in the area). "Hey, there is traffic at your 11 o clock, at 14k feet. Don't hit it."

The Reaper operators knew they was there, due to the camera aim. And then changing the camera angle for a better picture on the second pass. But not much they could do to avoid.

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    $\begingroup$ Wouldn't it have been far easier (and safer for the pilot) to just shoot it down? I mean, yeah, it'd be seen as an act of aggression and all that, but so was what they actually did, so I don't think that'd make any difference... $\endgroup$
    – Darrel Hoffman
    Mar 17 at 21:13
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    $\begingroup$ @DarrelHoffman - Absolutely easier. But also, an overt act of aggression. Russian fighter jet shooting down a US aircraft, over international waters? That is a LARGE escalation of the status quo. $\endgroup$
    – WPNSGuy
    Mar 17 at 21:57
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    $\begingroup$ Making two close passes to disrupt a drone is also an act of aggression. Is the argument here that the intent might not have been the same as the end result, of fully downing the drone? Agreed you couldn't make that argument if guns had been used. These close passes leave open the interpretation of it being an attempt to temporarily disrupt, downing being an accident (@DarrelHoffman). Perhaps that's why US generals have called it "reckless" flying by the Russians, rather than act of aggression, because they don't want an escalation, and their public reaction is part of determining that? $\endgroup$
    – Peter Cordes
    Mar 17 at 22:58
  • $\begingroup$ @PeterCordes Maybe it's just a matter of plausible deniability. IF the Russian government, or high ranks, did order to disable the drone, a direct gun hit would be undeniable. Using the "fuel dump attack" could be afterwards denied or justified by saying it was a reckless maneuver by the pilot, an initiative of the pilot himself to harass the adversaries (fighter jet pilots are not unknown of such jackassery, after all) or something like that. $\endgroup$
    – Lorenzo Donati support Ukraine
    Mar 19 at 8:39
  • $\begingroup$ @PeterCordes It could also be the truth: IDK the actual level of professionalism and competence of the average Russian jet fighter pilot, but given how the war in Ukraine is going for Russian troops, I could well see a (maybe inexpert), frustrated and pissed off pilot trying to "ruffle the feathers" of the drone, without knowing it could actually end this bad (the collision that could have bent the propeller blade could well be interpreted as a sign of inexperience of the pilot). $\endgroup$
    – Lorenzo Donati support Ukraine
    Mar 19 at 8:39
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Several possibilities exist as well as nonsense theories.

Possible scenarios

  1. The dumped fuel could cause the drone engine to fail.
  2. The dumped fuel could cause some sensors to fail and indeed obscure the cameras

What is unlikely though, if at high altitude where the fuel is way below flashpoint, that it would ignite.

The next questions for which I do not have answers are

  1. Does the Su-27 have fuel dump capabilities?
  2. If so, why are the dump nozzles right near the engine exhausts as opposed to most airliners, the dump nozzles are near the wingtips?

But in the video, the drone did not pass through the dumped fuel trail. Wake turbulence could have upset it as well.

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    $\begingroup$ "why are the dump nozzles right near the engine exhausts" – I can't tell you that, but it is not unique: The F-111 Lancer famously has its fuel dump nuzzles right between the engines and can light the dumped fuel with the afterburner. Totally useless, but a great airshow stunt. On the F-111, the location of the fuel dump nozzle is dictated by the fact that it is one of the first swing-wing fighters, and the designers apparently didn't want to put too much complicated stuff into the wings. $\endgroup$
    – Jörg W Mittag
    Mar 16 at 21:40
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    $\begingroup$ I don't understand the fuel bit at all. I see little chance of appreciable damage. Wake turbulence is another matter, though--from when I first saw this story I figured the pilot was trying to disrupt it that way. $\endgroup$
    – Loren Pechtel
    Mar 16 at 23:41
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    $\begingroup$ Discussion on another answer (since moved to chat where the useful info is much less likely to be seen) suggests that the SU-27 can "dump" fuel through its afterburner fuel lines, apparently without igniting it. metabunk.org/threads/… $\endgroup$
    – Peter Cordes
    Mar 17 at 22:48
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    $\begingroup$ @PeterCordes -- also addressed in this and other answers to a related question -- aviation.stackexchange.com/a/47997/34686 $\endgroup$
    – quiet flyer
    Apr 4 at 19:53
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The idea of lighting the drone on fire is really more of a joke, just because it isn't a feasible method, as you said, it would change state to a gas really quickly at high speed and altitude.

Some people are suggesting it could be to upset the sensors in the piston engine, the drone is through the cloud so fast that it is pulling in fresh air almost immediately, also, the MQ-( does not use a Piston engine, it is a TurboProp, which is a gas turbine engine.. basically.. it IS a jet engine but instead of producing thrust, it uses the rotation of the turbines to drive the propellor, allowing for a much smaller engine to drive a prop, which means the drone is more efficient an capable of flying at slow speeds, such as those speeds best for taking pictures/video. Pouring some jet fuel into a jet engine isn't gonna do much.

It is possible that they were trying to block communication, by disrupting the antenna direction, this is why the feed cuts out when the drone is buffeted, the antennas need a perfect line of sight. So perhaps the fuel cloud can disrupt communication just long enough to cause it to crash.

However.. I think it was more of a visual warning, saying these pilots are acting all kinds of crazy, and we best get out of here before we lose a $30 million dollar drone..

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  • $\begingroup$ "So perhaps the fuel cloud can disrupt communication", this is unlikely, this would have no more effect than rain. $\endgroup$
    – mins
    Apr 5 at 17:41

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