What happened to this engine? An odd AB shape & stall

Patriot · by **Patriot** » 06 Sep 2016, 10:10

Hey m8s!

I have a question to some engine guys or TEG specifically if he's arround.
Last friday during Zeltweg Airpower airshow in Austria the Belgian F-16 performed his routine show but seemed to have some issue with the afterburner... instead of having a full "carrot" plume it look like a tiny stream.. It looks pretty much like the MiG-21's 'leaking' afterburner.
To me it looks like some problem with the AB fuel injectors - like a set of them is off for some reason. Or is it maybe some deeper problem with the core of the engine? Is it possible that the ground crew or the pilot didnt have any feedback that there's something not quite right with the engine work? How they didnt noticed anything? The next day the aircraft experienced an apparent turbine stall but landed safely.

Friday's odd ab work:

http://www.airplane-pictures.net/photo/ ... ng-falcon/ TKOFF
http://youtu.be/oHU6krG6o0U High speed pass (4:10)

Saturday's stall:

https://m.youtube.com/watch?v=IdTRfV3zULo
http://youtu.be/x51UYlumFS8 (3:20 !!)

35_aoa · by **35_aoa** » 07 Sep 2016, 04:13

I'm not a mx expert, but I'd say that the first photo is somewhat inconclusive. You are right that segment 5 AB for example looks different, but that could have also been lucky timing during segment 1 AB light off or something too, which momentarily doesn't look "right" either. Not saying it isn't anything out of the ordinary, but it could certainly be the result of aerodynamics rather than engine malfunction as well. As for the second pic/actual event, it isn't totally abnormal to have a pop stall at slower speed/high AoA, especially with some sideslip/yaw due to inlet flow disruption. Generally these types of stalls quickly clear and don't develop into anything serious (i.e. stagnation). That being said, an engine malfunction such as DEEC problems, CIVV/RCVV damage/failure, FOD, etc is more likely to stagnate, and such an event would also be prefaced with a stall…….so in other words, it is hard to say not knowing more about it. Maybe some of the old hands around here with much more experience can chime in if I'm off base.

Patriot · by **Patriot** » 07 Sep 2016, 10:49

Apart from the cause of saturday's afterburner stall, the AB clearly had some issues the day before.
I was wondering if the odd burner on Friday led to the failure on the next day or these two occurances havn't anything to do with each other?

Boman · by **Boman** » 07 Sep 2016, 17:57

If there had been an issue with the AB, then the pilot would have aborted according to standard operating procedure. Even moe so at airshows as the negativity of a potential crash is worse than an aborted display. You don't need a repeat of Ramstein -86 or similar

The photos misleads you to think there is something wrong. Otherwise the pilot would have aborted as mentioned.

tjodalv43 · by **tjodalv43** » 08 Sep 2016, 02:45

I disagree. There are many pictures of the Belgian F-16 with a normal AB plume. I agree with Patriot that these pictures depict an abnormal plume. The AB stall/blowout would seem to support that, and the pilot did apparently abort the demo after it. From the cockpit I would assume there'd be almost no way to monitor whatever problem would cause that weird plume unless there was a noteable degradation in thrust or the engine kicked out an PFL or MFL. Which I guess wasn't the case. I would also think it plausible that if it was very sunny, anyone observing from the ground (MX, safety, etc.) might not be able to see the odd plume as apparent as it appears in high-res still shots. Maybe that's why they didn't KIO sooner. Just a theory though.

That_Engine_Guy · by **That_Engine_Guy** » 09 Sep 2016, 03:30

The PW220 engine has issues with not draining the augmentor spray manifolds correctly. When the fuel remains within the manifold 'coke' aka carbon deposits will form when the fuel is cooked within them void of oxygen. It will coat the internal surfaces of the manifold and begin to reduce the internal passages available for fuel-flow. (Think iron pipes with rust deposits that limit water flow after decades of continual use)

When a manifold becomes 'coked' it will affect it's ability to equally distribute the fuel around the circumference of the manifold. This will create high fuel-flow areas, and low fuel-flow areas within the augmentor. These fuel flow imbalances will then cause hot/cold streaks, or overly rich/lean areas of combustion. This can be seen in the exhaust plume in the appearance of streaks. It will also cause overly sooty or clean combustion patterns on the augmentor duct liner.

One way to correct (or mitigate) this known issue was a recurring 'bake' of the augmentor manifolds in a kiln. In extreme cases of coking found during borescope inspection, the manifold(s) would require replacement. The process of baking used pressurized air attached to the manifolds, piped into the kiln to help oxidize the coke as it was heated to 1200*F (If memory serves) This would bake the coke and make it release from the manifold's internal surfaces. The resulting powder could then be 'removed' by 'tapping' the manifold against the floor and shaking it. The process could be repeated until a borescope inspection of the manifold revealed it to be clean. Otherwise the manifold required replacements.

What I see in the photos of this post would lead me to believe the augmentor spray manifolds are 'coked' but without augmentor anomaly, (MFL/PFL or pilot reported) there is no reason to troubleshoot the streaking. Then again, they may have already been inspected and found to be acceptable according to installed engine (aka flight line) limits.If there was an issue, it may require an inspection and/or an out of cycle 'bake'. I'm sure that this behavior would exceed the JEIM limits, either during the borescope inspection of the manifolds or when the test cell run is accomplished post-JEIM.

My :2c:

Keep 'em flyin' :thumb:

TEG

35_aoa · by **35_aoa** » 09 Sep 2016, 07:59

Thanks for the info TEG, good stuff!

sprstdlyscottsmn · by **sprstdlyscottsmn** » 09 Sep 2016, 17:51

I second that! Thanks TEG, I got to learn something today.

m582 · by **m582** » 10 Sep 2016, 21:18

I've seen this many times before but not during a demo flight. The video (https://www.youtube.com/watch?v=x51UYlumFS8) shows an AB related engine stall at 3min20sec. Looking at the photo's and video of the AB flame pattern it is most likely a cracked/leaking no.4 augmentor spray ring. That would cause an uneven fuel distribution in the augmentor resulting in abnormal flame in the lower half of the augmentor. The engine can run for a while without producing MFL/PFL's. If it gets worse and you continue long enough the engine will have a AB blowout or a stall like it did in the video due to the increasing amount of fuel leaking from the manifold.

ghettobird · by **ghettobird** » 13 Sep 2016, 17:56

I always love the knowledge bombs that TEG drops, I've been off the bird/line for 16 years and still learning ha-ha

03fomoco · by **03fomoco** » 14 Sep 2016, 03:39

TEG is the man.

I would like to further add and just speculate that when we see this after an AB blowout event (with or without the stall) we typically find a cracked spray ring at what we call the "triple tree." It may not be easy to see as the spray rings have heat shields and if they are not melted away can hide this pretty well. It shows up right away during troubleshoot when fuel pressures are taken and analyzed.

TEG talked about flight line versus in shop limits and basically most shops have a local policy to bake all spray rings on each shop visit. There is a new modified version of the spray rings and location of the fuel feeds and so far it has outstanding performance. Not sure which FMS customers have decided to purchase that modification.