PT6 Compressor Stall....HELP!!!!

[ybp]

We have a PT6-27 in a DHC-6 that started compressor stalling. It has flown 16 hrs since we changed hot section and PT stator. It only stalls when the FCU is controlling the acceleration and it is above 80%. We did a coke bottle test on the bleed valve and it is closing at 82%.

So far we have changed the bleed valve and cleaned it, compressor and CT wash, changed FCU and fuel pump, inspected the 1st and 2nd stage of the compressor, changed the hot section, changed the complete power section and we are still having the same problem.

Currently, it has the same hot section and power section that had been running together in the #1 position. The PT stator has been changed, but we didn't have any problems with it for 16 hrs.

If I had any hair, I would be pulling it out!!! We have been in contact with our OH shop and PWC and they are stumped too. We have a new power section and hot section on the way but they currier went "mechanical" so we don't know when we will be getting more parts or if that will even fix it.

Any advice would be appreciated,

YBP

[GA MX Trainer Dude]

Hi

Had a similar problem a lot of years ago - finally fixed it when someone mentioned that they cleaned both engine bleed valves at the same time - in the same cleaning basket - and when they re-installed the C/D orifice in the bleed valves they had swapped them from side to side - didn't make any difference to one engine other than the bleed valve closed late but on the other side the bleed valve closed early - causing surge and stall. I don't know what temp you were doing your coke bottle test so don't know if your close point is late enough.

The small PT6 will stall and surge at higher speeds if the bleed valve closes to early - but will very often have no stall tendancy above about 92% when the valve is closed - in other words it will be transient from the point of early closing to a point where the engine has reached a speed that the valve should be closed.


By the way - the C/D nozzle is a classed part so is matched to the bleed valve on the bench - also a "dirty" nozzle will cause the bleed valve to close early as well.

I also had a problem with one engine with re-ingesting P3 air from a leaking airframe pickoff point gasket and bad seals between the compressor section and the back of the engine. No full stall but lots of rumble and surge.

On a bit of a funny note - I did an engine change one dark night (not a DHC-6) and I was the only one available to do the engine run so put my helper outside to watch - look for oil leaks etc. Running the engine at idle and the prop in feather he noticed an air leak from around the bleed valve and stuck his finger over the C/D nozzle and we had an immediate compressor stall - he had to go and change his shorts!!!!!

Hope this may help in your search.

Mx

[ybp]

Do you mean ITT or OAT for the coke bottle test? I was running the coke bottle so I don't know what the ITT was but the OAT was about -5 or -10.

It will ONLY stall above 80%....when the bleed valve is closed.

So far, at the boss' request, we have tried three other bleed valves, all of which came off engines that were running fine. We have cleaned the original one and it is now back on the engine.

This bleed valve is closing quickly at 82%. Someone suggested that it should close slowly up around 90% but we can't find anywhere in the manual that says that.

YBP

[twnotter]

I had something similar on a -50 engine. We had removed a couple of the lower diffuser tubes because they had let go. Once we temp installed them everything went back to normal. The manual states that you can have a given number (maybe 3 or 4) missing given performance is not affected. As a last resort you can split the engine and check your diffuser tubes. Hope this helps

[brownbear]

well I am curious to this problem.

I would consider trying a borescope into the hole where the compressor bleed valve goes. See if you can spot any damage,

Is it popping real bad all the time? Or only when accelerating? How fast is it accelerating?

Is the T5 jumping up, fuel flow doing anything?

Are all the diffuser tubes healthy and connected?

You should be able to borescope and see the centrifugal compressor.

There might just be something wrong internally with the compressor.

[ybp]

I boroscoped the first few stages of the compressor and didn't see anything. We checked all the diffuser tubes when we did the hot.

It pops about 4 times as it goes past 80-85% only when the FCU is controlling the acceleration.

When it stalls, the ITT and fuel flow jump, but other than that everything is stable.

At this point we are assuming it is the compressor too, but have to wait for parts.

YBP

[tiggermoth]

The really strange thing about this problem is that, like ybp mentioned, is that it started suddenly. The airplane flew fine the day before and then the pilots were doing their pretakeoff checks and pop pop pop!!

It seems like something may have let go somewhere but we can not figure out what or where. Like YBP said, the only peice left that we have not tried changing is the Gas gen, which is expensive and a lot of work. If anyone at all has even the remotest idea, PLEASE help.

[Wrench]

If you never used the same pt stator class when you changed it. It could be chocked causing compressor stalls.

[GA MX Trainer Dude]

Some more thoughts.


I would force the bleed valve to stay open and see if the stall still happens - you could do this by mechanical means or by removing the c/d nozzle from the valve. This can't hurt the engine as it will act like a stuck bleed valve in the open position which I have had a couple of times. See what happens with that. For some reason I think your valve is closing too soon - I seem to recollect that it should close at 85 - 90% depending on the OAT but don't have my training notes with me right now. You could do the bottle test on the other engine to see at what speed the valve closes - if there is a big difference then I would talk to P&W.

I know that you mentioned that you changed the FCU - the fault isolation chart - 72-00-00 - page 115 in my maintenance manual states to change the FCU as the last step with surge problems??? It is possible that the fuel schedule is too high at that point and causing the problem. Since the fuel schedule during acelleration is controlled by the "chrome dome" setting - it might be worth setting it down to slow the engine acelleration schedule and see if that brings any relief. If the FCU that you installed has the same differential bypass presure setting - which is what you are adjusting when you set the acelleration then you will still have the same problem. Try turning the acelleration schedule down - takes longer for the engine to spool up - and see what that does. This does not change any of the steady state conditions or starting conditions and you are only allowed to change it 3 clicks +- from the original position.

Also - is it possible that you have the "wrong" fuel nozzles installed - just a wild chance but sometimes things get messed up during nozzle maintenance. Nozzles or even just 1 nozzle could cause you problems with excess fuel flow - just like a faulty FCU.

One other little thing is (If I Remember Correctly) there is an orifice in the P3 line elbow to the FCU - if that is missing it makes the FCU very sensitive. Is it possible that this has been left out???

Try running the engine with the P3 airframe bleed blocked off - that way you know for sure that you don't have excessive airflow through the compressor. Make up a blanking plate and install it on the engine flange. A power check would most often show this as high fuel flow - high ITT - and high gas generator speed. At the lower temps it might not show as there is a lot of reserve built into the compressor. I had a -27 that made TO torque at -10 F and colder with no problem - when it warmed up to above freezing we couldn't get full power - the compressor was truly trashed on that engine.

One last thing - is the gas generator loaded up?? Is the generator load really high and causing the fuel flow to be high to begin with and then the FCU just cannot cope? Again just a wild shot - if it does it with no power extraction from the Gas Generator then that is not the issue.


Hope this helps - know what it is like to have these types of problems - I started working on the PT6 in 1973 so have been around a bunch of them.

Regards,

Mx

[ybp]

Mx, we changed fuel nozzles, checked to FCU orifice and adjusted the 'chrome dome' down as much as allowed...(before we changed the FCU) with no difference. We did not coke bottle the other engine but we did come to the conclusion that this valve is closing a little early.

Pratt says that the CT class may be too tight so we have a hot getting re-class right now. We have a 13.12 class in there now and they are opening this one up to 13.3. I don't know where they came up with that number but that is what they are recommending. They are saying that too tight of a hot, might be causing a P3 pressure increase therefore the bleed valve is closing too soon.

Then we will probably have to change our PT too, to get the engine to stay within parameters so the fun will continue, but at least maybe we can stop the stalling. I will keep you posted.

Thanks for the help,

YBP

PS: You say you have been around PT6's since 1973? Are you still working on them or do you have an office job now? Our experienced guy has been in the office for too long and can't remember all the tricks (Sorry NG).

[ybp]

Wrench,
I can't remember how close the class was on the PT stator but it flew fine for 15 hrs. It is possible that no one firewalled the throttles quickly in that time but there is no way to avoid the stalling while in reverse.

I am relatively new at this, but from now on I will check acceleration speed after doing engine work. It kinda sucks but I guess we live and learn. (Especially if they won't send me on an engine course)

YBP

[GA MX Trainer Dude]

YBP

It sounds like you are eliminating all of the things that could cause it externally - so it looks like we will have to wait and see what happens with the new PT vane.

I have an "office" job but have about a dozen engines 10 feet from my door

What keeps me current on the engines is the type courses I do that have the engine element in them. It really is true that if you want to really learn a subject the best thing is to teach it. In addition I keep very current with the problems through the interaction of the classroom participants who are working on them every day. I always learn something from every course that I instruct - and I write the info down in my training notes. I don't do much hands on stuff but do quite a lot of on the floor consulting when we have problems and a lot of rigging training as it such a mystery for some of the guys.

None of this is rocket science - you can do the same thing - keep a book on the engine and all the things you hear about it and you will be surprised at how much you will pick up. I worked on the engine a tiny bit when I went to school - and it was 4 years before I had my first course on one - which wasn't all that good as the students knew more than the instructor about the engine - it was not a P&W course. I think I did my first P&W course about 10 years into my career - so you are not alone in not getting the training. Company I work for now has invested a lot of time and money into developing their own courses - Flight Safety gets cranky if you use their copyrite manuals.

Keep pushing for the training - although some companies just won't do it.


Let us know how you make out with the engine - one of the nice things about threads like this is that there are a lot more people out there than you think that are reading this stuff.

Mx

Edited for typo

[ybp]

Well, the hot section has been sitting in Memphis at Fedex for two days now , first due to weather and then they noticed that it was labeled "Hot Section" and valued at $20,000 or something like that and wouldn't clear it through customs. ("Can't let those stolen sections go across the border")

It will be next week some time before we get the engine put back together again....much to the boss' dismay!!!

[ybp]

Well the Gypsy hot section finally showed up this morning so with great anticipation, we installed it in the engine........

The hot section we removed was 13.12 class, I think, and Wood Group scrapulated from our DPS numbers and PS class that we needed a 13.30 class hot section installed. So after two weeks of consoling the boss, the infamous hot section, now classed at 13.31, was installed and bets were wagered.....

Under a beautiful blue sky and light, warm wind, we poured the pickles to the old girl and she popped and farted just like before.....AND I WON THE BET!!

We coke bottled the engine and found that the bleed valve is now closing at 86%. Once again, we sent WG the numbers and we are waiting for there advise....

.....anyone have a spare -27 kicking around

[smartwrench]

This may be a long shot, but, is it possible that you might have gotten some of the lube that is put on the splines on the snap diameter of the CT wheel? This happened to me a few years back and as soon as I cleaned up the snap diameter and the recess on the shaft, the engine stopped popping.

[MCRS]

Ybp have you heard anything back on the problem yet? be interested to know what they have to say.
Had a similiar problem years ago. found the rubber in the bleed vlve had a hair split in it that when the valve was in just the right spot it would stall the compressor, other times it worked fine. That was a pain in the arse to find too!
That was back when Pratt didn't have a problem with you disassembling the Bleed valve, in fact they had a spares kit for fixing them too! Did it right out on the Apron in 40 degree heat while looking out over the ocean............ Boy, it's nice working in Africa!.......

[ybp]

We are waiting for a new engine, and then we will send this one out for repair. Pratt figures it is something in the compressor, or maybe a combination of diffusers. I will keep you posted when I find out more.

I didn't know Pratt had a problem with you disassembling bleed valves now... We will just say that this one has already been inspected.

[ybp]

WE WIN

The airplane was supposed to be leaving on contract soon with OH engines so we just changed the whole damn engine....and surprise surprise, it fixed the compressor stall!!!

Now we will have to wait for the OH report to find out what was causing the stalls in the first place.

I will keep you posted, when I know more,

YBP

[GA MX Trainer Dude]

ybp

Did you ever get the report back as to the cause???


Regards,

MX

[sprayrail]

Your bleed valve was closing too early! It should close at about 91% any earlier and you'll get a compressor stall.

[bilko]

Hey zues did you read any of the postings.

Non no no of course he never checked his bleed valve.

[tiggermoth]

Sprayrail, yup we checked the bleed valves, in fact we tried 3 different ones, they all did the same thing.

Anyways, we haven't gotten back a report yet because we still have the Gas Gen sitting in the box in our hanger. Not sure why we haven't sent it out yet, but thats why I am on the shop floor and not making the big bucks in the office

[ybp]

The "Banger" as it is known, is out for tear-down. It should be in the test cell any day now. I will let you know when we find something out.

YBP