Sikorsky S-76: Ask Nick Lappos
DCA (Droop Compensator Amplifier) check from memory:
Set Nr at 100%. Use trims to get a 2% N2 split. Quickly raise collective about an inch and the N2's should rejoin.
Set Nr at 100%. Use trims to get a 2% N2 split. Quickly raise collective about an inch and the N2's should rejoin.
Join Date: Sep 2004
Location: Back of Beyond
Posts: 80
Likes: 0
Received 0 Likes
on
0 Posts
This problem brings back a few memories of long days on the old A model.
We encountered this problem when we went to the enhanced -C30s, it happened during the DCA check mostly, but sometimes on a normal accceleration at start up, one engine would start popping.
Sometimes an FCU change would cure it, other times we would change both the FCU and the Gov, on one occasion the #2 was popping, as we had used up our spares, we swapped the FCU and Gov between 1 and 2, it cured # 2 and # 1 was okay.
We never really got an answer from Allison why this was happening, what we put it down was that on the contract we had at the time the C30 had to be high spec , to get the high spec, the O/H shop were working to very close tolerances plus the fact that the compressor was running a bit faster, the fuel scheduling was very critical.
Although we were able to cure it most of the time, on some occasions if it was doing it during the DCA check, we had to live with it if it wasn't too violent.
What are the specs on your engines?
Good Luck.
Tynecastle.
We encountered this problem when we went to the enhanced -C30s, it happened during the DCA check mostly, but sometimes on a normal accceleration at start up, one engine would start popping.
Sometimes an FCU change would cure it, other times we would change both the FCU and the Gov, on one occasion the #2 was popping, as we had used up our spares, we swapped the FCU and Gov between 1 and 2, it cured # 2 and # 1 was okay.
We never really got an answer from Allison why this was happening, what we put it down was that on the contract we had at the time the C30 had to be high spec , to get the high spec, the O/H shop were working to very close tolerances plus the fact that the compressor was running a bit faster, the fuel scheduling was very critical.
Although we were able to cure it most of the time, on some occasions if it was doing it during the DCA check, we had to live with it if it wasn't too violent.
What are the specs on your engines?
Good Luck.
Tynecastle.
Last edited by Tynecastle; 25th Oct 2004 at 13:07.
Join Date: Jan 2003
Location: TI
Posts: 217
Likes: 0
Received 0 Likes
on
0 Posts
Allison C30 Surge
Could be the things as described already but also have a look at the engine against a power check chart. If it is making a lot of power and is really strong could be a turbine nozzle problem as Tynecastle says i.e. too tight in area hence the cough. Have seen this in an AS350 Soloy conversion - engine had all the power in the world but would snort off ground idle, nozzles changed, no more snorting and no more power as well - long faces all round. Only one of many things of course. Your Allison shop should know what's wrong with it and of course being a twin that's what the other engine is really for, a supply of alternative components.
Aside from that you say you have had this problem for 18 months. What happens when the other one stops and you have to rely on this engine??????
Aside from that you say you have had this problem for 18 months. What happens when the other one stops and you have to rely on this engine??????
We have seen it a few times, and assuming there is no other obvious problem, most often it is because the engine shop can not set up the C30 for an S76 installation. It is mostly in the compressor, or rather with the relationship between the compressor and turbine. Did it come after engine change or module change?
It seams to be a difficult installation to get right because of the inlet design, why I do not know. We actually had to change engine shop until we found one with S76 "experience". I do not think Rolls-Royce will admit there is a diffrence, but in the end result, there is a diffrence.
What about the stall check? it must fail that one big time if it pops during DCA check.
Like said above, this is serious, if you loose the good engine in flight and the bad one have to spool up quickly there could be a real problem.
CB
It seams to be a difficult installation to get right because of the inlet design, why I do not know. We actually had to change engine shop until we found one with S76 "experience". I do not think Rolls-Royce will admit there is a diffrence, but in the end result, there is a diffrence.
What about the stall check? it must fail that one big time if it pops during DCA check.
Like said above, this is serious, if you loose the good engine in flight and the bad one have to spool up quickly there could be a real problem.
CB
Join Date: Oct 2002
Location: 1 Dunghill Mansions, Putney
Posts: 1,797
Likes: 0
Received 1 Like
on
1 Post
The 250 technical support team in Indy should be able to help ([email protected] / (317) 230-2720).
I/C
I/C
Join Date: Feb 2001
Location: Canada
Posts: 579
Likes: 0
Received 0 Likes
on
0 Posts
On the 250 C-30S engine it is not uncommon. The engine manufacturer won't acknowledge the problem.
It was a standing joke where I worked. Every time there was an engine change we would get compressor stall/popping/surge, when conducting the engine stall check.
The engineers would call the tech rep.
They would suggest: And the drill went something like this:
Try it with the AI and heater on, done,
Change the Bleedvalve, done
change the FCU, done
Then they would send the tech rep and tweak the N1 nozzle and then it would be fine; until the temperature dropped.
The engine guys told me that the test cell does not do a very good job of replecating the engine intakes of the 76. Therefore it would test fine in the test cell but fail the stall check in the aircraft.
(the stall check [from memory] was to advance the throttle from idle to full throttle and then back within a couple of seconds, the check would also test the blade dampers during acceleration and deceleration).
Once the engine spooled up there was never any problem with power.
It was a standing joke where I worked. Every time there was an engine change we would get compressor stall/popping/surge, when conducting the engine stall check.
The engineers would call the tech rep.
They would suggest: And the drill went something like this:
Try it with the AI and heater on, done,
Change the Bleedvalve, done
change the FCU, done
Then they would send the tech rep and tweak the N1 nozzle and then it would be fine; until the temperature dropped.
The engine guys told me that the test cell does not do a very good job of replecating the engine intakes of the 76. Therefore it would test fine in the test cell but fail the stall check in the aircraft.
(the stall check [from memory] was to advance the throttle from idle to full throttle and then back within a couple of seconds, the check would also test the blade dampers during acceleration and deceleration).
Once the engine spooled up there was never any problem with power.
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Stalls should not be experienced on throttle slams, nor on normal flight maneuvers. They have nothing to do with the inlet (in spite of the tech rep's assertion otherwise!), and if they are common to your operation, they are because your fuel controls are set up too hot. IHL's comment on having to reset them with big seasonal swings is quite correct, and also with big altitude changes.
The garden variety stall (on a clean, unworn engine) is due to the fuel control having too much accel gain, due to the metering valve position. After cranking the puppy in as far as it will go, then some shimming might be necessary.
I would be very surprised to find that someone could turn the heater on and or the accel bleed valve and get a stall (it takes a calendar to time the accel with the heater on!) Is that what you wrote, IHL?
Contact me if you have persistent stalls when you do the checks, I will try to help iron it out.
The garden variety stall (on a clean, unworn engine) is due to the fuel control having too much accel gain, due to the metering valve position. After cranking the puppy in as far as it will go, then some shimming might be necessary.
I would be very surprised to find that someone could turn the heater on and or the accel bleed valve and get a stall (it takes a calendar to time the accel with the heater on!) Is that what you wrote, IHL?
Contact me if you have persistent stalls when you do the checks, I will try to help iron it out.
Re the intake doors/airflow on the S76. ISTR a few (many?) years back an incident occured offshore, the outcome of which was the S76A was cleared for a ferry flight back to the beach with the intake door(s) removed.
Apparently a substantial increase in engine performance was noticed, borne out by a much improved power assurance figure. Much grumbling in the crewroom for the mod. to be made permanent
Apparently a substantial increase in engine performance was noticed, borne out by a much improved power assurance figure. Much grumbling in the crewroom for the mod. to be made permanent
Join Date: Nov 2003
Location: Jakarta, Indonesia
Posts: 27
Likes: 0
Received 0 Likes
on
0 Posts
My thanks to everyone who has responded to this subject. Let me just set the record straight on one point. Over the last 18 months or so we have had about 5 or 6 engines surge. In every case we carried out the standard checks/replacements and if we could not fix it we pulled the engine. We did not continue operation with a surging engine.
We have been working with our AMC and they in turn with RR Indianapolis to fix this problem. As has already been suggested in earlier posts the most likely engine to surge is one that demonstrates a very high power assurance margin. I can confirm that has been the case with our engines.
RR are also of the opinion that over the years the various efforts to extract more power from this engine has resulted in a deterioration of the stall/surge margin. The "fix" (at least supposedly for our engines), is to adjust the diffuser vanes and change the airflow through them. This will reduce the power but also eliminate the surging. At the same time RR have developed some new test cell procedures which are supposed to identify an engine which is likely to surge.
It seemed that we were finally getting to grips with this but just a couple of days ago I fitted an engine which came out of another helicopter and had been runing perfectly. The engine was run on a test cell at an AMC to confirm power and also went through the surge testing with no problem. The first run we did with it in the helicopter we had a surge. Right now we are going over the same old ground of component replacement. Again this was a powerful engine, +14Q at 780 T5 and N1 of 102.7%
It really throws me why this engine should now decide to surge on this airframe when it was okay on the previous and in the cell. I will keep you informed of the outcome.
By the way, we have operated these engines for about 16 years and all of a sudden we get this big problem right out of the blue.
We have been working with our AMC and they in turn with RR Indianapolis to fix this problem. As has already been suggested in earlier posts the most likely engine to surge is one that demonstrates a very high power assurance margin. I can confirm that has been the case with our engines.
RR are also of the opinion that over the years the various efforts to extract more power from this engine has resulted in a deterioration of the stall/surge margin. The "fix" (at least supposedly for our engines), is to adjust the diffuser vanes and change the airflow through them. This will reduce the power but also eliminate the surging. At the same time RR have developed some new test cell procedures which are supposed to identify an engine which is likely to surge.
It seemed that we were finally getting to grips with this but just a couple of days ago I fitted an engine which came out of another helicopter and had been runing perfectly. The engine was run on a test cell at an AMC to confirm power and also went through the surge testing with no problem. The first run we did with it in the helicopter we had a surge. Right now we are going over the same old ground of component replacement. Again this was a powerful engine, +14Q at 780 T5 and N1 of 102.7%
It really throws me why this engine should now decide to surge on this airframe when it was okay on the previous and in the cell. I will keep you informed of the outcome.
By the way, we have operated these engines for about 16 years and all of a sudden we get this big problem right out of the blue.
Join Date: Dec 2001
Location: Gold Coast, Queensland
Posts: 943
Likes: 0
Received 0 Likes
on
0 Posts
Gcmoir
From your description, these surges do not occur during an engine test, such as a DCA test, but in normal running.
It might be too obvious but if these are old 76s, the over speed electrical system should be disconnected. I had several surges when this system played up and had to grap the CB in a hurry.
Whoops!!
I see you did get it in the DCA check, not in flight!
From your description, these surges do not occur during an engine test, such as a DCA test, but in normal running.
It might be too obvious but if these are old 76s, the over speed electrical system should be disconnected. I had several surges when this system played up and had to grap the CB in a hurry.
Whoops!!
I see you did get it in the DCA check, not in flight!
Last edited by Nigel Osborn; 26th Oct 2004 at 05:54.
Join Date: Nov 2003
Location: Jakarta, Indonesia
Posts: 27
Likes: 0
Received 0 Likes
on
0 Posts
Nigel,
I think with one exception where we could get a surge when making an N2 beep change, every surge event has been while performing the DCA check. We could sit there all day pumping the power levers back and forth as quickly, (or slowly), as you wanted and there would not be even the hint of a cough. It is purely related to N2 demand.
I think with one exception where we could get a surge when making an N2 beep change, every surge event has been while performing the DCA check. We could sit there all day pumping the power levers back and forth as quickly, (or slowly), as you wanted and there would not be even the hint of a cough. It is purely related to N2 demand.
The Stall Damper Check is the only official check for the stall.
I have encountered this as well with compressor sections that are machined to such high tolerances and therefore "overpowering" the rest of the turbine.
We had all the same issues and just ended up replacing the engines. The replaced engines were all +12 as well.
If you think it is location, why don't you swap engines left to right in the airframe and see if it duplicates.
I have encountered this as well with compressor sections that are machined to such high tolerances and therefore "overpowering" the rest of the turbine.
We had all the same issues and just ended up replacing the engines. The replaced engines were all +12 as well.
If you think it is location, why don't you swap engines left to right in the airframe and see if it duplicates.
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Actually, some of that might be the standard to which the vibration system is maintained, and some to the nature of the aircraft.
You might want to have the mechanics toss on the vibe gear and take some measurements to compare to its history. Doesn't hurt to ask.
Generally, the job it does and the weight it operates at don't affect the 4P (20Hz) very much. I'm going to bet it has more 1P (5Hz) than it should. You can almost count 1P and it bobbles you more, 4P is more like a rumble on the floor and in the seat.
You might want to have the mechanics toss on the vibe gear and take some measurements to compare to its history. Doesn't hurt to ask.
Generally, the job it does and the weight it operates at don't affect the 4P (20Hz) very much. I'm going to bet it has more 1P (5Hz) than it should. You can almost count 1P and it bobbles you more, 4P is more like a rumble on the floor and in the seat.
After my reply above, we actually had the experience again, it's been awhile since the last one.
No problem with current installation, except a bit low on power (+4%) with a loaner turbine.
Turbine change after OH of our own- fails DCA check and stall check.
Swapping bleed valves - DCA ok, stall fail.
Swapping FCU - DCA fail, stall fail.
Engineers
Installing loaner compressor - DCA ok, stall ok
Since the turbine came from OH it was probably "tight" in the nozzle, and since the engine was a bit low on power the compressor was maybe not the best. New compressor cured the problem. The new compressor and OH turbine is +12% in pwr margin.
The problem engine was #2, during all swapping #1 was checked as well every time - no problem!
Just wanted to share this with you.
CB
No problem with current installation, except a bit low on power (+4%) with a loaner turbine.
Turbine change after OH of our own- fails DCA check and stall check.
Swapping bleed valves - DCA ok, stall fail.
Swapping FCU - DCA fail, stall fail.
Engineers
Installing loaner compressor - DCA ok, stall ok
Since the turbine came from OH it was probably "tight" in the nozzle, and since the engine was a bit low on power the compressor was maybe not the best. New compressor cured the problem. The new compressor and OH turbine is +12% in pwr margin.
The problem engine was #2, during all swapping #1 was checked as well every time - no problem!
Just wanted to share this with you.
CB
Our 76B in VIP rigout is very smooth - the engineer works carefully to take it all out.
Sometimes, though, on a cold morning it is a bit like a gravel road between 120-140 kt until the dampers and the rest of the head warms up.
Soundproofing is pretty good, and the rear double-glazed windows with tinting stop the gawking spectators while taxying around or at the FBO.
Sometimes, though, on a cold morning it is a bit like a gravel road between 120-140 kt until the dampers and the rest of the head warms up.
Soundproofing is pretty good, and the rear double-glazed windows with tinting stop the gawking spectators while taxying around or at the FBO.
Leave it to Nick to talk about a rumble in the floor....Gunnies never change!
76 type rating
