Compressor stall in a 206L

[RVDT]

Couple of points here -

The procedure to turn on the anti-ice is 2 fold.

1/ You may have inlet icing in the first instance.
2/ Relieve the pressure downstream by using the anti-ice as a secondary manual bleed valve.

Pay attention as to when to use ant-ice. Less than 5 degrees C and "visible moisture". The key being visible moisture. i.e. if you can see a cloud is a start point. If it is less than 5 degrees having the anti-ice on is no issue for performance.

[bront]

Originally Posted by Ascend Charlie

Geez, Bront, why then is a bleed valve or a bleed band fitted to turbine engines? To remove the excess air that can build pressure at the back of the compressor, and cause the flow to break down and stall the compressor blades.

A C20 on start-up has the bleed valve open to allow acceleration of the compressor without getting too much air into the combustor. It stays open, closing gradually, until around 92% N1 when the engine can take all the air OK. It opens again on sudden accelerations. To let out the back pressure and allow the wheels to spin up faster.

Try leaving the wedge in the bleed valve when you go to start next time, hear the stalls start very early on. Too much back pressure.

Anti-ice is still turned on in hot countries too! Because it removes some of the excess air and pressure, not because there is ice in the intakes.

We are not talking about operating at low N1 speeds, we are talking about operating at high N1 speeds where the bleed valve will be closed so what the engine does during starting is irrelevant.

[SASless]

Bront,

Are these numbers familiar to you re N-1/Ng Compressor Speeds for the C30 equipped Bell 206L series?

1-18. GAS PRODUCER RPM


]Continuous operation

63 to 105%

Maximum 105%

Maximum transient 106% (Do not exceed 10
seconds above
105%)

If as we have been told the Compressor Bleed Valve works up to about 92% at which time it is closed.....can we assume it is open sometimes during Normal Operations?

So, in fact, there is a bit more to this Compressor Bleed Valve operation thing than you post would suggest.

Am I wrong in thinking that?


If perhaps the Bleed Valve is malfunctioning or incorrectly adjusted....and remains closed at start that would. present a problem. If it was mis-rigged and was opening and closing incorrectly that too could cause a problem re possible Compressor stalls too...right?

[LRP]

Originally Posted by SASless

Bront,

Are these numbers familiar to you re N-1/Ng Compressor Speeds for the C30 equipped Bell 206L series?




If as we have been told the Compressor Bleed Valve works up to about 92% at which time it is closed.....can we assume it is open sometimes during Normal Operations?

So, in fact, there is a bit more to this Compressor Bleed Valve operation thing than you post would suggest.

Am I wrong in thinking that?


If perhaps the Bleed Valve is malfunctioning or incorrectly adjusted....and remains closed at start that would. present a problem. If it was mis-rigged and was opening and closing incorrectly that too could cause a problem re possible Compressor stalls too...right?

The Bleed Valve on the 250 series engines has no adjustment/rigging that I know of, it either works properly or it doesn't and you replace it. It doesn't cycle as a function of N1/Ng, there is no direct link from RPM to cycle. The values are representative of the Pc (compressor discharge pressure) and are variable based on ambient temperature.

[SASless]

It has been a very long time since I was punished by having to fly a Jet Ranger....but my point about the valve operation is correct in that it is can be open in varying amounts during normal engine operation....correct?

It starts fully open....and is fully closed by about 92% I am thinking.

Rapid acceleration of the engine will also cause it to open unless I am mistaken.

I have far more experience with PT-6's and Lycoming Engines than I do Allisons.

[Ascend Charlie]

So, Bront, in a hot country on a hot day, when there is no ice at all, no restriction to the intake, why is the pressure low enough, in your mind, to cause a stall? And why does low pressure cause a stall?

The blockage is at the back of the compressor, where too much air is jammed up and trying to get through to the combustion chamber, the flow is slowed and the angle of attack gets too high, and flow breaks down over the surface of a compressor wheel. The breakdown can spread forward over the wheels, and in the worst case the airflow blasts back out the front, in a surge. This can be very serious, bending compressor wheels and maybe causing the engine to fail.

Open a bleed valve for a heater or some anti-ice, anything to take away some of the excess high-pressure air.

If, as you surmise, there was TOO LITTLE pressure at the compressor causing the problem, opening a bleed valve would make the problem worse, n'est-ce pas?

Sassy, the old T-53 bleed bands would open and close according to acceleration schedules, and caused problems if it stayed open (from start) making it impossible to get full power, or stayed closed, causing slow acceleration and popping.

[bront]

As far as I know there are only 3 way to get a compressor stall and they are; damaged engine, partially blocked inlet or disrupted airflow to the inlet. The most likely reason for a blocked inlet would be icing, however it could be a heap of grass or snow across the front of the particle separator or filter or a very dirty filter.

If the bleed valve is functioning incorrectly that falls under a damaged engine. LRP is correct on how it functions.

And just to correct the OP the heater also draws it's air from the compressor and not the turbine.

All I'm really trying to get across is that the correct action is to lower the collective to reduce the power demand and that rolling the throttle off is not the correct procedure and actually dangerous IMO.

[bront]

Originally Posted by Ascend Charlie

So, Bront, in a hot country on a hot day, when there is no ice at all, no restriction to the intake, why is the pressure low enough, in your mind, to cause a stall? And why does low pressure cause a stall?

.

In the situation you describe I would not expect a stall unless the engine was damaged so I'm not sure what you are getting at.

[bront]

[QUOTE=Ascend Charlie;10364609]

If, as you surmise, there was TOO LITTLE pressure at the compressor causing the problem, opening a bleed valve would make the problem worse, n'est-ce pas?

QUOTE]

You are quite correct but if you have icing the only way to get rid of it is to open the anti icing and accept the reduction in power until the ice has cleared.

[John Eacott]

Originally Posted by Weads

I have a couple of questions about the emergency procedures for compressor stall in the 206L. The POH says that if you have the indications of a stall or a stall itself, then you are to turn on the engine anti icing and (if installed) the heater. I’m having a hard time understanding this because antiicing takes hot P3 air and uses it in the compressor support housing thus increasing the temp of the incoming air through the turbine. Your cabin heat takes air away from the turbine and redirects it as cabin heat? Both of those seem counter intuitive to helping cool an engine. Any insight would be great thanks guys!

Weads, maybe you could specify what type of 206L you are operating and what engine? The 206L1 with the C28 was well known for poor performance and also for compressor stalls, I operated one that was a nightmare for stalls which always seemed to happen at just the wrong time, until the problem was fixed. Dropping the lever is the immediate action, but in some cases it just can’t be achieved fully without putting the machine and occupants at risk.

I’ve never heard of rolling back the throttle as an action and would be quite reluctant to do so.

[LRP]

Originally Posted by SASless

It has been a very long time since I was punished by having to fly a Jet Ranger....but my point about the valve operation is correct in that it is can be open in varying amounts during normal engine operation....correct?

It starts fully open....and is fully closed by about 92% I am thinking.

Rapid acceleration of the engine will also cause it to open unless I am mistaken.

I have far more experience with PT-6's and Lycoming Engines than I do Allisons.

Here is a screen shot of the valve sequencing for a C47 engine (Bell Training Handbook). I believe the C30 is the same or very close. I think the 92% value is related to the inducer bleed port (suck/blow).

[RVDT]

RR250 Series compressor stall.

1/ ICE?
2/ Dirty Compressor? (Power check)
3/ Damaged compressor? (Power check)
4/ Bleed valve jet blocked or dirty - valve closes too soon?
5/ FCU acceleration schedule too high?
6/ Duplex Fuel nozzle?
7/ #1 turbine nozzle area / flow dimension? (Is your power check too good to believe)

Something ain't right. Stop and fix it.

[Saint Jack]

A correction, if I may, to LRP's Post #24, the compressor bleed valve closing point is adjusted at a repair/overhaul facility by fitting the appropriate sized nozzle/jet to control the rate of air escaping from the upper side of the diaphragm. This, in turn, determines the point (N1) at which the pressure build-up closes the bleed valve. The nozzle/jet size is a calibrated setting and therefore NOT field-adjustable. There is no "rigging" associated or required with this compressor bleed valve. RVDT's Post #32 mentions the nozzle/jet in Item 4/. But more importantly, take note of his statement, "Something ain't right. Stop and fix it." This is absolutely correct - and please let us all know what you found during the troubleshooting and rectification process.