Compressor stall in a 206L
 

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!

The compressor stalls because of back pressure in the compressor, so any way of bleeding off that pressure a bit by taking that pressurised air away will help.

How about something really novel....and reduce Collective or perhaps roll the Throttle back but stay in the Green Arc for Rotor RPM if in flight?

FYI

3-3-E. ENGINE COMPRESSOR STALL/
SURGE
• INDICATIONS:
1. Engine pops.
2. High or erratic TOT.
3. Decreasing or erratic N1 or N2.
4. TORQUE oscillations.
• PROCEDURE:
1. Collective — Reduce power,
maintain slow cruise flight.
2. TOT AND N1 — Check for normal
indications.
3. ENGINE ANTI-ICING switch — ON.
4. PARTICLE SEP PRG switch (if
installed) — ON.
5. HEATER switch (if installed) — ON.
NOTE
Severity of compressor stalls/surges
will dictate if engine should be shut
down and treated as an engine
failure. Violent stalls/surges can
cause damage to engine and drive
system components, and must be
handled as an emergency condition.
Stalls/surges of a less severe nature
(one or two low intensity pops) may
permit continued operation of engine
at a reduced power level, avoiding
condition that resulted in
compressor stall/surge.
If pilot elects to continue flight:
6. Collective — Increase slowly to
achieve desired power level.
7. TOT and N1 — Monitor for normal
response.
8. Land as soon as practical.
If pilot elects to shut down engine:
9. Enter autorotation.
10. Throttle — Closed.
11. FUEL VALVE switch — OFF.
12. Accomplish autorotative descent and
landing. (Refer to AUTOROTATIVE
DESCENT PROCEDURE, paragraph
3-3-A-3).

LRP,

The Original Poster noted what the POH had to say and asked for some additional input.

If the Engine gives three or four low intensity pops....what do you recommend doing as you posted an excerpt from some unknown source as you did not specify from where it comes from.

First action is to reduce power demand by lowering collective but rolling off throttle has to be one of the craziest things I've ever heard! There is only 1 justification that I can think of for playing with the throttle in flight and that is dealing with tail rotor emergencies just before touch down. After reducing power demand then turn on engine anti icing in case engine inlet icing is starving the compressor of air resulting in a compressor stall.

Bront,

What happens when you reduce Engine RPM to Minimum Power On Nr momentarily and roll the throttle back up to full throttle as set before the Compressor Stall occurred?

If you lower the Collective first and attempt to cure the problem that way and it does not work.....the next thing you are told to do is turn on the bleed air heater and anti-ice.

What does that action do for you?

Can you not get much the same reaction by slowing the engine RPM....and reducing the flow of air to the compressor?

I had an occurence once with a passenger flight.
Reduced collective and it stopped, until I raised it again.
Nursed the bird back to base with the occasional surge again - looking for autorotative reachable spots all the way... calming down the passengers ;-)
When they striped the engine they found, that parts from the compressor housing, there where the compressorwheels spin, went through the engine.
After seeing the amount, which went through, an outside landing would have been the better decison....
If you don´t have knowledge of i.e. salty environment causing an engine surge, better treat it as an engine failure on its way....

A surge on a healthy engine would be unusual these days, eliminating external or environmental factors it is safe to presume that the primus may be about to shuffle off its mortal coil.

Who on earth has ever experienced a compressor stall in an L-model? Nobody I've ever heard of.

A compressor stall is caused by the compressor not being able to supply enough air at a high enough pressure to the combustion chamber. The pressure in the combustion chamber then pushes back into the compressor causing it to stall. If nothing changes this will cycle continuously until something fails. This lack of air can be from a number of reasons, partially blocked intake, damaged compressor or air leaking between the compressor and combustion chamber.

Rolling off the throttle and then rolling it on again will do the following IMO. Firstly as the engine slows it will probably stop the stall briefly but once stabilized at the lower RPM the engine will still need to produce about the same amount of power if you have not lowered the collective and I would expect it to stall again. Then if you open the throttle again the engine will try to speed up to increase the rotor RPM and this will demand the most power yet and it will stall again.

I don't know for sure if slowing the RPM will do what I believe because I have never experienced it but I know for a fact that accelerating the rotor will cause it to stall as I have experienced this myself. In my case my L3 had a corroded compressor housing which caused the magnesium to have large raised areas like blisters that were rubbing against the impellor. Whilst taking off in the dark and trying to move the movable landing light forward with my left thumb, I accidently put my finger on the rotor RPM beep trim button instead and was inadvertently trying to accelerate the rotor RPM to a higher value. Each time I pushed the button forward I was rewarded with a load bang and an extremely violent yaw and the bystanders witnessed a good 6 feet of flame out the tailpipe. Very impressive apparently!

If you lower the collective and that doesn't fix the problem, then you have a very serious mechanical fault and the only option is to enter autorotation and shut down the engine. If lowering the collective stops the stall, then turning on the engine anti ice will hopefully remove any ice (if that is the problem) that has built up in the inlet and causing the lack of airflow. Turning on the particle separator, if you have one, will hopefully remove whatever is blocking it. I have no idea what turning on the heater would do except rob the compressor of more air so I do not understand why that is part of the procedure. If anything I would have thought it should say 'Turn it off.'

The stalls I experienced were extremely violent and I'm convinced that if they had been allowed to continue for any length of time would have destroyed the drive train in seconds.

Your idea of rolling off throttle is extremely dangerous IMO. For starters it is not part of any procedure in the flight manual and if memory serves me correctly is in fact prohibited, except for entering autorotation and for tail rotor emergencies. And secondly the rotor RPM and every other gauge will be all over the place and it would be very hard to control, if not impossible. There is only one option initially and that is the same for pretty much every other emergency in a single engine helicopter IMO and that is to lower the collective and only then try to work out what is going on.

This is completely incorrect. It stalls because of a lack of pressure, not because of too much pressure. Turning on anti ice is to remove any ice that may be blocking the inlet causing low air flow and low pressure.

It's too simplistic to talk about too much pressure or too little pressure - it's related to pressure ratio (exit/inlet) and the mass flow. Turning on the heater is opening a bleed - as has been stated - which then modifies the pressure ratio and increases the surge margin. Some aircraft have a switch on the collective that is used to force open the normal bleed valve, even though the Ng is higher that the value at which it normally closes, if it is anticipated that the aircraft will enter conditions conducive to disrupted airflow - typically turbulent airflow on landing or flying through hot gas emissions. Ice is definitely nothing to do with it although it's true that ice can cause surge by disrupting the local airflow.

It's a copy/paste from the L-3/L-4 RFM. Same procedure in the 407 (except the reference to TOT, N1, N2 changes to MGT, Ng, Np). Been the same procedure for years. First step in all is collective reduce.

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.

One would think the RFM would. have OAT temperature ranges if there was concerns over Icing alone being the cause of the Compressor stall.

It does not....thus it must be intended to be used at all Tems as part of the Emergency Procedure..

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.

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.

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.

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.

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.

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.

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.

[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.

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.

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).



https://cimg2.ibsrv.net/gimg/pprune....0fb0353c5f.jpg

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.

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.