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Old 18th Jan 2019, 13:37
  #14 (permalink)  
Join Date: Apr 2008
Location: cape town
Posts: 42
Originally Posted by SASless View Post

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?
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.
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