H125 As350 HYD Cut-Off
Thread Starter
H125 As350 HYD Cut-Off
I need some clarification on the Hydraulic Cut off switch on B3 models.
If you read the Flight Manual of B3 B3+ & B3e and the report in the link bellow they all state that when switching the HYD cut-off switch in the off position the pedals should retain hydraulic assistance.
quoting from the RFM section 7
I flew many B3 from old ones to brand new ones and, as far as I can remember, every time I tested the HYD cut-off switch on the collective the pedals lose hydraulic assistance
Quote from the report
During the extensive system testing, it was found that the operation of the…“HYD” cut-off switch activation, resulted in the yaw load compensator solenoid valve energisation [too], which was not expected by design (loss of the complete hydraulic assistance on the yaw control).
The related wiring configuration was checked and a discrepancy was detected
Surely all the B3s I've flown can't all have been wrongly wired??
What am I missing here?
Fire-Fighting AS350 Hydraulics Accident: Dormant Miswiring
https://aerossurance.com/helicopters...ics-miswiring/
If you read the Flight Manual of B3 B3+ & B3e and the report in the link bellow they all state that when switching the HYD cut-off switch in the off position the pedals should retain hydraulic assistance.
quoting from the RFM section 7
- A single-body yaw servo
- A load compensating system to reduce, in the event of a hydraulic pressure loss, the yaw pedal feedback loads for an indefinite period.
- The load compensator pressure can only be dumped by selecting the accumulator test switch to TEST position (down)
I flew many B3 from old ones to brand new ones and, as far as I can remember, every time I tested the HYD cut-off switch on the collective the pedals lose hydraulic assistance
Quote from the report
During the extensive system testing, it was found that the operation of the…“HYD” cut-off switch activation, resulted in the yaw load compensator solenoid valve energisation [too], which was not expected by design (loss of the complete hydraulic assistance on the yaw control).
The related wiring configuration was checked and a discrepancy was detected
Surely all the B3s I've flown can't all have been wrongly wired??
What am I missing here?
Fire-Fighting AS350 Hydraulics Accident: Dormant Miswiring
https://aerossurance.com/helicopters...ics-miswiring/
Last edited by MitchStick; 24th Oct 2023 at 04:31.
Back in the day I undertook my type rating on a B3+, but since then, I've had the privilege of flying SD, SD2, and now FX2 variants. Intriguingly, they all exhibited similar behavior, at least in terms of hydraulics
Collective Switch Does not de-energize tail rotor load compensator. HYD-TEST Switch on the console does.
On my maiden flight in the FX2, a seasoned instructor, post-shutdown, opted to discharge the Tail Rotor Load Accumulator using the collective switch. This was instead of the standard HYD-TEST Switch located on the console. Initially, I chalked this up as an FX2 peculiarity and didn't dwell on it
However, the following morning, I was taken aback to see the tail-rotor at its maximum deflection. My first thought was that perhaps we hadn't adequately adjusted the pedals back and forth, leading to an overnight drift to max Angle Of Attack. That could have been it; but, upon landing back at base, our engineer highlighted the importance of performing the TLA Discharge post-flight via HYD-TEST switch on the console, not on the collective
Drawing attention to this Service Bulletin https://ad.easa.europa.eu/blob/EASA_..._2021-0123-E_1 , it's explicitly stated that the pedals' functionality remains unaffected by the collective hyd switch (well, until the accumulators have nothing more to give). The collective switch Given that the co-pilot I was flying with is an astute professional, your observations, coupled with my experience, hint at the possibility of non-compliant AS350s in operation
It's noteworthy that the bulletin encompasses B, BA, B2, B3, and B3e models, emphasizing that they all should exhibit uniform behavior in this context
Overall to cut to the chase, and if I were to sum this all up there is one important takeway
The main concern identified by the bulletin is that certain AS 350 helicopters had wrongly embodied the modification specified in the ASB, which led to a condition where the solenoid of the tail rotor load compensator was de-energized when the “HYD” cut-off switch was activated. This has the potential to lead to a loss of hydraulic power in TR control, potentially resulting in a loss of control of the helicopter
I know this doesn't answer your question directly, but I hope it helps somewhat
Collective Switch Does not de-energize tail rotor load compensator. HYD-TEST Switch on the console does.
On my maiden flight in the FX2, a seasoned instructor, post-shutdown, opted to discharge the Tail Rotor Load Accumulator using the collective switch. This was instead of the standard HYD-TEST Switch located on the console. Initially, I chalked this up as an FX2 peculiarity and didn't dwell on it
However, the following morning, I was taken aback to see the tail-rotor at its maximum deflection. My first thought was that perhaps we hadn't adequately adjusted the pedals back and forth, leading to an overnight drift to max Angle Of Attack. That could have been it; but, upon landing back at base, our engineer highlighted the importance of performing the TLA Discharge post-flight via HYD-TEST switch on the console, not on the collective
Drawing attention to this Service Bulletin https://ad.easa.europa.eu/blob/EASA_..._2021-0123-E_1 , it's explicitly stated that the pedals' functionality remains unaffected by the collective hyd switch (well, until the accumulators have nothing more to give). The collective switch Given that the co-pilot I was flying with is an astute professional, your observations, coupled with my experience, hint at the possibility of non-compliant AS350s in operation
It's noteworthy that the bulletin encompasses B, BA, B2, B3, and B3e models, emphasizing that they all should exhibit uniform behavior in this context
Overall to cut to the chase, and if I were to sum this all up there is one important takeway
The main concern identified by the bulletin is that certain AS 350 helicopters had wrongly embodied the modification specified in the ASB, which led to a condition where the solenoid of the tail rotor load compensator was de-energized when the “HYD” cut-off switch was activated. This has the potential to lead to a loss of hydraulic power in TR control, potentially resulting in a loss of control of the helicopter
I know this doesn't answer your question directly, but I hope it helps somewhat
Last edited by nikoel; 23rd Oct 2023 at 10:38.
Thread Starter
The wrong modification was my first thought but the fact that I flew aircrafts assembled in 2018 and 2020 in the past which is after the 2004 SB made me discard that option, I also flew an SD2 years ago and they all had the same behaviour.
I don't understand how can all this aircrafts be different from what's written in the RFM
I don't understand how can all this aircrafts be different from what's written in the RFM
I flew many B3 from old ones to brand new ones and, as far as I can remember, every time I tested the HYD cut-off switch on the collective the pedals loose hydraulic assistance/
If you want to feel what is like with no compensation, pretty sure HYD OFF and HYD TEST at the same time will show you what it is like!
A few have tried this in flight with a HYD failure and activated HYD TEST instead of HYD OFF due to confusion and it is not reversible once done. Make sure your seatbelt is tight as you will lift yourself out of the seat trying to push the pedal!
From the OP's linked report:
I think the key difference (to answer OP's question) is that 7daN of pushing force is what 'feels like' pedals without hydraulic assistance to us (compared to normal operation), whereas this is actually still being augmented by the load compensator to bring it to an acceptably controllable level for a 'no-hover landing'. Without the load compensator assisting, the force required to overcome the zero-pitch-return moment is basically impossible to overcome and tail rotor control is effectively lost (58daN in the report). Of course these calculated values from the report are for a 'hover' condition but the magnitude difference between load compensator vs. no load compensator is clearly illustrated.
the pilot “effort” load to be applied on the right pedal to maintain the heading in stationary is about 58 daN. Note: With the yaw load compensator working properly and charged with only 5 bar of nitrogen (recorded pressure), the load was estimated at 7 daN. The near 60 kgf represent eight times the value that the pilot would expect.
I think the key difference (to answer OP's question) is that 7daN of pushing force is what 'feels like' pedals without hydraulic assistance to us (compared to normal operation), whereas this is actually still being augmented by the load compensator to bring it to an acceptably controllable level for a 'no-hover landing'. Without the load compensator assisting, the force required to overcome the zero-pitch-return moment is basically impossible to overcome and tail rotor control is effectively lost (58daN in the report). Of course these calculated values from the report are for a 'hover' condition but the magnitude difference between load compensator vs. no load compensator is clearly illustrated.
• Right accumulator: 14 bar
• Forward accumulator: 4 bar
• Left accumulator: 0 bar
• Yaw load compensator (tail rotor): 5 bar
• Forward accumulator: 4 bar
• Left accumulator: 0 bar
• Yaw load compensator (tail rotor): 5 bar
Thread Starter
They do to an extent - what you would be feeling is the equivalent to a HYD failure - the TR accumulator is connected mechanically and not to the hydraulic actuation and is downstream in the control path to the TR unlike the MR accumulators. It has an over centre bell crank that compensates mechanically. It will be heavier but you can still move the pedals.
If that's the case it is a bit confusing the way is written in the FM as it does not specify that but only says the pedals will retain assistance for an indefinite period of time.
I applaud your desire to learn more about your aircraft. And don’t feel embarrassed, my experience is that few pilots truly understand the AStar hydraulics (including the yaw load compensator). In addition, with the advent of the B3 there are also single and dual hydraulic systems that further muddy the waters. Also, your use of an accident report is a great asset to expand your understanding, however, this forum is the last place. Time spent with a knowledgeable instructor is the best method to learn this information. There is no substitute for a factory course or an enviroment like FlightSafety, etc.
Having said that, you might find these reports useful after obtaining adequate instruction:
Hydraulic cutoff switch: (Canadian report) A05F0025
Accu Test Switch: (Canadian report) A06P0123
Dual hydraulic accident: (US report) CEN15MA290 (includes information of pedal loads)
Having said that, you might find these reports useful after obtaining adequate instruction:
Hydraulic cutoff switch: (Canadian report) A05F0025
Accu Test Switch: (Canadian report) A06P0123
Dual hydraulic accident: (US report) CEN15MA290 (includes information of pedal loads)
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Thread Starter
In a perfect world yes a knowledgable instructor or even better a factory course would be the best. However I don't think I have to tell you how the real world works, I've been flying AS350s for 7 years now (Never flown dual hydraulics), no company ever offered a factory course and I sure was never able to afford it myself (23000$ from Airbus back in 2017), all the instructors I've flown with where line pilots that knew probably same thing I knew or less but flew the aircraft daily.
I am not surprised that the accumulators had low nitrogen pressure. It’s not that unusual, especially in aircraft that lack adequate maintenance and attention. However, at this level of charge/imbalance, a pilot should have detected a noticeable jolt in the controls during the performance of a proper hydraulic check.
However, the yaw load compensator charge is especially alarming at 1/3 of the prescribed level, it would have significantly increased the pedal loads if hydraulics were lost.
Per my (above) referenced accident report, pedal loads change significantly between (1) normal operation, (2) hydraulics off with, and (3) without yaw load compensator (YLC). However, the pedal load is also dependent upon the amount of pedal displacement.
The normal AStar hydraulic system operates at 40 bar. Note in the attached diagram, the pressure relief valve for the YLC actuates at 55 bar. To fully understand the YLC operation you should understand why this is set at a much higher pressure:
Last edited by JimEli; 25th Oct 2023 at 15:57. Reason: corrected grammar
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