Will Main Rotor Actuators be Stucked?

[HoverFort]

I'm a student studing helicopter dynamics and I read it in "UH-60 Flight Control and Hydraulic Systems" that there are mainly two failure types of main rotor actuator types:
The first one is that a breach of the hydraulic chamber would cause pressure loss, which is can be seen in many accident reports.
The second failure type is jamming of the piston within the cylinder such that the servo actuation displacement is fixed. But this is confusing for me because there's very few documented accidents that are caused by such failure.
Have you seen any documented accidents casued by the stuck/jam/locked-in-place of the main rotor servos? Or does it just more oftenly happen for RC helicopters (with electric servos) ?

[gulliBell]

First example: Applies to loss of hydraulic fluid pressure however so caused. If there is no fluid, or the hydraulic pump is kaput, there is no pressure to drive the piston.
Second example: It's not the power piston getting jammed in the cylinder, it's the pilot valve that directs the pressure to the extend or retract side that gets jammed (because of a particle/dirt in the hydraulic fluid). Have a look at the Copterline S76 accident report.

[HoverFort]

Thanks a lot! I've seen the Copterline S-76 accident report and that is exatly what I meant for the second type. There is also another accident report I can find today is the Eurocopter AS350-B2 helicopter accident. In that accident, the servo control input rod got disconnected from the input lever because of bolt failure. But I'm not sure whether this kind of failure would lead to a locked-in-position of the servo.
As a result, maybe the stuck failure is rather rare compared to the first failure type.

[CTR]

The first one is that a breach of the hydraulic chamber would cause pressure loss, which is can be seen in many accident reports.
The second failure type is jamming of the piston within the cylinder such that the servo actuation displacement is fixed. But this is confusing for me because there's very few documented accidents that are caused by such failure.
Have you seen any documented accident ?

The failure modes described appear to be for ballistic damage to a actuator hydraulic cylinder. Under normal operation boost actuators rarely fail structurally. If loss of pressure occurs it is almost always due to leakage in a hydraulic line, or failure of the hydraulic pump.

For helicopters rotor actuators there is no way to mitigate the failure of a jammed piston in a cylinder. For this reason extreme care is taken to ensure this doesn’t happen from normal operation. In fact for civil certified aircraft the probability of this occurring exceeds once in every 1 X E9 flight hours. Which explains why you will not find much evidence of the securing, short of battle damage.

Hydraulic boost actuators like the H-60 use a servovalve to control boost direction. Servo valves can sometimes jam from fluid contamination. When they jam they can result in a hard over command in either direction, or the equivalent of a stuck actuator piston. To appear to be a stuck actuator piston the servo valve needs to stick at the exact center position, which is very improbable. So you will find many accidents that describe the actuator going hard over, but next to none describing a jammed condition.

Because the jamming of a servo valve is a significantly probable event, modern large helicopters like the H-60 incorporate a bypass function in the actuator in this event occurs.

[rotormatic]

There’s also jack stall..


https://www.skybrary.aero/index.php/Servo_Transparency

[[email protected]]

Jackstall is completely different..

Have a look at the AS365 main servos, they have an inner and outer distribution mechanism so the in the event of either jamming, the other retains the function of the servo.

[Reely340]

There’s also jack stall..


https://www.skybrary.aero/index.php/Servo_Transparency
Meep!
"Jack Stall" is the reaction of the hydraulics to the pilot trying to fly outside the specified capabilities of the hydraulics.
The second he stops doing so, "jack stall" goes away.
No Pistons jammed, no foreigen particles to be removed, no damage, the kit is airworthy all the while.

[Rigga]

Its a bit before my time but I believe early Chinook actuators were prone to jamming, the reasons for which I don’t know. However, from the D model onwards, “Dual Actuators” were fitted where if one half jammed the other would continue without loss of control. These actuators were also fitted with “Jam Simulator” buttons with which to periodically test that capability.

[chinook240]

Its a bit before my time but I believe early Chinook actuators were prone to jamming, the reasons for which I don’t know. However, from the D model onwards, “Dual Actuators” were fitted where if one half jammed the other would continue without loss of control. These actuators were also fitted with “Jam Simulator” buttons with which to periodically test that capability.
Rigga, aren’t you thinking of the Lower Control Actuators, as opposed to the Upper Boost Actuators on the Chinook? The BOI for ZA721 suggested a form of jam in the UBA might have contributed to the subsequent crash.
https://webarchive.nationalarchives.gov.uk/20121031034809/http://www.mod.uk/NR/rdonlyres/BED68A4D-44B2-4754-AA76-93C08C86892C/0/maas87_05_chinook_hc1_ZA721_27feb87.pdf

[[email protected]]

There was also a suggestion that a RN Wessex crash (into a sea wall I think) could have been caused by a servo jam or runaway and led to the Wessex fleet adopting the 'If in doubt, Primaries out' terminology for dealing with a control malfunction.

This was based on the fact that the primary hyd system powered the main rotor actuators and the secondary system did ASE inputs and TR. The primary system was more powerful and so a runaway or jam in that system was much worse.

A similar idea was adopted by the Sea KIng force where coupled or uncoupled indications of a control malfunction was used to diagnose and select the appropriate system off.

[212man]

There was also a suggestion that a RN Wessex crash (into a sea wall I think) could have been caused by a servo jam or runaway and led to the Wessex fleet adopting the 'If in doubt, Primaries out' terminology for dealing with a control malfunction.

This was based on the fact that the primary hyd system powered the main rotor actuators and the secondary system did ASE inputs and TR. The primary system was more powerful and so a runaway or jam in that system was much worse.

A similar idea was adopted by the Sea KIng force where coupled or uncoupled indications of a control malfunction was used to diagnose and select the appropriate system off.

How could you fly with no MR servos, or am I misreading it?

[[email protected]]

How could you fly with no MR servos, or am I misreading it? I probably didn't make it very clear. Neither aircraft could fly without MR servos. On the Wessex, with primaries out the secondaries could push the MR jacks no problem but would struggle to oppose a primary jack hardover/runaway - if you had a control problem and incorrectly took the secondaries out you would allow the primary hardover free reign and probably die.

[212man]

I probably didn't make it very clear. Neither aircraft could fly without MR servos. On the Wessex, with primaries out the secondaries could push the MR jacks no problem but would struggle to oppose a primary jack hardover/runaway - if you had a control problem and incorrectly took the secondaries out you would allow the primary hardover free reign and probably die.

ok - yes I knew you couldn’t fly them hence my puzzlement.