Jump to content

Hydraulics System Test


Recommended Posts

Had a student ask this today. What is the purpose of a hydraulics system test (switch off, move cyclic, switch on, move cyclic)? I didn't have a good answer for him. Best guess is that the manufacturer just wants to see the hyd sys cycled. Make sure it will come back on again. Never heard the why behind this test.

  • Like 1
Link to comment
Share on other sites

First, you gently move the cyclic around with Hyd ON and watch the disc to ensure it follows your inputs - if the disc goes up when you move the stick forward, shut down and carefully run away from it.

 

Then turn the Hyd OFF. Move the stick forward left at 45 degrees, then back right, then from the middle again go front right and back left. The effort involved should be the same, if not one of the servos is going bad. (Get your mate to do this while it is shut down, and you are up on the step watching the servos - you will see that only one servo moves at a time.)

 

Holding the cyclic, raise the collective about an inch to make sure the effort involved is not ridiculous. Push it back down - feel the interconnect as the cyclic wants to move too. You will probably NOT get the lever all the way to the floor - but when Hyd is back ON it will go down again.

 

If you wish (it is not in the checklist) pull the HYD circuit breaker, confirm that hydraulic power is restored. This is a test of the fail-safe solenoid, which ensures that if you are flying Hyd OFF and have an electrical failure, the hydraulics come back on.

 

Hyd ON, CB IN, ready to roll.

 

I always did this test at idle, but the flight manual says to do it at 100%

  • Like 1
Link to comment
Share on other sites

First, you gently move the cyclic around with Hyd ON and watch the disc to ensure it follows your inputs - if the disc goes up when you move the stick forward, shut down and carefully run away from it.

 

Then turn the Hyd OFF. Move the stick forward left at 45 degrees, then back right, then from the middle again go front right and back left. The effort involved should be the same, if not one of the servos is going bad. (Get your mate to do this while it is shut down, and you are up on the step watching the servos - you will see that only one servo moves at a time.)

 

Holding the cyclic, raise the collective about an inch to make sure the effort involved is not ridiculous. Push it back down - feel the interconnect as the cyclic wants to move too. You will probably NOT get the lever all the way to the floor - but when Hyd is back ON it will go down again.

 

If you wish (it is not in the checklist) pull the HYD circuit breaker, confirm that hydraulic power is restored. This is a test of the fail-safe solenoid, which ensures that if you are flying Hyd OFF and have an electrical failure, the hydraulics come back on.

 

Hyd ON, CB IN, ready to roll.

 

I always did this test at idle, but the flight manual says to do it at 100%

 

That sounds like the Bell 206 procedure which is not, repeat- not! universally applicable. Follow RFM procedures to the letter.

Link to comment
Share on other sites

What is the purpose of a hydraulics system test (switch off, move cyclic, switch on, move cyclic)?

 

As already quoted in the post above and from common RFMs (Bell 206B/L, 407, 212, etc.) the basic purpose of the Hydraulic System Check (during run-up) is to determine proper operation of hydraulic actuators (servos) for each flight control system, check for abnormal forces, unequal forces, control binding, or motoring that would require maintenance action. Additionally, as in any other hydraulically operated, electrically controlled system, the check verifies the continuity of wiring and operation of electrical components.

 

As a side note, the term “switch off the hydraulics” is a slight misnomer, since in most system the hydraulics are not truly turned off, an example is the Bell 206 and AS350.

 

In the Bell 206 when the hydraulics switch is placed in the off position, a solenoid valve is energized, up stream from the servos, which redirects hydraulic fluid back to the reservoir, thereby bypassing the servos. The AS350 incorporates a solenoid valve at all three of the main rotor servos. When the hydraulics switch is placed in the off position, all three solenoid valves are energized to open the pressure line of the three servos direct to the return line, thereby bypassing the servos and directing hydraulic fluid back to the reservoir.

 

In both systems, even with the hydraulic switch off, the hydraulic pump continues pumping fluid in a continual loop from reservoir, pump, filter, solenoid valve, and back to the reservoir. So you have more of a bypass action that isolates the servos and protects the hydraulic pump.

 

Since hydraulic systems very between models, you’ll have to refer to each specific models technical reference. As an example, the AS350 incorporates accumulators into the system, which are also tested during run-up. Moreover, larger helicopters like the S-64 have as many as six or more general hydraulic systems.

 

PagesfromRotorcraftFlyingHandbook_2012_z

 

 

Screenshot2014-04-18at24410PM_zps675641f

Link to comment
Share on other sites

As iChris mentioned the AS350/355 hydraulic system and since it illuminates type specific procedures-

The pilot doesn't lift the collective in that airframe during the hydraulic system check, it remains locked down until you intend to fly. You check that status twice: prestart and before the hydraulic system test.

Edited by Wally
Link to comment
Share on other sites

  • 2 weeks later...

On the Bell 206, turning the hydraulic system off is also a check on the swashplate uniball shimming. If it is shimmed too tightly, it will cause unnecessary wear on the uniball. That shimming also determines the amount of force required to move the controls in the event of a hydraulic failure. I have had 206"s come out of maintenance with the shimming too tight. With the hydraulics off, I couldn't move the controls. Needless to say it went back into the hangar. Since I am an aircraft mechanic as well, it was easy fix.

Link to comment
Share on other sites

From above...

 

"Holding the cyclic, raise the collective about an inch to make sure the effort involved is not ridiculous. Push it back down - feel the interconnect as the cyclic wants to move too."

 

​What is the purpose of this? I noticed this effect in the R-44 and with hydraulics restored there is no trace.

Link to comment
Share on other sites

On the Bell 206, turning the hydraulic system off is also a check on the swashplate uniball shimming. If it is shimmed too tightly, it will cause unnecessary wear on the uniball. That shimming also determines the amount of force required to move the controls in the event of a hydraulic failure. I have had 206"s come out of maintenance with the shimming too tight. With the hydraulics off, I couldn't move the controls. Needless to say it went back into the hangar. Since I am an aircraft mechanic as well, it was easy fix.

That's an excellent note...

 

I've had a Bell 206 come out of maintenance with the shimming done improperly too.

 

Following uniball maintenance and after a successful ground run-up, an inflight check with the hydraulics off should also be accomplished.

Link to comment
Share on other sites

Hello, i'm just entered in this forum, i think that is an optimun channel to learn new information and talk with other people to helicopters.

I fly with a agusta A109, is a beatufil machine and now talk about the hydraulic system .

It has 2 hydraulic system, accordingly two tank,the primary and secondary.

The primary govern the cyclic and collective and pedals,the secondary govern the command and the services whithout pedals control.

Cyclic, collective, rotor brake, nose weel lock,parcking brake and the lamding gear.

The hydraulic test is used according to manual to try the correct sistem function before any flight.

For the test there is a specific interruttor to exclude the system 1 or 2.

I hope to be a help.

I'm very happy to talk with other people that share my passion.

Have a nice flight....!!!!

Link to comment
Share on other sites

  • 6 years later...

Everyone, I know this is an old thread, but I hope people are still tuning in. After reading all the explanations, I noticed that there is some fundamental lack of understanding of hydraulic systems and what each manufacturer is looking for on their respective checks. In reference to the Bell 206/407(single hydraulic servo systems) and as well, the 205/212 (dual/single systems), Bell has universally asked to have 2 separate hydraulic systems check performed: One at idle -the "initial Hydraulic check", and the other one done at 100% NR- the"Hydraulic servo check". No where in the above answers did I see anyone differentiate the two which leads to my belated response to explain the two separate checks. The "Initial" hydraulic check is performed at idle NR to check that the means of disabling(bypassing) the transmission-driven hydraulic flow is operable and capable of bypassing the system in case of a servo pilot-valve jam(hard-over) caused by contamination or un-even wear within the pilot-valve slider itself. That system includes an electrically actuated solenoid and a spring-loaded bypass valve manifold assembly allowing flow to be redirected to the inlet side of the pump or reservoir itself. Fluid never stops moving, it just gets re-directed away from the servos. Some of this valving is internal to the pump body and some can be externally mounted on the transmission deck and interconnected with hard and flexible lines. If the system incorporates a pressure-activated Caution Light and switch, this procedure tests both their functionalities as well. Here is what was missing above: The Initial check also serves as a pre-warning of a malfunctioning servo valve. Pilots are supposed to remove frictions enough to have free controls and only "monitor" (NOT move) the controls for any un-commanded movements - i.e. jumping or motoring in any direction, which would indicate a possible servo-valve malfunction BEFORE flight NR rpm is achieved or flight takes place. The second "Actuator check" is done at representative "flight" rpm to simulate normal force-feedback, and is performed by moving the controls in each servo axis( X-pattern in 206; + pattern in 407; up/down for collective servo in both) to ensure there is no binding within the individual servo bodies and that trapped fluid is allowed to be pushed manually between the chambers with a normal, expected force. If fluid cannot be moved between the chambers, a servo would essentially lock-up and be immovable. One may be able to feel excessive force from a possible mis-shimmed uni-ball but unless you are familiar with the actual drag-scale numbers you really won't be able to tell whether you are in spec or not so it isn't part of the flight manual required check criteria. The Bell 206B actually had a flight manual change to include a post-landing servo actuator check to see if any servo force feedback changed from the start to the shutdown. If excess force, different from what was felt at start up, was noticed, it could indicate that the bushings within the servo had expanded and began to bind, not something you would want to find out after a hydraulic pressure loss! Apparently, some servos were experiencing heat-related binding enough to get Bell to ask pilots to check for it on every shutdown. The 350 single and dual hydraulic systems have their own specific requirements driven by their design and should be treated accordingly(another discussion al together). There are not any real "universal" just-do-this procedures for these aircraft. Each should be checked based on the designer/manufacturers procedures and performed with a real understanding of how each system works and is supposed to function. If you don't understand the system, you need to get real technical advice because you won't be able to identify when something is wrong, and that is really the whole point of the checks.

Good Hunting,

Jon

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...