Hydraulic Test at Idle or 100% in a Bell 206

[glasairnz]

Interested in your views - the Bell 206 flight manual calls for a Hydraulic Off Test at 100%. We are testing here for a hard over of course - but in the case of an actual hard over where you are not holding on very tight then isn't the machine going to fly off the ground and hurt us? Wouldn't it be better to do a hard over test at Idle or would that not be a valid test?

[Nearly Retired]

Glasair, you might be a little confused. The latest-and-greatest version of the 206 RFM states that an initial hydraulic check should be performed at idle. Once you get the engine started and the rotor spinning, you shut the switch off to check for unwanted motoring. Then, up at 100% you do the conventional hydraulic check/servo test - switch off and move the cyclic in an "X" pattern to check for binding and stuff. (Don't forget to check the collective servo as well.)

 

Even at 100%, a hardover resulting in full deflection of the cyclic will not cause the ship to do anything. However the blades might flex enough to strike the tailboom or damage the transmission isolation mount. A two-blade, underslung system cannot generate enough cyclic power to roll the ship over on the ground. A hardover in the collective servo would be another story.

The 206 has one of the best hydraulic systems ever designed and installed in a helicopter. In all my years in this industry I don't think I've *EVER* heard of a hardover in a 206. Doesn't mean there's never been one, they're just very, very rare. As are total hydraulic failures - almost unheard of.

[AngelFire_91]

Keep in mind that without any collective input in, there is going to be very little if any actual thrust in the direction of the cyclic. The blades will deflect that direction yes, but without collective input to provide thrust in the direction of deflection there isn't going to be enough force to roll the helicopter.

[Eric Hunt]

It would make sense to do the test at idle - anybody know why it is done at full noise? (apart from just saying "It's in the flight manual")

[AngelFire_91]

It would make sense to do the test at idle - anybody know why it is done at full noise? (apart from just saying "It's in the flight manual")

My guess, and this is only a guess, so take it for what it's worth. They want to test it at full operating pressure. The pump is driven from the M/R transmission. Therefore at idle the pump will not be turning at it's operational speed. As said above when you switch the hydraulics off you're looking for motoring and binding of the servos which would indicate a failed seal. At idle there may not be enough pressure to push through a broken seal... again. just a guess.

[lelebebbel]

Here's the background story I was told:

 

1) manual used to say do the check at 60% idle.

 

2) person crashes helicopter

 

3) lawyer sues bell, says it could have been a hydraulic failure

 

4) bell says hydraulics are checked every flight

 

5) lawyer says: ... But only ever at 60% on a cold system, they could have seized once they got hot

 

6) Bell pays settlement and amends flight manual to do another check at 100% after the flight

[AngelFire_91]

Here's the background story I was told:

 

1) manual used to say do the check at 60% idle.

 

2) person crashes helicopter

 

3) lawyer sues bell, says it could have been a hydraulic failure

 

4) bell says hydraulics are checked every flight

 

5) lawyer says: ... But only ever at 60% on a cold system, they could have seized once they got hot

 

6) Bell pays settlement and amends flight manual to do another check at 100% after the flight

Probably a more likely story than my guess. haha

[iChris]

Interested in your views - the Bell 206 flight manual calls for a Hydraulic Off Test at 100%. We are testing here for a hard over of course - but in the case of an actual hard over where you are not holding on very tight then isn't the machine going to fly off the ground and hurt us? Wouldn't it be better to do a hard over test at Idle or would that not be a valid test?

 

As already quoted in the post above and from common RFMs 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.

 

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

 

As a side note, the term “switch off or “turning 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.

 

Check a prior post on this issue at the link below:

 

Hydraulics System Test

 

 

Always use the “Search Forums” function at the top of the page to find additional information, we’ve covered hundreds if not thousands of issues.

 

Edited March 31, 2016 by iChris

[rotormandan]

This was stated above but just to make it simple....

 

The 206b is checked at idle and 100% as per the checklist

 

Once at idle the switch is turned off. Check for hard over and weird motoring felt in the controls. And as i just learned the uniball.

 

Once at 100% the x pattern to check the servos.

[iChris]

We are testing here for a hard over of course - but in the case of an actual hard over where you are not holding on very tight then isn't the machine going to fly off the ground and hurt us?

 

Wouldn't it be better to do a hard over test at Idle or would that not be a valid test?

 

 

This was stated above but just to make it simple....

 

The 206b is checked at idle and 100% as per the checklist

 

Once at idle the switch is turned off. Check for hard over and weird motoring felt in the controls. And as i just learned the uniball.

 

Once at 100% the x pattern to check the servos.

 

The term “Hard-Over” refers to a servo being driven to one of its two extremes by hydraulic pressure. However, that’s somewhat immaterial in this case, since in neither of these hydraulic test are the servos actually being pressurized.

 

When you turning the hydraulic switch to the off position, you’re actually turning the switch on (closing its contact) to energize a solenoid valve that routes hydraulic pressure away from the servo valves and back to the reservoir.

 

 

 

 

 

 

 

 

 

Edited April 1, 2016 by iChris

[Nearly Retired]

A *little* bit misleading, Chris. The Preliminary Hydraulic Check (which only appears in the 206B-3 RFM but curiously not the 206A or "straight" 206B) is done at IDLE. If it passes this test, then you roll up to 100% and do the "regular" hydraulics check.

[iChris]

A *little* bit misleading, Chris. The Preliminary Hydraulic Check (which only appears in the 206B-3 RFM but curiously not the 206A or "straight" 206B) is done at IDLE. If it passes this test, then you roll up to 100% and do the "regular" hydraulics check.

 

It is a little bit confusing in the page excerpt format. In the post above, you were looking at two separate pages that appeared to be one page. However, the preliminary hydraulics check is done at 70% N1 (idle) and the hydraulics check at 100% Nr. Full page format below:

 

 

 

 

 

 

Edited April 1, 2016 by iChris

[rotormandan]

I guess i've only read the b3 manual then

[AngelFire_91]

 

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

I forgot about the Uniball thing! I was taught that once a long time ago. Amazing how much you forget sometimes. Now.... what was I doing...