Wati! Wait! Don't Tell Me.

[jlunseth]

We have had two recent threads on vacuum pumps and mention was made that when they fail, carbon from the vanes can get into the gyro and damage it.  According to some A & P's apparently.  And according to a company that makes non-carbon vaned pumps.  So having just recently had a pump go and gotten it replaced I was worried and thought I should quick get my pump lines and gyro checked out.

And then it dawned on me.  Doh!  Its a vacuum pump.  The very reason it is a vacuum pump is to ensure that air from the pump does not get into the gyros.  Rather, the pump pulls clean air through the gyros and then from the gyros to the pump, which is pulling a vacuum. 

So now, pray tell, just how does carbon dust from the vacuum pump travel all the way the wrong direction through the outlet line and get into the gyros.  

Doesn't make any sense to me. 

[AndyFromCB]

When the pump fails, the gyro keeps on spinning, the flow reverses, this time around sucking carbon into it. It's called physics...

[jetdriven]

The gyro cases have 5" of vacuum in them, and every time you shut down, and especialy when the pump suddenly fails, carbon dust gets sucked into the gyros.  Thats how I understand it. The reason they developed the CV1J4 filter was to stop this carbon dust.  Correct me if I am wrong, but the filter darkens with accumulated hours.

We bought one and are waiting to install it next week, after overhauling the DG and soon to be the atitude gyro as well.

 

http://www.aerotechcomponents.com/products.html#history

 

[jlunseth]

So the pendulous vanes, which are set to cup in the direction from which the flow is coming, i.e. the clean air, inlet side, then act as a fan to reverse the flow, and blow air out the clean air inlet side??  Sorry, sounds like some good salesmanship to me. 

I don't mean this personally though, I am sorry if it sounded that way.  It just strikes me that some vendors have come up with a reason to sell something that is unnecessary. 

I was sorta wondering why my gyros were still working fine, despite the failure of the old pump. 

I used to have a diesel Suburban.  Got the oil changed, and went back to the same service place a couple of weeks later, a guy comes running out to show me how dirty the dipstick is, I should not have let the oil change go on that long, it needs to be changed immediately and the engine inspected for damage.  No, I said, you changed it two weeks ago.  Its a diesel, did you notice that?  The oil goes black the first time the engine is started.  That's how diesel's work.

Oh?  He says.     

[jetdriven]

I think it has more to do with vacuum inside the cases, and reverse flow when the pump fails. I have heard of gyros going bad within a hundred hours after the pump goes.  Call AQI or TGH and ask them if they see carbon in gyros sent in for overhaul. I bet they do.

[jlunseth]

I will ask my Avionics guy.  Thanks folks.

[AndyFromCB]

You might be right but why wouldn't they act like a fan? If they keep on spinning, they keep on moving air? Wish someone would post a diagram of a gyro's vanes.

[David Mazer]

Okay, so I'm just a stupid chemical engineer who hasn't practiced as and engineer for many years and has no particular knowledge of these systems but it would seem appropriate that there would be a simple valve between the pump and the gyros to prevent any back flow.  Assuming there is an unobstructed inlet, and I believe there must be for the gyros to spin at all, then the pressure would equalize from the inlet side rather than the vacuum side.  And, assuming the dust is damaging, why wouldn't they install the valve from the beginning.  Also, if the pump is any distance at all from the gyros, the carbon dust probably wouldn't make it that far back as the pressure would equalize from both sides with the predominance coming from the inlet side since the pump would act as a resistor.  Again, from nothing but a theoretical perspective.

[sleepingsquirrel]

Brownian motion !

or as they claim  in Quantum physics particles can disappear and reappear anywhere. (Quantum tunneling)

[David Mazer]

My recollection of Brownian is more along the lines of random rather than Star Trek transport travel but it would make lots of other things make sense.

[N601RX]

It wouldn't take very much flow of air flow at all to move the carbon dust around inside the vaccum system.  As soon as the pump stops sucking the Gyros and Brittain Vac servos are going to act as a vac storage tank and begain sucking air in from the pump.  Even after these storage areas equalize there is probably some natural reverse flow of air from the pump to the instruments due to the small pressure difference from under the cowling and inside the cabin where vacum filters are located.

[John Pleisse]

The engine stops, the vac pump stops immediately, the gyros continue to spin. This can create a modest backflow. This is exasserbated by a clogged or obstructed intake filter or CV1JV and/or a high time carbon vane pump.

[N601RX]

Quote: jlunseth

We have had two recent threads on vacuum pumps and mention was made that when they fail, carbon from the vanes can get into the gyro and damage it.  According to some A & P's apparently.  And according to a company that makes non-carbon vaned pumps.  So having just recently had a pump go and gotten it replaced I was worried and thought I should quick get my pump lines and gyro checked out.

And then it dawned on me.  Doh!  Its a vacuum pump.  The very reason it is a vacuum pump is to ensure that air from the pump does not get into the gyros.  Rather, the pump pulls clean air through the gyros and then from the gyros to the pump, which is pulling a vacuum. 

So now, pray tell, just how does carbon dust from the vacuum pump travel all the way the wrong direction through the outlet line and get into the gyros.  

Doesn't make any sense to me. 

[jetdriven]

Since 1995.

[Hank]

So if the picture below is an accurate although VERY simplified picture of a spinning vacuum gyro, if it is still spinning after the pump dies, which everyone agrees on, how does the air flow change direction? Or does the backflow start due to pressure differential somewhere after the rotation stops? [Quickly draw in the vacuum pump]

Or since the pump craps out, does it stop spinning, or slow down a lot, causing a pressure rise in the lines between the still-spinning gyro and the failed pump, and THAT is what pushes 'carbon dust' toward the spinning cups which then scatter it through the guts of the instrument? This actually sounds plausible.

Vacuum pump stops pumping, restricting air flow through the former pump. Gyro continues to spin, pushing air towards the restricted pump, pressure rises and in time [a couple of minutes?] rises enough to stop the spinning gyro and push it backwards as the higher pressure air escapes back out the way it came in. Drawing v.4 seems to have it at last.

 

[jetdriven]

I think so, N601RX (Mike) might have it, there is higher pressure in the cowl than the cabin, so after the pump fails, carbon dust and air flow from the pump to the gyros. 

[N601RX]

I don't believe that a spinning gyro will act as a fan and continue to push air through it.  At lest I don't think the ones that I have saw the insides of would not do that.  I have the guts out of one at home, I'll post a picture when I get home tonight.  I think that once the pressure difference is equalized from the servos and instruments the cabin has a slight vaccum compared to under the cowling and this will continually draw air through the pump and into the instruments.

 

[74657]

If this is the case, wouldn't the gyro and AI be sucking up carbon every time the engine shuts down?  The pump "fails" or stops pumping then correct?  I believe that there would be carbon dust that breaks down during the life of the pump through normal wear, not just at failure.....

[jetdriven]

I have no data but I think over the course of 500 or 1000 hours some dust makes its way into the gyros from normal shutdowns.  When you shut down, the pump is putting out low vacuum at idle, and when you kill it, the engine winds down over a few seconds.  Failure in flight is more a sudden stoppage from 5" or more of vacuum. That could cause some real reversion.  

AMTonline explains some.   http://www.amtonline.com/publication/article.jsp?pubId=1&id=1426&pageNum=1

[jlunseth]

Here is what I found online, again, if you can believe it.  May be sales "material," you need to buy this filter because....

http://www.aerotechcomponents.com/products.html

Seems to me that carbon would have a long way to go with very little to drive it.  The instruments would not create a vacuum because the pendulous vanes face the wrong way, and any leftover vacuum in the instruments would be satisfied quickly by the intake air, which is close to the instruments.  Carbon would have a long way to go, would have to make it through the regulator to get to the gyros.  But if there is a problem, a filter on the vacuum side will cure it.

[takair]

Please be careful here.  Most of the discussion on the Aerotech sight refers to an air PUMP.  Most Beech aircraft use an air pump.  Essentially identical to a vacuum pump, but the airflow is pushed into the instruments.  This mandates a filter for even normal operation.  The common Beech filter is metalic and can not be inspected.  This filter allows inspection.  In other cases (and as seen in the Aerotech drawings) the filters are used in twin engine installations.  In the event of one pump failing, the other will continue to draw air until the valve closes.  Also, when only one engine is running it can draw air through the non running pump.  This is another case for the filter.  Most newer Mooneys have a stand-by pump.  Like the twin, a filter can prevent cross suction from pulling carbon out of the non running pump and into the operating pump. 

In the case of our singles, the filter may do more harm than good.  Vacuum pumps are already running hot and under load.  Adding a filter just adds another point of stress on the system.  Keep in mind that the vacuum regulator does a pretty good job of preventing backflow of particles in the unlikley event that vacuum remains on the system after pump failure.  Note that your vacuum decreases rapidly when you shut down.  The argument that you need this filter in case your pump fails and the carbon manages to get in the instruments may be offset in that you may now have a higher incidence of vacuum pump failure.  You also add another restirction to your airflow.  Airflow is as important to a vacuum system as the suction. One possible benefit of this filter might be to prevent junk from being sucked into the pump when a line is breaking down.  However, lines should be changed when they become brittle.  Some recommend every seven years. When you vacuum pump fails, be sure your mechanic checks for dust in the line.  If there is, he should back track until the lines are clean.  I think you will find that it is rare that the dust gets beyond the regulator.  Please be concider these things before modifying your certified product.  What sounds good in an ad may cost you with more frequent pump failures, possibly when you least want one to fail. This is to not meant in any way as a knock on Aerotech.  It appears to be a fine product when used in the proper application.

[carusoam]

I think squirrel has hit on something...

Brownian motion, or random walk theory, or drunken navigation.

When the pump turns to carbon dust, their isn't much driving force to keep that dust from spreading in all directions.  Upstream or downstream of the pump.

Keep in mind that the pump drive keeps turning even after the failure.

Gyros are assembled in a clean room environment because of their sensitivity to dust.

Now would be a good time to see how yours is plumbed.  Filters and back-up systems too.

As for what pendulus vanes look like, consult an IFR text book.  Its mandatory trivia learning for the rating.  Somewhat like buckets on a water wheel....oldschooool.

 

Best regards,

-a-

 

[MARZ]

I have to add to the AAHHHH NOPE side -

The input of air flow on a vacuum system to the gyros comes from a filtered source (little or no restriction) from within the cabin of the plane (in some cases screwed directly to the back of the gyro - in others remote mounted and piped via rubber tube)   The vacuum pump is routed from outside the plane - through a regulator and then to the gyro.  A failure of the pump - catastrophic or gradual will equalize from the side of least resistance - the filtered end piped into the gyro. 

Any naysayers try this - remove the hose from the end of the regulator direct to your gyro and inhale - there just isn't enough restrictive force to pull contaiminates from the pump through the regulator and the longer plumbing to the gyros.

[M016576]

Quote: sleepingsquirrel

Brownian motion !

or as they claim  in Quantum physics particles can disappear and reappear anywhere. (Quantum tunneling)

[jlunseth]

Anyone know which position the regulator would go in the event of a pump failure, open or closed?  I would think open, but not necessarily.  If the failure position is closed, that would sink it for the carbon dust theory. 

For those who don't understand, there is a vacuum regulator between the vacuum pump and the gyros, which makes sure that the vacuum does not go over a certain limit, probably about 5.2".  It is a spring loaded regulator, so in the event of a failure of the suction pump, it is going to go in the direction the spring tells it to - either fully open or fully closed.  If it fails in the closed position, that would close off any flow from the pump to the gyros.  And if the failure position is closed, then if the gyros are trying to pull air back from the pump, that suction in the wrong direction would help the regulator close.