Greg_D

Alternate Static Source Blockage

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I have a 1994 Ovation with a KFC150 AP.  It works great down low (below 4000 ft).  Above that altitude, I get a wild ride with big pitch oscillations.  I’m pretty sure it’s a static source blockage somewhere.  Opening the alternate static source valve in flight has no impact on the airspeed indicator , VSI, or altimeter.  Not even a slight wiggle in any of them, which is what leads to to believe the problem is somewhere in this system. There is a drain valve for the system below the left wing. Are there any others?  Does anyone have ideas on blowing out the system with some compressed air or other techniques to drain or troubleshoot the system?

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Blowing things out of the instrument systems... is highly NOT recommended...

By anyone, for any reason, at any time....

Unless your O1 has gone all digital... the old mechanical gauges are still very sensitive to compressed air... :)

 

Phew... got that out of the way...

At the back of the BK APs is a hose that connects the AP box to the static system...

Depending on who did the install, and what hose type they used... its proper connection to the static system may have been lost...

See if you can get a look at the back... if you see a yellowish broken rubber hose hanging down...

That has been a common cause for APs that have lost control of their altitude hold....

Other things to double check... static air drain... sometimes the drain loses its seal....

Go O1!

PP stuff I have read about on MS, not a mechanic...

Best regards,

-a-

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Disconnect the static line coming from the alternate static valve. You should be able to blow it out from there. Double check ther3 are no static devices in the tail.

You should have your static system re-certified afterwards. But to that end, having your avionics guy troubleshoot it is probably your best bet.

 

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8 minutes ago, carusoam said:

Blowing things out of the instrument systems... is highly NOT recommended...

By anyone, for any reason, at any time....

Unless your O1 has gone all digital... the old mechanical gauges are still very sensitive to compressed air... :)

 

Phew... got that out of the way...

At the back of the BK APs is a hose that connects the AP box to the static system...

Depending on who did the install, and what hose type they used... its proper connection to the static system may have been lost...

See if you can get a look at the back... if you see a yellowish broken rubber hose hanging down...

That has been a common cause for APs that have lost control of their altitude hold....

Other things to double check... static air drain... sometimes the drain loses its seal....

Go O1!

PP stuff I have read about on MS, not a mechanic...

Best regards,

-a-

Well, the service manual says to blow low pressure air (10-25 psi) through the lines from the disconnected lines at the airspeed indicator to static ports, so I’m guessing someone at the factory thought it was a recommended way to clear a clogged static line.  I know not to blow anything towards the instruments.  Mainly, I was curious if there were other drains besides the one in the wing or if anyone else had seen this issue before.  The AP static hose is a black  rubber hose and it is connected properly and in good shape.  I don’t think that hose being disconnected would cause the alternate static system to stop functioning altogether.  Unless maybe it’s just open and leaving the alternate system permanently on.  I’m thinking of opening the storm window on the next flight to see if I get any changes with that before opening up the static system.  Thoughts?

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Kind of odd that it is altitude sensitive...  You have documented the situation really well...

The issues I wrote about were more system leak oriented...
 

Wish I could have helped a bit more... :)

 

On the other hand...

We have a really good AP guy around here that has some really good insight.... let’s invite @Bob Weber to the discussion...

Best regards,

-a-

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The Ovations have 2 static drains, one under the left wing close to the fuselage the other is located on the bottom of the fuselage left side aft of the wing by your access panel.

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Static system is plumbed to two static ports in the sides of the fuselage.  The static system drain is under the main fuselage, just aft of the wings (the pitot system drain is under the wing root and not part of the static system).

Autopilot pitch is controlled by the attitude indicator (electronic signal for pitch and roll reference) and altitude reference (static pressure).  When you transition to the “wild ride” can you look to see if the attitude indicator leads or causes this?  Not likely, but something to check.  Not likely if your attitude indicator seems to be working and the autopilot is stable at lower altitudes.

The altitude is referenced to the static system - as you suspect, this seems to be the most likely issue.  The Alt Static source is sealed by a couple of small o-rings that get old and cracked effectively defeating the alt static source selector (sometimes you can feel this if the alt static source is hard to move or doesn’t feel smooth when moved).  Theoretically, a change in attitude could cause a change in cabin pressure and a resulting change in the static pressure reference leading to oscillations.  A static system check can quickly determine if your static system is tight and eliminate or confirm this possibility.  The lack of change in the system pressure when you open or close the alt static source is a pretty clear indicator that you have a significant static system leak somewhere.

It does seems odd that it is attitude and altitude sensitive but all I can think is to start by eliminating the two potential causes above.

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10 hours ago, Warren said:

Static system is plumbed to two static ports in the sides of the fuselage.  The static system drain is under the main fuselage, just aft of the wings (the pitot system drain is under the wing root and not part of the static system).

Autopilot pitch is controlled by the attitude indicator (electronic signal for pitch and roll reference) and altitude reference (static pressure).  When you transition to the “wild ride” can you look to see if the attitude indicator leads or causes this?  Not likely, but something to check.  Not likely if your attitude indicator seems to be working and the autopilot is stable at lower altitudes.

The altitude is referenced to the static system - as you suspect, this seems to be the most likely issue.  The Alt Static source is sealed by a couple of small o-rings that get old and cracked effectively defeating the alt static source selector (sometimes you can feel this if the alt static source is hard to move or doesn’t feel smooth when moved).  Theoretically, a change in attitude could cause a change in cabin pressure and a resulting change in the static pressure reference leading to oscillations.  A static system check can quickly determine if your static system is tight and eliminate or confirm this possibility.  The lack of change in the system pressure when you open or close the alt static source is a pretty clear indicator that you have a significant static system leak somewhere.

It does seems odd that it is attitude and altitude sensitive but all I can think is to start by eliminating the two potential causes above.

The attitude indicator isn't doing anything abnormal.  I"m pretty sure it's something in the static line since nothing happens when I open the alternate static source.  My next step will be to check the o-rings and if those look OK, then disconnect the static line at the instruments and blow some low pressure air into the lines towards the drains.

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22 hours ago, Greg_D said:

I have a 1994 Ovation with a KFC150 AP.  It works great down low (below 4000 ft).  Above that altitude, I get a wild ride with big pitch oscillations.  I’m pretty sure it’s a static source blockage somewhere.  Opening the alternate static source valve in flight has no impact on the airspeed indicator , VSI, or altimeter.  Not even a slight wiggle in any of them, which is what leads to to believe the problem is somewhere in this system. There is a drain valve for the system below the left wing. Are there any others?  Does anyone have ideas on blowing out the system with some compressed air or other techniques to drain or troubleshoot the system?

The drain valve under the left wing is for the pitot system. The static drain is on the aft belly kind of near the avionics bay access panel. 

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Hmmmm.

The Alt static doesn’t show any difference?

That might be a hint of something being amiss...

Something like the Alt static system is always open...

+1 static system check

The cabin is always under a lower pressure than outside... during flight...

Moving the Alt static valve should show the difference in reading...

PP thoughts only, not an instruments guy...

Best regards,

-a-

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On 7/29/2020 at 10:18 PM, Greg_D said:

I have a 1994 Ovation with a KFC150 AP.  It works great down low (below 4000 ft).  Above that altitude, I get a wild ride with big pitch oscillations.  I’m pretty sure it’s a static source blockage somewhere.  Opening the alternate static source valve in flight has no impact on the airspeed indicator , VSI, or altimeter.  Not even a slight wiggle in any of them, which is what leads to to believe the problem is somewhere in this system. There is a drain valve for the system below the left wing. Are there any others?  Does anyone have ideas on blowing out the system with some compressed air or other techniques to drain or troubleshoot the system?

I assume when you say a wild ride you’re referring to when the autopilot is On and in Altitude Hold mode?  If so an open in the static system is a possibility.  Does opening and closing the cabin air vent or heater vent make it worse?  I’ve seen this in an Ovation with a system 55X when there was an open line in the static system.

If your going to blow out the alternate static valve, I’d remove it from the plane and do it on the bench.

Clarence

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On 7/30/2020 at 7:24 AM, buddy said:

The Ovations have 2 static drains, one under the left wing close to the fuselage the other is located on the bottom of the fuselage left side aft of the wing by your access panel.

I believe the one on the wing is the pitot drain.

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