Altimeter / ROC problem

[whiskytango]

I have a '68 C model that just developed a problem where the altimeter swings back and forth over a range of about 300 feet, and the ROC indicator swings back and forth in the same sequence and at the same or worse rate.  Clearly a problem with the static system.  The problem goes away when I select Alternate Static Source.  I took the plane in to have an IFR Certification done and told the mechanic about the problem.  The plane passed the IFR Certification with no repairs needed, but the problem remains.  I have operated the static drains, but no water ever comes out. ( I did this when the temperature had been above freezing for several days).  Any ideas what is going on here?

[carusoam]

Can you tell if the static drain is leaking?

There is one or two threads regarding changing seals and other parts...

Some of the tubing might be getting old as well.

Best regards,

-a-

[Hank]

Seems there was a revision to the static system at some point. Peek in the tail cone and make sure that the tubing connecting the two static ports goes up and across, and not down and across. Check it for leaks and cracks, and make sure the connectors are in good shape.

[whiskytango]

I will check the static drain and tubing routing / connectors.  This didn't start happening until right after the annual.  Thanks for the info!

[carusoam]

Check to see if you have a static drain...

I believe my C had a 'dead-leg' that got emptied at annual.

Best regards,

-a-

[Piloto]

Next time try push and hold the static valve knob when in flight. Vibration on the static valve may be causing it.

José

[takair]

Even with the drain at the theoretical low point, there is a strong possibility that there is another low spot in the line. I had one in the tail where the two static port lines came together, but it can be anywhere. It does not take much. What can happen, is you get enough water in there that it is almost closed off. It acts as a check valve. When presseure changes with altitide, it eventually "bubbles" through.....this is the jump you see. Also, when this water freezes, it can close off the system all together. They may not have noticed this during the certification, because the attitude of the aircraft may be different and because it is not a truly dynamic test. Have your mechanic look for the low spot....start at the tail...I don't think it would take long.

[takair]

Just re-read your post....I had assumed this happens in a climb or descent. Is this also in level flight?

[Andy95W]

One of your 2 static ports could be blocked. As the airplane slightly yaws left and right, the air pressure increases then decreases and cannot equalize because the other port on the opposite side is blocked.

Just a guess, but I've seen weird things like that happen in similar circumstances.

[N201MKTurbo]

Remove the static line from the alternate static valve and blow it out with compressed air. Just make sure you don't put compressed air to the instruments, you will be rebuilding them all.

[DXB]

I have a '68 C model that just developed a problem where the altimeter swings back and forth over a range of about 300 feet, and the ROC indicator swings back and forth in the same sequence and at the same or worse rate.  Clearly a problem with the static system.  The problem goes away when I select Alternate Static Source.  I took the plane in to have an IFR Certification done and told the mechanic about the problem.  The plane passed the IFR Certification with no repairs needed, but the problem remains.  I have operated the static drains, but no water ever comes out. ( I did this when the temperature had been above freezing for several days).  Any ideas what is going on here?

Sorry to dumb down and divert this excellent discussion for a sec, but I have a '68C and am completely unaware of having an alternate static source other than smashing the VSI.   Is this an aftermarket addition, or do I just know dangerously little about my plane?

[whiskytango]

Just re-read your post....I had assumed this happens in a climb or descent. Is this also in level flight?

Yes it occurs in level flight also.  I had the autopilot engaged in altitude hold, and it was a wild roller coaster ride until I pulled the Alternate Static knob. After doing that, the altitude was steady.

[Marauder]

Sorry to dumb down and divert this excellent discussion for a sec, but I have a '68C and am completely unaware of having an alternate static source other than smashing the VSI. Is this an aftermarket addition, or do I just know dangerously little about my plane?

Later vintage Mooneys are fitted with an alternate air. Mine is on the lower left panel. You should be able to retrofit this valve. Try calling Lasar or the folks at the factory.

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[takair]

Yes it occurs in level flight also.  I had the autopilot engaged in altitude hold, and it was a wild roller coaster ride until I pulled the Alternate Static knob. After doing that, the altitude was steady.

Then N1395W may be right, blockage on one side. The frustrating thing with water and ice is that it can be gone by the time you get to a shop and come back when you most need it to work....in rain. Answer is usually the same, a low point in the lines that allows Ayer to collect. I have seen the same thing with kinked rubber hoses, but they are rare in static systems in a Mooney.

[carusoam]

65C didn't have a static system drain or alternate static source...

Plan B was to empty the dead leg at annual and break the VSI if need arises.

Dev,

How many months do you have now? Got all the gear issues worked out?

Some much to learn...,

[Hank]

Pretty sure our 65 C ... DOES have an alternate static source. It is right next to tne cowl flaps handle.

Wonder if it was an option?

My 70C doesn't have movable cowl flaps. I know I've got Precise Flight standby vacuum, will have to look for alternate static.

[carusoam]

Daver what does the control knob Look like on yours?

Best regards,

-a-

[Hector]

My 67C does not have alternate static source.

[Marauder]

My 70C doesn't have movable cowl flaps. I know I've got Precise Flight standby vacuum, will have to look for alternate static.

I had the Precise Flight stand-by vacuum system removed after I figured out what MP pressures you needed to fly to keep the vacuum in the correct range. It is a good "get down" system but is challenging if you need to rely on it for for steady flight.

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[Hank]

I am mistaken. It says "PULL STBY VAC ON."

Yep, that's what goes to my Precise Flight unit.

[mooniac15u]

I must confess, quick search, can't find anything in owners manual, not sure exactly where this goes to and what it does?

The standby vacuum is an STC'd add-on. There should be a flight manual supplement. It runs off the difference between manifold pressure and atmospheric pressure. It is critical to understand the operational limitations before you try to use it.

[mooniac15u]

Details available here:

http://www.thevacsource.com/svs.php

[Marauder]

The standby vacuum is an STC'd add-on. There should be a flight manual supplement. It runs off the difference between manifold pressure and atmospheric pressure. It is critical to understand the operational limitations before you try to use it.

As Mooniac is pointing out, don't be fooled into thinking this is a complete standby vacuum system. The amount of power reduction needed to provide the differential is pretty sizeable. I will try to find my old placard that I made to show how much power reduction was needed to get acceptable vacuum.

It is much better than having nothing, but is far less capable than an electric standby vacuum pump that the later models sport.

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[mooniac15u]

Exactly! At high altitude and/or cruise power settings it does nothing for you. Fortunately it works best down low at approach settings. If you have to go missed you can say goodbye to your vacuum until you are able to reduce power again.

I've never used mine in actual IMC but I had a vacuum pump fail while VFR and it was a useful experience to see how it behaved at various power settings. If I ever have to use it on an instrument approach I will advise ATC in advance and request a nonstandard missed procedure with a straight-ahead climb.

[mooniac15u]

Unfortunately you have to disconnect your vacuum pump if you want to try it.