Replacing faulty directional gyro

[JoeFFG9]

Hello. I'm co-owner of a 1974 Mooney M20F. Currently, the avionics are still steam gauges with the directional gyro and attitude indicator. Also have a GNS 430W and an STEC 30 auto pilot. The current plan is to eventually replace the system with either glass or near glass (GI 275s to replace the vacuum gauges).  We were hoping to have the current avionics work for a couple more years but now I'm seeing issues with the directional gyro.

This recently happened while entering the pattern for landing. The directional gyro started to spin like a top. Once landed, I was taxing straight and the gyro was doing this: (The low vacuum light was on but the rpms were near idle so that is expected. The attitude indicator was not tumbling. Question: Is this a bad gyro and if so, what would be the best option? 1) Replace with same type of refurbished gyro? 2) Send out for refurbishment  3) Replace with Gi275?   .  I'm leaning towards #3 since that will start us on the path of replacing the avionics (gradually) but can still be used with STEC30 and GNS430W.   (I'm looking to use this plane for IFR training so would need something that can be used in real IMC.)   Thanks.

 

[DCarlton]

I'm no expert but I had trouble with mine too.  Did you check to see if the two spring loaded knobs/shafts were sprung back out into the correct position and not binding anywhere on the panel?  Mid Continent overhauled mine; the thing came back like new.  

[M20F]

I like G5’s personally and find the 275’s small and cluttered.  
 
The answer to your three questions though is how much money do you have?  You will get almost 100% back out of an overhauled vacuum DG.  You might get 50% on a G5/Gi275.  
 
These questions  are always three sided.  Dollars versus capability versus resell. 

The fact you are a co-owner means a transaction is in your future.  You will generally lose on that transaction, my advice would be spend as little as needed. 

[Rwsavory]

Especially if you’re going to be flying IFR, get a G5 or GI275.  I have been flying with a G5 HSI now for a few months, and I don’t see any reason to get the fancier model. If you have the money for it, get the G5 AI as well. Then you can delete your vacuum system, and you will have full attitude redundancy.

[JoeM]

Another option (which we just used to keep our plane flying while we get ready to install dual GI 275s) is to buy one of the many serviceable DGs available on the used market which other owners have removed to upgrade. This would keep your plane IFR capable, reduce downtime, reduce expense, and give you and your partners time to make future decisions. 

[AndreiC]

If it was tumbling and spinning like crazy I think it may be more likely the vacuum pump. The AI does not need to go crazy. Easiest to check would be to turn the engine back on, revv it up to 1500-1800 rpm, and look at your vacuum gauge. If it is not showing 5” your pump is toast. 
I was in your situation not long ago, and chose to replace my DG with a good used one. (In my case it was the DG that was bad, but only in the sense that it had a lot of precession). I paid about $400 for a used one, and compared to the cost to install a G5, especially as a DG to interface with the autopilot, I felt it was a steal. But I am happy to stay with steam gauges for a while, people have been flying with them for decades and planes were not falling put of the sky because of this. 

[N201MKTurbo]

The cheapest thing you can do is get the gyro overhauled. The cheapest replacement would be a G5. I think the G5 will work with the STec autopilot. 
 

I replaced my DG with a G5 HSI so I would have a legal IFR indicator for my Avidyne. I have had it for 8 years now. It works great and you get GPSS which is kind of majical. 

[Yetti]

I have an RC Allen DG out of a 75 F model that was working when removed.  $100.00 plus shipping.   Does not have a heading bug.

[M20F]

38 minutes ago, N201MKTurbo said:

The cheapest replacement would be a G5. I think the G5 will work with the STec autopilot. 

As of Wednesday it worked with mine. 

[markgrue]

My opinion:  Do not piecemeal avionics.   You will pay 2 or 3 times the labor.  Replace the dg or more likely the vacuum pump.   Save up and do all avionics at once.  If you are set on the garmin stuff get the 275. The G5 will not display traffic when you get that far.

[JoeFFG9]

2 hours ago, AndreiC said:

If it was tumbling and spinning like crazy I think it may be more likely the vacuum pump. The AI does not need to go crazy. Easiest to check would be to turn the engine back on, revv it up to 1500-1800 rpm, and look at your vacuum gauge. If it is not showing 5” your pump is toast. 
I was in your situation not long ago, and chose to replace my DG with a good used one. (In my case it was the DG that was bad, but only in the sense that it had a lot of precession). I paid about $400 for a used one, and compared to the cost to install a G5, especially as a DG to interface with the autopilot, I felt it was a steal. But I am happy to stay with steam gauges for a while, people have been flying with them for decades and planes were not falling put of the sky because of this. 

Interesting. I need to look at this. I know during run up at 1700 rpm, the gauge is below the green area at around 4.2”. Thanks

[N201MKTurbo]

1 hour ago, Yetti said:

I have an RC Allen DG out of a 75 F model that was working when removed.  $100.00 plus shipping.   Does not have a heading bug.

He has an autopilot. It needs a DG with a heading bug and an autopilot pickup coil.

[Yetti]

1 hour ago, markgrue said:

My opinion:  Do not piecemeal avionics.   You will pay 2 or 3 times the labor.  Replace the dg or more likely the vacuum pump.   Save up and do all avionics at once.  If you are set on the garmin stuff get the 275. The G5 will not display traffic when you get that far.

Has anyone done a feature comparison between the G5 and the 275?

I believe the only vacuum instrument in the Eagle is the Artificial Horizon.    Was thinking about replacing it and pulling the vacuum.

[AndreiC]

1 hour ago, JoeFFG9 said:

Interesting. I need to look at this. I know during run up at 1700 rpm, the gauge is below the green area at around 4.2”. Thanks

Unless you have a wet vacuum pump (unlikely, from what I know), the dry pumps either work or they don’t. When they fail they fail catastrophically. So if it still shows 4.2” it means the pump is probably still good. 

[N201MKTurbo]

On the old Cessna I just sold, it had Venturies. At run up it would make about 1” of vacuum. It would get the DG going.

[Matthew P]

Have you found a DG, I have one from a recent avionics upgrade $75.00 plus shipping (Cessna 3110C No Heading Bug) shipped from 35757

[JoeFFG9]

27 minutes ago, Matthew P said:

Have you found a DG, I have one from a recent avionics upgrade $75.00 plus shipping (Cessna 3110C No Heading Bug) shipped from 35757

Would need a heading bug for auto pilot. Thanks.

[Bolter]

When faced with the same decision years ago, I went with the G5 (had 430w and KAP150) and had no regrets on costs or function.  Assuming you have to pay someone to do the work, it does not take long before labor costs make a do-it-once solution the cost competitive option.  

-dan

[DCarlton]

5 hours ago, JoeFFG9 said:

Interesting. I need to look at this. I know during run up at 1700 rpm, the gauge is below the green area at around 4.2”. Thanks

Is it remotely possible you have a loose hose or a vacuum leak?  I would personally do a little basic troubleshooting before I started ordering parts or removing the DG.  It’s a monster to remove.   Check the DG knobs/shafts to make sure they’re not binding on the panel (mine were at one point); if the clutches don’t engage when the knobs are released (at least for the heading bug) weird things happen.  Check to make sure you don’t have any loose or cracked hoses etc (I’ve had both over the years).   

[M20F]

11 hours ago, Yetti said:

Has anyone done a feature comparison between the G5 and the 275?

I believe the only vacuum instrument in the Eagle is the Artificial Horizon.    Was thinking about replacing it and pulling the vacuum.

G5 vs GI275

Personally I find the GI so small that all the stuff it does just clutters it.  Plus I really don’t need anything other than an HSI and ADI.  I don’t care for any of the other stuff (that’s me).  G5 is solid and clean.  

[47U]

22 hours ago, JoeFFG9 said:

The directional gyro started to spin like a top

 

10 hours ago, DCarlton said:

I would personally do a little basic troubleshooting before I started ordering parts or removing the DG.

I had a DG doing that once upon a time… coming out of an annual inspection.  Except, mine was rotating CCW.  The vacuum hoses were hooked up backwards on the DG.  

[vik]

What about NSD-360 (or NSD-1000 if you gyro is electric)? It will work with GNS430 and STEC30, cost much less than 275 or G5, but a bit more than having your gyro overhauled. This is assuming you have enough space behind the panel and is willing to swap a dozen of wires from your CDI, which you would be able to pull out and sell. HSI is much better for IFR flying anyway.  Let me know if you are interested. I have both units available for sale.

G5 is a better unit, but realistically, you are looking for double installation at $10K + after all said and done.

A singe GI 275 in HSI config will set you back close to $5K after taxes + same for the installation probably.

The cheapest option is still to have your DG overhauled. 

Vik