Newly rebuilt DG precessing

[BigBadLou]

I just paid through my a** ... err .. nose ... to replace my DG with a rebuilt unit.
The original reason was due to unacceptable precession rate (had to adjust it after every U-turn or after a few minutes).

Flying with the newly rebuilt DG, I do not see any difference, it precesses "like crazy", i.e. 10-15 degrees after a U-turn and easily 30 degrees after a turn (standard rate). Again, that is unacceptable.

I spoke to the shop that replaced the DG. The owner said that it is not unusual that a rebuilt DG will precess badly and needs to be rebuilt again. To my question "don't they test the rebuilt units before they release them back to the customer?", he replied "they should".

So question #1:
If the unit was rebuilt and tested good at the avionics shop, how can it become "double plus ungood" within a few flight hours? (extra points for catching the reference)

The filters are clean and changed regularly, the vacuum pump has <300 hours, vacuum gauge shows correct readings (per POH).
I cannot give you information on the location and direction of all the hoses. I can only assume that if the vacuum gauge shows good, the DG and AI have enough airflow.
The AI works without a glitch, btw.

So question #2:
Could the plumbing be the cause of DG precession?

Any advice and information is appreciated.

 

[Henning]

I just paid through my a** ... err .. nose ... to replace my DG with a rebuilt unit.
The original reason was due to unacceptable precession rate (had to adjust it after every U-turn or after a few minutes).

Flying with the newly rebuilt DG, I do not see any difference, it precesses "like crazy", i.e. 10-15 degrees after a U-turn and easily 30 degrees after a turn (standard rate). Again, that is unacceptable.

I spoke to the shop that replaced the DG. The owner said that it is not unusual that a rebuilt DG will precess badly and needs to be rebuilt again. To my question "don't they test the rebuilt units before they release them back to the customer?", he replied "they should".

So question #1:
If the unit was rebuilt and tested good at the avionics shop, how can it become "double plus ungood" within a few flight hours? (extra points for catching the reference)

The filters are clean and changed regularly, the vacuum pump has <300 hours, vacuum gauge shows correct readings (per POH).
I cannot give you information on the location and direction of all the hoses. I can only assume that if the vacuum gauge shows good, the DG and AI have enough airflow.
The AI works without a glitch, btw.

So question #2:
Could the plumbing be the cause of DG precession?

Any advice and information is appreciated.

Question 2 answers with a resounding yes if there are rotten hoses leading to the DG, either side really as one loses vacuum, and the other contaminates the gyro.

 

[Silvaire]

The first question is: did the original unit always have this problem or is it something it just started doing? Vacuum gauge doesn't tell you everything about what's going on.

 

[Clark1961]

Are you sure it's the HI? My compass went wonky on me and I thought it the HI had a problem.

On to the other questions, supposedly shipping can be rough on a HI. If it gets dropped then the bearings may be damaged. I have no idea how much truth there is to that story.

 

[Henning]

Are you sure it's the HI? My compass went wonky on me and I thought it the HI had a problem.

On to the other questions, supposedly shipping can be rough on a HI. If it gets dropped then the bearings may be damaged. I have no idea how much truth there is to that story.

I would think it would take more than a roughly handled box to hurt the gyro. They do see some rough handling in the plane on occasion.

 

[Silvaire]

Had an AI seemingly go bad once, vacuum gauge showed in the green. Turned out the filter had gotten wet during a very heavy rain storm.

 

[UngaWunga]

Who rebuilt it?

 

[brien23]

Rebuilt or new, rebuilt is cheaper but you get what you pay for. It also has to do with who made it some are better than others for rebuild. If you start with a cheap gyro and rebuild it you end up with a cheap rebuild gyro, just how long do you expect it to last.

 

[Henning]

Rebuilt or new, rebuilt is cheaper but you get what you pay for. It also has to do with who made it some are better than others for rebuild. If you start with a cheap gyro and rebuild it you end up with a cheap rebuild gyro, just how long do you expect it to last.

If the shop will let it out the doors without an express caveat on the order of, "We did what we could, but it's not long for life." then I expect 5 years/1000hrs out of a pneumatic gyro overhaul. If the internal parts are worn, they get replaced.

 

[BigBadLou]

Question 2 answers with a resounding yes if there are rotten hoses leading to the DG, either side really as one loses vacuum, and the other contaminates the gyro.

If the hoses leaked, they would allow air to be pulled into the vac pump and the gauge would not show in the green, IMO.
There have been at least 2 mechanics inspecting behind the panel this year and reported no rotted plumbing but I can take a look myself to check the hoses.

The first question is: did the original unit always have this problem or is it something it just started doing? Vacuum gauge doesn't tell you everything about what's going on.

The original unit was fine until about this spring when it slowly started precessing more than usual. Hence the rebuild.

Are you sure it's the HI? My compass went wonky on me and I thought it the HI had a problem.
On to the other questions, supposedly shipping can be rough on a HI. If it gets dropped then the bearings may be damaged. I have no idea how much truth there is to that story.

Correct, we checked and the compass is good, the HI/DG is the one that shows incorrectly.
The rebuild was local so I hope nobody dropped the DG on the floor after the rebuild. That would leave a mark on the bearings!

Had an AI seemingly go bad once, vacuum gauge showed in the green. Turned out the filter had gotten wet during a very heavy rain storm.

Filters are new from annual but I will double check them.

Who rebuilt it?

A reputable local avionics shop. However, how does one quantify "reputable"?
They have the unit now, testing it first, supposedly. Haven't heard anything in 2 days so my assumption is that they are rebuilding it again.


Thank you for the feedback, gentlemen. I will let you know the outcome.

 

[Henning]

If the hoses leaked, they would allow air to be pulled into the vac pump and the gauge would not show in the green, IMO.
There have been at least 2 mechanics inspecting behind the panel this year and reported no rotted plumbing but I can take a look.

It depends where the leak is in relation to the gauge pick up, just something to peek at, or really best, feel. You can feel bad rubber easier than you can see it often. It all looks nice, but turns your fingers black...

Another thing to look at that causes gyros to precess is the mounting. Is there a chance the panel is tweaked slightly and that by tightening the screws you are causing a slight distortions in the instrument case? One thing to try is backing off the screws slightly and seeing if that has an effect on the precession.

 

[BigBadLou]

It depends where the leak is in relation to the gauge pick up, just something to peek at, or really best, feel. You can feel bad rubber easier than you can see it often. It all looks nice, but turns your fingers black...

I will check myself this week.

Another thing to look at that causes gyros to precess is the mounting. Is there a chance the panel is tweaked slightly and that by tightening the screws you are causing a slight distortions in the instrument case? One thing to try is backing off the screws slightly and seeing if that has an effect on the precession.

I will ask the shop about that.

Thank you.

 

[GlennAB1]

Gyro's precess, or drift. Here's a pretty good read, scroll down to "Operation"
https://en.wikipedia.org/wiki/Heading_indicator

 

[Silvaire]

Precession is actually a very complex thing. In my Sperry textbook "Navigational instrumentation for Aerospace Vehicles" from The Spartan School of Aeronautics it is described as being "beyond the scope of this text"

 

[GlennAB1]

Yeah, should have the Northrop book.

This one:

https://openlibrary.org/works/OL7134799W/Electricity_and_electronics_for_aerospace_vehicles

 

[Henning]

Perhaps the OP could quantify the rate of precession? I am assuming he understands that there is inevitable precession and that every 15-20 minutes you need to tweek a few degrees unless you have a slaved unit. I was assuming he's seeing something on the order of >5° every 5-10 minutes which most people including myself would consider a problem and seek a solution. Personally I would use it as an excuse to install a G-500.

 

[BigBadLou]

To repeat from my first post, the precession is easily 30 degrees after a 360 turn or 10-15 degrees after a U-turn. (flying timed turns in holds)

That, in my book, qualifies as excessive precession.

Didn't get to go see the plane today and couldn't reach the shop. I will try tomorrow.

 

[Dan Thomas]

Dropping a gyro one inch onto the workbench can impart enough shock to the bearings to damage them. They tell you to "handle like eggs" for good reason.

Did this airplane have a vacuum pump fail in flight? Broken vanes and rotor and all that? The vacuum in the instruments can suck carbon debris all the way back from the pump and contaminate all the hoses and instruments. Connecting a new instrument to a system like that is just pouring money down a hole.

 

[Henning]

Dropping a gyro one inch onto the workbench can impart enough shock to the bearings to damage them. They tell you to "handle like eggs" for good reason.

Did this airplane have a vacuum pump fail in flight? Broken vanes and rotor and all that? The vacuum in the instruments can suck carbon debris all the way back from the pump and contaminate all the hoses and instruments. Connecting a new instrument to a system like that is just pouring money down a hole.

If that's all it takes, how the hell do the things stand up to turbulence and student landings?

 

[Dr. O]

My DG is an AN - a great big old derelict that looks like hell and works like heaven.
I'm so used to it that when I fly other planes I am always caught by surprise when I notice the DG heading has gone off in the weeds.

Anyway, mechanical gyros are obsolete.

 

[jsstevens]

If that's all it takes, how the hell do the things stand up to turbulence and student landings?

If the bench is a hard surface that's all it takes. Acceleration/Deceleration is speed change over time and a metal on metal contact is a _very_ short time.

The structure in the airplane will take some of the immediate shock by flexing.

I worked with a set of classified computers where we had to store the removable hard drives in a safe. We had multiple hard drive failures until I put a thin sheet of foam down inside the steel drawer where we stored the hard drives. The metal on metal shock of setting them down was killing them.

John

 

[Henning]

If the bench is a hard surface that's all it takes. Acceleration/Deceleration is speed change over time and a metal on metal contact is a _very_ short time.

The structure in the airplane will take some of the immediate shock by flexing.

I worked with a set of classified computers where we had to store the removable hard drives in a safe. We had multiple hard drive failures until I put a thin sheet of foam down inside the steel drawer where we stored the hard drives. The metal on metal shock of setting them down was killing them.

John

A drop of 1" will take approx .07 seconds and reach a velocity of ~2.4fps. Let's figure it takes .03 seconds to come to a stop gives you ~2.5g. I just can't believe that spec on a gyro going into an aircraft is going to be that shoddy.

 

[jsstevens]

A drop of 1" will take approx .07 seconds and reach a velocity of ~2.4fps. Let's figure it takes .03 seconds to come to a stop gives you ~2.5g. I just can't believe that spec on a gyro going into an aircraft is going to be that shoddy.

You're off by at least an order of magnitude on your deceleration time. (Did some quick googling of studies on these kinds of impacts.) Probably 2 orders of magnitude. If it's .0003 now you've got 250g.

John

 

[Henning]

You're off by at least an order of magnitude on your deceleration time. (Did some quick googling of studies on these kinds of impacts.) Probably 2 orders of magnitude. If it's .0003 now you've got 250g.

John

I can't imagine anything that fragile making certification for a life critical airplane installation.

 

[Dan Thomas]

I can't imagine anything that fragile making certification for a life critical airplane installation.

If you've ever had one of those things apart you'd be amazed at how tiny those bearings are. A shock could easily Brinell the bearings. The airplane NEVER imparts that sort of shock to the instrument, even in the worst turbulence or hardest landing. You'd have to fly it into a stone cliff to do it.

 

[Henning]

If you've ever had one of those things apart you'd be amazed at how tiny those bearings are. A shock could easily Brinell the bearings. The airplane NEVER imparts that sort of shock to the instrument, even in the worst turbulence or hardest landing. You'd have to fly it into a stone cliff to do it.

They take knocks and smacks during maintenance that would easily replicate a 1" drop.

 

[Dan Thomas]

They take knocks and smacks during maintenance that would easily replicate a 1" drop.

I never treat them like that, and they last a long time. Some owners might be replacing gyros regularly because their mechanics are rough with them. Moreover, a mechanic would have to be really rough to replicate a one-inch drop onto a solid table.

Starting them up in cold weather is also hard on them. The tiny bit of very fine grease in those bearings hardens in cold weather and gets squeezed out so that the bearings are dry for a while until the airplane warms up inside. A cabin preheater is a big help, or a heated hangar.

 

[Dan Thomas]

Here are some interesting numbers relating to the handling of gyro, both when they're installed and when they're not.

https://www.casa.gov.au/sites/g/files/net351/f/_assets/main/airworth/awb/31/008.pdf

A quote, from page 3 of the above:

Gyro instruments cannot withstand the shock of being dropped, jarred, or
struck by pieces of equipment. Shocks cause the ball bearings to indent the
race and brinell (indent) the bearings. Releasing one end of a typical gyro
instrument from about 7mm (¼ inch approx.) onto a hard surface can result
in something like a 14G shock to the gyro and brinell the bearings and result in
vibration, accelerated bearing wear, loss of gyro RPM and excessive heading
drift in directional gyros. While some directional and vertical gyro designs may
incorporate vibration isolation absorbers, the rate gyro (turn and bank) may
not, making it particularly sensitive to even light handling shocks.

 

[BigBadLou]

UPDATE:
Went to check on the plane last night. The instrument shop tested the gyro and returned it as "good" so the shop put it back in the airplane.
Using a bright flashlight, I climbed up (down?) under the panel and checked all the hoses. All are stiff, none brittle, squishy or leaving residue. So the hoses seems pretty darn good. (no, I did not remove a hose to check the inside)
Filters are brand new from the annual, as I mentioned earlier.
The vacuum gauge feed seems to come from the Y that connect the AI, DG and pump hoses and the hose itself is much thinner and seems to be in good order as well.

One thing I was surprised at were the massive hoses. I understand that they need to be rigid to not collapse under the 5" of vacuum but I think we now have much better materials for vacuum hoses that are less bulky and also much lighter. What I saw under the panel looked like the oil-cooler hoses.

I have another IR lesson tonight so we will check the precession again and see if it has changed. I will report then.

 

[BigBadLou]

Starting them up in cold weather is also hard on them. The tiny bit of very fine grease in those bearings hardens in cold weather and gets squeezed out so that the bearings are dry for a while until the airplane warms up inside. A cabin preheater is a big help, or a heated hangar.

Good point. However, I have never had a problem in winter time. And we are still in the 70's here in TX so we should be good on grease.

 

[AggieMike88]

Lou, there is one silver lining....

Think of all the practice you are getting doing IFR maneuvers with partial panel.....

 

[Henning]

When you held the gyro up in place behind the panel, could you get it to wobble at all, or was it solid flush and true?

 

[BigBadLou]

Lou, there is one silver lining....

Think of all the practice you are getting doing IFR maneuvers with partial panel.....

When you held the gyro up in place behind the panel, could you get it to wobble at all, or was it solid flush and true?

Negative, I did not touch/remove/install the gyro, our on-field shop is doing that.
I would assume the mechanics would know what to look for when a customer is complaining about a malfunctioning gyro. But then again, what do I know. If airplane mechanics are anything like car mechanics, we are all doomed, DOOMED, I tell ya!

All kidding aside, the shop owner is a very nice gentleman, I will be talking to him more today. If the gyro still precesses badly tonight, I will be leaving the plane with them to troubleshoot for a full week and see what they find.
I wonder, would taxiing in circles be enough to get the gyro to precess? Though I don't think there will be enough air flow for the gyros since the pump produces only 2" (or so) of vacuum at idle.

 

[Henning]

Negative, I did not touch/remove/install the gyro, our on-field shop is doing that.
I would assume the mechanics would know what to look for when a customer is complaining about a malfunctioning gyro. But then again, what do I know. If airplane mechanics are anything like car mechanics, we are all doomed, DOOMED, I tell ya!

All kidding aside, the shop owner is a very nice gentleman, I will be talking to him more today. If the gyro still precesses badly tonight, I will be leaving the plane with them to troubleshoot for a full week and see what they find.
I wonder, would taxiing in circles be enough to get the gyro to precess? Though I don't think there will be enough air flow for the gyros since the pump produces only 2" (or so) of vacuum at idle.

Holy crap no. I literally had a shouting match with one of the best twin Cessna shops in the country over a turbo problem to get them to pull the Vernatherm because I worked out what was causing the issue was a contaminated oil cooler. If my old IA boss hadn't been there to step in and say "Listen to him, he's really good at diagnostics." it would have gone on longer and probably disinvolved the shop to get repaired.Your average mechanic has very little diagnostic skill once you eliminate the basics, and most only know how to fire the parts gun at things to figure out what's broke. As soon as things get complicated, you need to put your own brain to work.

 

[BigBadLou]

That's what I was afraid of.
I will keep y'all posted.

 

[Kiddo's Driver]

If the shop will let it out the doors without an express caveat on the order of, "We did what we could, but it's not long for life." then I expect 5 years/1000hrs out of a pneumatic gyro overhaul. If the internal parts are worn, they get replaced.

Warning, thread creep...
What should you expect out of a NEW gyro instrument? I ask for two related reasons.
1st: I put a new attitude indicator in my Tampico about 4.5 years ago.
2nd: When I go to replace instruments in the future the expected life of the gyro should/will play into the math on going to some glass.

-Jim

 

[Henning]

Warning, thread creep...
What should you expect out of a NEW gyro instrument? I ask for two related reasons.
1st: I put a new attitude indicator in my Tampico about 4.5 years ago.
2nd: When I go to replace instruments in the future the expected life of the gyro should/will play into the math on going to some glass.

-Jim

I think 10 years and/or 1000+ hrs is a pretty common average from what I've noticed inspecting log books, but I haven't really been paying more than cursory attention to it. It does parallel my personal experience though. If you fly aerobatics, these numbers are not relevant.

The cost of a gyro is inconsequential in the decision to go glass. If you are repairing an old King mechanical HSI which gives some of the benefits in information as with glass, then financially you can justify the glass on cost comparison. With a simple gyro there is no way to create a financial justification or even argument, I can operate and replace pneumatic gyros for the rest of my life and not spend what the install of the glass costs, much less the actual equipment.

The argument on glass or not is won on the value of the extra information received. "Is that extra information, and the new delivery system of the information worth it?" That's a question only you can answer, but my answer after my first IPC with a G-600 and SVT was a resounding '**** yes, this is worth even more than the high asking price." which is why the next plane I bought I dropped off after my first flight to have the minimum equipment I needed to get the information delivery system I wanted, a G-500, 430w, and JPI engine monitor with fuel flow, all coupled together so I could optimize my speed using the GPS coupled to the fuel flow using a real time in flight automated calculation. Fuel is the leading mistake pilots make that gets them killed. In IFR conditions when you are dealing with distractions and problems, The SVT method of delivering attitude and situational awareness leaves you a hell of a lot of spare processing time for the problems by making processing the information to fly the plane with so much faster because the panel now takes over the process of turning multiple numeric and 2D visual data streams (both instruments and charts) into a 3D composite in real time which formerly pulled resources from your brain's ability to work on the other problems. First time I flew with AV and the G-600, I knew Imwould not own another traveling plane without it.

The only people I know who marginalize the result of SVT is ATPs and such who fly 1000 hrs a year IFR and like to fly approaches by by hand.

 

[Tom-D]

If that's all it takes, how the hell do the things stand up to turbulence and student landings?

1" to a metal bench will generate 100g, ever see that in flight.
The bearings in that gyro will have been brinelled (sp) and never operate correctly again.

 

[GlennAB1]

It's probably way more complicated... if the gyro is spinning, is the "shock" load distributed differently in the bearing, verses, if the gyro isn't spinning?

 

[BigBadLou]

UPDATE:
Went for an instrument lesson last night. Gyro not gyrating, artificial horizon not artificially horizoning, suction gauge not sucking and on top of that, the TB not TB-ing. (it is electric, btw - I have a red flag but no popped breaker)
Monkeys, I tell ya, complete monkeys.
I checked hoses, clamps, connections, nothing out of the ordinary behind the panel. *shrug*

So I canceled the lesson, logically. At least I got a good ground lesson out of it with a knowledgeable CFII (who also used to be a DPE). Having feedback from the "other side" sure helps.

I spoke to the shop owner and he said they'll look into it.
Given the progress of events, I am not sure what to expect.
1) Gyro was precessing a little too much.
2) Shelled out a grand for a rebuild. (used to be $300 just last year)
3) Gyro started precessing much more.
4) Had gyro checked.
5) Gyro does not work anymore. AI dead. Suction gauge says 0". And the TB is dead.
With this progression, I wonder what else will be dead the next time I climb into the cockpit.

<rant>
See, kids, THAT is exactly why I normally do things myself. People do not understand when I say "why should I pay somebody a lot of my hard-earned money to screw it up if I can screw it up myself for free??" *shrug*
I restored my car. I rebuilt my transmission. I rebuilt and installed a different rear axle. I fixed the electronic tachometer and speedometer. I designed and built my engine. I reprogrammed the ECU for better power and economy. I welded my own exhaust. But I just can't fix my own damn airplane.
</rant>