Any vacuum instrument experts out there?

[Curlytips]

Experienced some problems a couple of times. The Artificial Horizon develops a "lean" despite my being straight and level. This eventually corrects itself, but shortly after, the DI started revolving. Suction gauge showed no problems. (Also had more serious precession on the DI than I would expect, recently). Any experts out there with a theory of what's wrong? Any serious suggestions appreciated!

[Pilot DAR]

The gyroscopes are spinning, but not fast enough. And/or, the erecting system in each instrument cannot operate because of low air flow.

Easy thing first, replace the vacuum system filter, and check the air supply side of the system for obstructions. If the air source is obstructed/limited, the vacuum pump will pull, and the indicator indicate an adequate vacuum, but the flow of air at that pressure (vacuum) may not be adequate to run the two gyros.

Next thing, is the whole vacuum system in conformity with the original design? Sometimes components are swapped. they seem to work, but really don't work with each other. If you're running a working vacuum pump, varied gyro instruments should work with it. But, if you've a venturi tube, it will not run the newer smaller gyros properly, a venturi will only run the older, larger "AN" style gyro instruments. Sometimes the older instruments are replaced for new (which is a good idea) but the venturi tube is not replaced with a vacuum pump and system. The whole system must be in harmony.

As both gyros seem to have erecting problems, it's probably not the erecting mechanism in both the instruments. Were it to be only one instrument acting "drunk" and not erecting, and there is no other reason, it should go to the shop, it's erecting air passages/blades are sticking or blocked, and should be corrected. Otherwise, there's not much else to check for, vacuum pumps themselves either work or they don't, and the suction gauge is telling you that!

[Curlytips]

Thanks for that. Driven by regular vacuum pump rather than venturi. Both instruments are original 1969 fit. Both are quite noisy when gyros are running down, but don't think that's the problem. Need to get the system checked. Eventually G5 potential, but need to save up! Regards, Curlytips

[Pilot DAR]

If the instruments are original 1969 (and lots are), have never been rebuilt, and you can hear anything louder than a fine whine as they wind down, plan for the G5 sooner than later. I'm quickly coming to the opinion that it's not really worth the cost to rebuild mechanical gyro instruments. Soon, they will be as welcomed as incandescent light bulbs!

[Romeo Tango]

What PD said

[India Four Two]

plan for the G5 sooner than later.

And the bonus is, you will get an increased useful load! ;)

[double_barrel]

Easy thing first, replace the vacuum system filter, and check the air supply side of the system for obstructions. If the air source is obstructed/limited, the vacuum pump will pull, and the indicator indicate an adequate vacuum, but the flow of air at that pressure (vacuum) may not be adequate to run the two gyros.


When reading for my AGK exam, I noticed that the Pooley's book suggests that a blocked inlet filter would result in reduced vacuum reading. That does not seem right, I would expect an increased vacuum reading (unless the relief valve opens well within the green?) with a reduced airflow. What is typically seen ?

[Pilot DAR]

I noticed that the Pooley's book suggests that a blocked inlet filter would result in reduced vacuum reading.

I'm not sure what a Pooley's book is, or it's authority, so can't comment about that part. Thinking your way through the circuit: The vacuum pump either runs, and produces a vacuum, or it does not - like a household vacuum cleaner. (It is possible for a vacuum pump to be really worn out, and produce less vacuum, but usually they fail before the vacuum drops noticeably.) The vacuum is pulled past a vacuum relief valve, (which has it's own foam filter). That valve is adjusted to prevent too high a vacuum being produced to the instruments. From there, the vacuum source is connected to the attitude indicator, DG, and suction gauge. So the vacuum pump spends it's whole operating life trying to take out air from these three instruments.

To operate, the AI and DG require air to enter, to blow as a jet across the gyroscopes (the vacuum gauge is just a diaphragm, like a very simple altimeter). To the AI and DG, that air comes from within the cabin. It is wise to not draw crap into these instruments, so that air is filtered. The instruments will, however, operate just fine with no filter, other than the risk of becoming gummed up. (AI and DG's have had much longer lives operating in the panel since smoking was disallowed in cockpits, they used to look brown and gummy inside from smoking planes). Considering the filter as not being restricting of the airflow, there is no other restriction between cabin air and the gyro jets inside the AI and DG.

If no air is allowed into the AI and DG (blocked filter), it'll be like holding your hand over your household vacuum cleaner nozzle, the vacuum cleaner will still produce a vacuum, just no flow. The pressure of the air in the hose is reduced to the efficiency of the vacuum pump. Note that "volume" type vacuum pumps (which the plane has) do not produce a high vacuum/low pressure, but rather lots of lower pressure air. The type of vacuum pump used for calibrating altimeters produce a high vacuum, very low pressure, to simulate high altitudes, and could wrap a vacuum system vacuum gauge right around, but would not produce enough flow to run even one vacuum gyro instrument.

So if the vacuum system air filter were to be blocked, no air could enter the AI and DG, and they could not operate for lack of differential pressure across the jets for the gyros. The vacuum pump will still draw air, so it will now come through the vacuum relief valve as needed to maintain the vacuum setting, and the suction gauge will still indicate that. So, the vacuum gauge is not an indicator of flow, or a filter obstruction. The suction gauge will indicate three things: A complete failure of the vacuum pump, as no vacuum at all, a failing pump, as a lower vacuum (Maintenance may be able to reset the vacuum relief valve to capture that last efficiency before it fails entirely), or, a case or hose leak in the instruments or plumbing, which allows air entry on the vacuum side, where it should not be. This could be a reduction in vacuum, or total loss, depending upon the size of the leak.

Most modern flight manuals show a schematic of the vacuum system (usually in section 7), so you can follow the bouncing air molecules yourself.

And, once you have all of this firmly understood, enjoy it briefly, it'll all be useless when airplanes all go electronic!

[Vilters]

These old instrument run on small bearings, and when you hear them clearly after engine shutdown? These small bearings are worn out.
If suffufficient flow? These worn bearings can start to resonate internally and slow the rotors down at random. They go in at out of resonating frequencies. Time for an overhoul.

[Andrewgr2]

I’m amazed to hear of gyro instruments still working since 1969! In my experience 10 years would be a good lifetime!

[Jim59]

In my glider I use a T&S manufactured in 12/1962, overhauled in 2006 and again this year. Works fine. My horizon is also well over 10 years old. I would guess 20+ years old and certainly not overhauled in the 13 years I've had it. Probably last much longer in gliders becasue they don't run continuously - only when needed.

[Pilot DAR]

I believe that the DG in my 150 is original since 1975, and still works perfectly fine. I did have to replace a gyro bearing in the AH about 25 years ago, but no problems since. Most GA electric turn co ordinators are a different matter, they have a finite life, as the motor for the gyroscope has brushes, which wear. When they wear, they also fill the inner workings of the instrument with dust. When I bought my 150 in '87, I replaced the factory turn co ordinator with a brushless one, and no problem for 3000 hours so far. It's worth noting that in most GA SEP planes, the AH and DG are vacuum, where the T&B or turn co ordinator is electric, so as to meet the requirement that there be two distinct power sources for gyro flight instruments. The newest electronic gyro replacement instruments get around this with internal batteries.

The only real problem with very old ('60's and earlier) instruments is that some had radium faces (so they would glow for night flight). That radium is now a hazard if inhaled (as dust) or ingested (as particles on your hands). Though you're allowed to leave these instruments in service if they're in the panel, nearly no instrument shop is allowed to work on them, and DIY people most certainly should not. When this came under regulation, I went through my, and two other friends stores of instruments, and disposed of the suspect instruments to the proper radioactive materials disposal. The owner of the instrument shop I used to work for died of cancer related to radium ingestion, so this should not be taken lightly. If in doubt, never disassemble an instrument, and don't put old ones on display on your mantle, where the kids can get at them.

[scifi]

I used to work on some very intricate apparatus, which required only a very minute amount of lubrication oil, which was applied by a dampened brush.
So I suppose that the same applies to gyro bearings, they will need regular servicing, with a miniscule amount of lubrication.
Also each Gyro will have it's own inlet gauze filter which should not be blocked by dust.
Some of my friends worked at RAF Sealand as Instrument Technicians, but they would not even tell you if it was raining, in case they fell foul of the Official Secrets Act.

[Pilot DAR]

which required only a very minute amount of lubrication oil, which was applied by a dampened brush. So I suppose that the same applies to gyro bearings, they will need regular servicing, with a miniscule amount of lubrication.

'Best to maintain the bearings as specified by the instrument manufacturer. The aircraft gyroscopic instruments I was trained to overhaul did not use oil on the bearings, but rather Aeroshell #6 grease. The amount of grease to be added to an instrument ball bearing was to be a drop about the same size as one of the balls for the gyros I worked on. There was no specified time in service to replace the grease, it was on condition.

Also each Gyro will have it's own inlet gauze filter which should not be blocked by dust.

Yes, the old large "AN" style gyro instruments, the ones about 6" cube in size, with a smaller round window, did use a gauze paper filter, and yeah, they tended to be filthy, particular in smoker's planes. If one of these older AN gyros requires service, it's worth considering replacing, before even spending the cost to take it to the instruments shop. Most of these were radium dials, and the instrument shop either will not touch them, should not touch them, or will charge a fortune to strip and repaint the dial (vintage warbird originality type stuff). It's not worth the cost and trouble unless you're trying to maintain an original panel appearance. The newer 3 1/8" gyros do not have their own filter, and require a central filter on the air inlet. Again, maintain in accordance with the manufacturer's instructions.