ARTIFICIAL HORIZON PROBLEM ?

[john ball]

Not sure if I understand what is happening with my Artificial Horizon in my Vans RV6. It is a standard vacuum unit that is driven by the vacuum pump on the back of Lycoming 0-360 180 BHP. The vaccum gauge reads just over 5 in the cruise at 2200 to 2400 rpm. Twice recently when I have been flying at 2000 to 3000 feet in temperatures of about 6c to 12c, after about half an hour, I notice that it is starting to show a progressive nose down indication, then it slowly over about ten minutes starts to rolls over to the left and steep nose down. As soon as I land and am taxying the unit seems to be correct with a level nose up indication (taildraggerRV6).
This is the second time this has happened, but seems to be only at higher altitude ?? The reason I say this because, it happened on Sunday going over the Channel to LFAT luckily in clear visability, but on the way back I flew at 1500 to 1700ft and it seemed OK - maybe a bit sluggish - not sure. I have previously taken the AH out and put is on a test rig and it seems OK at all extreme angles and the bearings sounded OK according Bill Wunderlich whose test equipment we used, but it was only for a few minutes. So to summerise, why is it after half an hours flying and why at Altitude or is this all coincidences. Thanks John

[S-Works]

I would suspect the gyro is on its way out.

[funfly]

My suggestion is that if you have any doubt at all about your artificial horizon then ditch it and replace with new, it could save your life some day and you must trust it 100%.

[Jetblu]

I experienced similar. As Bose X says and I did the same as funfly.

[A and C]

Before you go rushing into buying expensive bits of kit change the filters in the system, a blocked inlet filter can cause low airflow and poor adjustment of the regulator will mask this.

Fit new filters to both the inlet and regulator, these will only cost a few quid and will eliminate them as the sorce of the problem, once that is done you can investigate any other issues, my best guess is that the bearings in the AI are giving up but as you would want to protect a new AI with new filters it makes sence to change the filters first.

[Pilot DAR]

Diagnosing AH problems is somewhat beyond the scope of PPRuNe posts.

However, check the easy things first:

Is the gyro turning at all? Yes, if the AH indicates at all.

Is the gyro coming up to proper speed? Probably, if it works correctly in some phase of flight.

However, was it ever? Bear in mind that 3 1/8" AH and DG are not compatible with venturi tubes, they must be driven by a vacuum pump. I have seen incompatible setups, where the owner said it stopped working - no, it never worked, it just more doesn't work now.

However, gyro bearings on their way out can be a little difficult to diagnose. A first indicator would be noisy, and gyro winds down to a stop too soon following engine shutdown. If the AH gyro is not making that nice whirr, even after removal of the vacuum, something's wrong. Either it never came up to full speed (air problem), or the bearings are bad (it'll be noisy). This may be linked to a too quick spool down. Expect 5 to 7 minutes to wind down to a stop.

If you think it's an air problem, is there a DG? Is it operating properly?

If the air is good (both vacuum and filter), and the AH is not working, there is little you can do to fix it within "normal" owner capability. Yes, sending it to the shop will approach the cost of a new one. You decide which is better. If you opt for new, they may ask for your core (though I don't know why they think they should get it).

Otherwise, as A and C points out, air problems for a correctly arranged system are very easy to diagnose.

[jxk]

There is also a small filter within the instrument itself. You can see it if you remove the inlet union. Sometimes it is a thin gauze and sometimes foam.

[phiggsbroadband]

Hi... Does anyone know if they can be lubricated with oil? It seems strange that ball-race bearings or just plain phosphour-bronze bearings could last many years, without a drop of oil occasionally.

[A and C]

The bearings for the gyro are a very small ball race and it would seem that they are happy with the very light oil that is applied at manufacture or overhaul.

I would caution about buying overhauled gyros, most seem to last about half as long as a new unit.

The only exception to this have been the gyros I have had overhauled by Skysmart avionincs three of my aircraft have gyros overhauled by Skysmart and there product seems to last as long at factory new gyros.

Skysmart (http://www.skysmart.co.uk)

I have no interest in Skysmart except as a very happy customer.

[S-Works]

I would second Skysmart, they did my AI a few years ago after it developed the same problem as the OP. Good service and reasonable price.

[Whiskey Kilo Wanderer]

Given that it's a Permit aircraft, you could replace it with a Dynon D1. Bin the vacuum system with its related cost, complexity and weight.

[A and C]

An interesting idea but do you really want to hang your life on an uncertified bit of kit ?

It's a bit like my 30 year old Cessna 152 slow , over wieght, expensive to operate, much rather have a Sportcuiser fast, economical and state of the art !

The only problem is that I have seen sportcuiser's with more loose rivets after 300 hours than my Cessna that has 14,000 hours.

They say you can't polish a turd................ But you can roll it in glitter and fool the masses.

[cessnapete]

I second the suggestion of the D1 as a s/by instrument. Dynon have an excellent record of reliability in the LAA field of aircraft worldwide.
We have had one in our C180 for some months, fitted into a spare instrument hole with the Dynon mount supplied.
I would much rather use the D1 with its solid state AHRS horizon, speed and altitude(GPS), than a partial panel approach with an ancient turn and slip.

[Whiskey Kilo Wanderer]

Those of us who fly ‘Uncertified’ aircraft hang our life on them all the time…. In this specific instance at present you can’t legally / officially fly Permit aircraft on the gauges for real anyway.

The OP’s problem seems to be with what might be called certified instrumentation. Given that a proportion of the IR / IMCr syllabus is teaching people how to fly on partial panel when the AI quits, I guess that’s only to be anticipated. The new generation instruments do seem more reliable than the vacuum variety. Our friends in the heavy end of aviation have been relying on them for years.


Drifting the thread a bit, like a wandering AI…

In general with permit / homebuilt aircraft, the range of build and maintenance quality will vary. Thankfully this variation doesn’t step beyond the bounds of safety, before being caught by the inspection system. A builder / owner has more incentive to do a good job, because they will be flying the aeroplane once it’s been competed, unlike the guy on the Cessna production line. Thankfully the PFA / LAA inspection system keeps things reasonably safe.

The comment about loose rivets (not the celebrated Ppruner) in various home built aircraft is fair. My aircraft has just come back from a rebuild following an incident. Some of the rivets were found to have worked over the years. The aeroplane is reputed to be the highest hours homebuilt in the country, admittedly with a mere 3.5K hours on the airframe. It will be interesting to see what happens if we get to the stage of having 14K airframe hour Sportcruisers.

By pure coincidence yesterday I was re-reading Found at Pharisee, the Richard Bach short story in the A Gift of Wings collection. It is about a man who believes totally in the safety of certification. It’s obviously fiction, with suitable exaggeration to make the point.

I still think you are being a bit hard on the Cessna 152, calling it a polished turd rolled in glitter. They are nice old aeroplanes that most of us learned to fly in and remember with great affection…:ok:

[john ball]

OK, thank you for the many answers and thoughts about what to do.

I am going to take out the filter at the weekend and put in a new one. I had already put a new sponge filter on the regulator. But i do not think I have had the real answer as to why it happens after half an hour and at 3000ft as against 1500ft --- or am I missing something in the previous explanations. The AH has been tested on the bench and the suction gauge reads 5.

As for the comments about homebuilts --- they are often the only way some people can afford to fly. Not renting a spamcam at £200 ph !! I do virtually all my own maintence that saves me at least £5,000 a year ? I think the Vans RV6 is one of the best to fly and most efficient with a Lycoming. They are also very docile to fly compared to others. I think 8,000 in the world and 250+ in the UK says something.

[john ball]

OK, now totally sorted out. A good lesson --- the filter had looked fine to me, but on closer inspection the paper had gone to a sort of 'blotting paper' appearance. Thus it was partially blocking and only causing the AH to go funny at altitude. So ---- On Saturday flew with the new filter and what a difference ( but same vacuum gauge reading !!? ) I took the RV6 up to 3500ft and stood it on its ear each way, then pulled up and down in lots of swoops and plummets http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif--- totally perfect and accurate.

All for the price of a cheap filter, that I should have changed at the Annual :ugh:

[phiggsbroadband]

Hi John, which filter did you replace? and where is it located?

Our Cessna has had a similar problem, and I would like to check that out.

Pete

[john ball]

The Filter was in the cockpit above the left side rudder pedals behind the panel.

But in your Cessna it could be in a slightly different position.

The item is made by Rapco and is about 4 inches long and 3 inches in diameter. Looks like a cylindrical concertina of paper.

Easy to fit, jus unscrew the two facing plates, disconect the tubes and refit new one. A five minute job, except in my RV6 you need to be very small and slim to wiggle your way under the footwell. In fact I just took the seats out so I could lie in the aircraft stretched out.