Attitude Indicator Failure

[Charles Schefer]

I got to have a little partial panel fun Sunday afternoon when my Attitude Indicator (or to date myself and for those that prefer - "Artificial Horizon") tumbled as I was starting to descend thru an overcast layer. I was quite comfortable with it except for the headache and expense of fixing the problem

We (my DOM/IA and I) have now pulled the AI out and I'll probably just take it over to the Part 145 Instruments & Avionics shop on the field for overhaul. I did look on Spruce to see what a newer generation AI would cost - tho in my case the AI drives my Altimatic III autopilot so it would need to be compatible. As it stands I'm betting a simple overhaul is probably the best option.

It's funny... I was not really shocked by the failure at all because on the same morning I'd a couple times thought the AI looked ever so slightly off by showing wings level when the wings were ever so slightly banked. Then the autopilot (Altimatic III) wanted to turn off course slightly when the AI was level and the aircraft was already perfectly on course (i.e. - why turn?). These oddities were so slight and subtle that I wasn't convinced I was seeing it correctly so I just kept a close eye on it.

Another interesting point... on the last few flights my autopilot has been "porpoising" in altitude hold mode. It's always been finicky about setting the altitude with the fussy pre-select knob but once set it would always hold quite well. Last few flights I've been hand-flying at least the altitude hold because of the porpoising. Yesterday I searched the forums and saw a post from Pat commenting on her C2000 porpoising when the AI was on the way out (if I recall the post correctly).

Anyway... any thoughts or feedback on this topic from those who have been down this road before would be appreciated.

Best Regards,

- Charles

[N3322G]

Yup. My C2000 gets its input from the AI - if one is unhappy, so is the other.

[Don Nelson]

Mine too, failed a couple months ago. Sometimes slow to come alive on start up, sometimes very minor porpoising, sometimes wandering a bit on heading.
Same thing happened on my single with same autopilot several years ago.
Nothing to do but overhaul or replace.
Don

[Richard Lanning]

Get a firm quote on the overhaul before you commit. Mine went out a month or so ago. The overhaul cost about as much as a refurbished replacement. In my case though, a refurbished replacement would have been a much better option since my installed AI is an older style.

[William Hughes]

My old AI is also acting rather odd sometimes. It will sometimes stand up immediately, and sometimes oscillate wildly for several minutes before stabilizing on a strong bank and slowly standing up. Once it is up it will behave nicely, other than occasionally taking a slight lean that corrects itself after a few minutes.

The vacuum pump is new and the relief valve was checked and set. The filters are the canister type right on the AI and I haven't been able to look at them. The only other instrument on the system is a DI and it is new.

I was planning on having it removed and inspected during the annual in a few months, and then probably replacing it unless there is something obvious. Too bad, really. It has a really nostalgic B17 look about it that suits the aircraft.

[N3322G]

Another thought for those looking at AI is to be sure to replace all hoses from vacuum pump to instrument. These areas, instrument to firewall, firewall to valve, valve to pump was often overlooked. When we got around to this, we found multiple out of spec hose types and can you say brittle? Can't prove it but I suspect the hose debris caused double vacuum pump failure ... quit fun to fly that way ... as long as it was VFR and I was headed to the shop.

[Charles Schefer]

Thanks to everyone for all these comments and replies. Since my AI is tied to my autopilot I can't simply go out and buy a new one. My choices were really to do an overhaul exchange or to overhaul the one I have. I decided to do the latter. I took the unit around to Capital Instruments here on the field at HEF. Tom Kellet at Capital has been overhauling these types of gyros for over 35 years. It was fascinating to talk to him about the inner workings of the unit as he started disassembling it in front of me.

Tom told me that my unit was not an Edo-Aire unit (what he assumed I would have) but in AIM attitude indicator. He said all the Century AIs (whether via Edo-Aire or AIM) are extremely robust and well built. It was stamped 1993 (I think that's what he said) so it's obviously not the original but it did look old and dated. He said the AIM units are a little more involved to overhaul (vs Edo-Aire) but have the added advantage of having a built in filter in the back (which he showed me). My filter had carbon in it which he said was odd because that is more indicative of an AI that's been in a pressure system not a vacuum system. Vacuum pumps have carbon blades so if one fails in a pressure system the result is often carbon in the unit filter. It may be that this unit was in a pressure system before being put in my plane - all before I owned it.

It took a few days but Capital did a very nice job overhauling the unit. It has been completely rebuilt with a new, balanced pendulous mass, all new bearings and a new filter. They also offered to repaint the face which has made it look much more modern as you can see here (below). Not sure the sky blue should be "gloss" vs flat paint but nevertheless it looks very good. I've only ground tested it because I am doing other maintenance but I'll test fly in VMC later this week.

My AI after OH by Capital Instruments

Total cost for the OH was $800.

- Charles