Autopilot Debate

[Charles Schefer]

I've researched the forums extensively on this topic and most of the posts I've read have been helpful but are now fairly dated so I think it's worth asking for some present-day advice...

My PA-30 has an Altimatic III Autopilot. Here's a pic of the control head:

Altimatic III

Also related - it has a selector switch to govern what heading / nav source it's tracking and just under that is a separate on/off switch and trim adjustment for the backup "Autoflite" system. Here's another pic:

Altimatic III Selector & Autoflite

I've read the book several times and experimented with all this in flight. Basically the Autopilot itself is Attitude based and is based on the vacuum driven attitude indicator. As a backup Piper incorporated the "Autoflite" system which is based on an electric turn-coordinator type system and is really just a wing leveler. The idea being that if you are IMC and your vacuum-attitude based autopilot fails you can switch it off and turn on the Autoflite wing leveler as a secondary / backup system.

I have to say that from what I've read / experienced and understand, this autopilot was way ahead of it's time (perhaps not unlike Comanche aircraft in general). If working properly there is really nothing this autopilot won't do. It has altitude pre-select (albeit on a fairly "gross" scale), auto-trim, a glide slope coupler (where installed). and other features. It even incorporated an air-speed limiter to prevent the aircraft descending at such a rate it exceeded airspeed limitations (Vno / Vne) and on the climb it prevents climbing into a stall by swallowing the climb to avoid getting too slow. As an attitude based autopilot the turns are crisp and decisive. I am used to flying with a rate-based S-Tec 55X and have now come to realize how much more "crisp" the turn in and response are in an attitude based autopilot like this old Altimatic (also our S-Tec 55x in the Cirrus is not too good in turbulence being rate based).

On the down side... the altitude pre-select is on a pretty "gross" in it's scale. It's not like setting a digital altitude in tens of feet but rather turning a knob to rotate a drum (see the first pic). Also it has to be calibrated on each flight due to baro changes. The disadvantage of being attitude based is that it's based n a vaccum attitude indicator spinning 3x as fast as a turn coordinator. I've heard it argued that a rate based S-Tec and the turn coordinator gyro are more reliable.

My autopilot performed well on the test flight pre-purchase. I've read a post from Kristen Winter (pretty sure it was Kristen's post) where she advised the autopilot as an item to test thoroughly and carefully and I have to agree with this. I probably did not test mine enough pre-purchase and have discovered just how crazy expensive these things are to repair or replace.

Here's where I am today... my Altimatic III will track a heading or nav source (VOR/GPS) very well. On a LOC it will track but did seem to S-turn quite a bit before locking on. Probably an adjustment issue. I don't think I have a GS coupler or if I do it's not working. It will hold altitude if I play around with it. However I have never seen the autopilot adjust the electric trim so I think the autotrim is probably inop. Also, the altitude preselect works some of the time but not often and not with any precision. My impression is there are dirty contacts in there somewhere and I can get it to climb but not descend. Also it's out of calibration. The little round window with the horizontal line used for calibration is slanted slightly down when it thinks it's level. Finally... my Piper Autoflite system is completely Inop and I don't think it worked when I bought the plane. The prior owner said it did but candidly I did not test this and I've read alot about mis-understandings of how these systems work. For instance, if you don't turn the Altimatic Off before turning Autoflite on then the wing leveling you are getting is coming from the Altimatic not the Autoflite.

I spoke to Bob Furguson out at Autopilots Central. He said the systems can be made to work. He said the only part they can't get / replace is a "bellows" that is used in the altitude hold function. He said if mine holds altitude at all (which it does) then that is probably not a problem. He said to fix it they would really need me to fly the plane out to them in Oklahoma and let them pull the whole thing apart, overhaul it, then reinstall it all and test / calibrate. Cost wise he said he could only give me an "idea" but worst case it could approach $10K. So I might spend $4K if not much is wrong but maybe I'd spend close to $10K. If I went this route and everything was made to work like it should it seems like I would have a very capable autopilot with features that would cost more like $30K installed if I were going with a new S-Tec of similar capability. That said I worry that I'd be "throwing good money after bad" investing so much in a 45 year old autopilot with limited parts support.

Another idea is to spend say $15K installing a basic S-Tec 30 or 50. That would give me a new autopilot and while it would cost more (no knowing how much more since I really don't know if the Altimatic repair would be 4K or 10K) it would at least be a known outcome (or should be). it would have altitude hold but no vertical speed or altitude preselect or even a glideslope coupler but no big deal I'd just hand fly approaches, I often do anyway.

While I've had no problems with the S-Tec 55x in the Cirrus I have noticed quite a few posts on the web on different forums about S-Tec issues - particularly since they were acquired by Cobahn. I've read comment about lower quality components being used in servos and the like but I don't know if that's really true.

Another question... what about Century? A Century 2000 is attitude based and the costs seem lower but avionics suppliers and shops I've spoken to seem to recommend away from Century and towards S-Tec. Many seem to say that S-Tec is the "only game in town". I've heard shops say that Century won't work with shops they only work with owners so shops prefer S-Tec.

I've also called Avidyne and ended up emailing with the VP of Product Development in charge of the new DFC-90. Predictably he would not give me any info on if or when the DFC-90 would be available for PA-24/30/39 aircraft but he agreed it would be a good platform. He said a Mooney 201 or similar would probably be the next platform and that they need to get through their existing pipeline of commitments before considering other airframes. So for now the DFC-90 is not an option, maybe it will be one day.

Anyway that's a lot of commentary... I'd appreciate any thoughts or feedback from those who have already been down this path, especially recently...

Thanks,

- Charles

[N3322G]

FWIW. Century sponsored us with a C2000 for the world race. Still very happy with it. No pre-select altitude hold.

Agree on the work with owners but not shops comment.

[Charles Schefer]

[Kristin Winter]

I would recommend talking to John VanBladeran about installing a Century 2000, as he has done several of these installations. I would go that way rather than S-TEC.

One thing I am investigating to converting the Altimatic III to an Altimatic IIIC/Century III, which is the same unit. The servos are the same, as is the computer/amplifer. The control head and the altitude sensing device needs to be changed. This unit has the control head with four rocker switches, one roll knob and one pitch wheel in the same sized package as the Altimatic III. It has not altitude preselect or speed control. Basically, they removed features and simplifed to improve reliability. It has the altitude hold feature instead of the preselect. The coupler etc is the same.

Piper put the Altimatic IIIC in thousands of airplanes from about 1975 through about 1983 or so. Century sold it as the Century III to Beech, Mooney, Aero Commander, so they are common and I understand that all parts are available. I am going to call Autopilot Central one of these days and see what they think about making that conversion. Both are STC'ed for the PA-30 and I believe PA-24, but didn't specifically check that.

[9089P]

We went with an Stec 30 with gpss and could not be happier. The avionics shop worked with both Stec or Century so we had a choice but their strong recommendation was the Stec due to reliability issues with the Century. We were already leaning in that direction for our single as the Stec gets its input from an electrical gyro, not vacumn, so a vacumn failure will not disable the A/P. We have had 2 premature vacumn pump failures but never an electrical problem.

The Stec tied to a 430W greatly simplifies ifr.

Good luck with your decision, Don

[N3322G]

Charles - forgot to add, we also added GPSS after the first decade - wonderful function with 530W.

Funny to hear someone go with S Tec due to V pump failures but not electric - in the past our problem was electric but not v pump failures.

[Mark Anderson]

S Tec system 30 is the best GA light airplane AP I have used. With GPS steering it is dead on. The Altitude hold is definitely worth the money with the Comanche. The Comanche is heavy enough to hold the altitude without having electric trim. Unless I am in a lot of turbulence it hardy ever chirps at me. I believe even with an electrical failure it will hold the wings level. With a back up vacuum Attitude and DG I don't worry too much about it. Rock solid AP. Best saftey/avionics investment I made.

Mark Anderson

[9089P]

Hi Pat,

Didn't I just read an article about a twin Comanche having a double vacumn failure?:-)

Don

[N3322G]

Don,

Yes, you did - that's why I was so careful in my phrasing ... "in the past". After 43 years in and around 22G, the vast majority of problems have been electrical in nature. The worst being the double alternator failure in IMC - just 3 hours after annual, a belt and a bearing - no warning.

As you saw in the article, one vacuum pump failure, about 6 hours of flying before the second - I can only guess, bad batch. These situations are why when I'm flying for keeping engines happy on my weekly flight, I hand fly and hand fly all 90% of all practice instrument work. I do use the A/P on most trips and to make sure it is still working as expected on 10% of the practice instrument flying.

There are risks any way you go. Do know Mom replaced S Tec altitude hold and original Piper A/P to get the C2000 in. I haven't been faced with an A/P decision, I just know we're happy with the C2000 and the added GPSS with 530W.

[md11flyer]

I have the Altimatic 3 in my Twin and find it a rock solid unit even though it is 40+ years old. In my very limited experience with this autopilot ( only been caretaker of this twin for 4 years) I have found the only problems with this older autopilot manifests itself after "mucking about with the wiring behind the panel and harness that runs under the floor boards.
I had one problem a cold solder joint in the wire harness to the radio coupler and one pulled wire from another connector under the floor board, both happened at different upgrade times. Once when removing 40 years of avionic wiring upgrade remains under the floor and once after redoing the panel. I was lucky to trace them without much trouble, but when people have problems with this older equipment often the answer is the cabling/ connectors. And regardless of who the shop is mistakes can be made so look around for the obvious and the not so obvious.
Also I believe that a lot of units out there were sent out for overhauls over the years and just put back into the airplane without doing the inflight setup. Its not hard to do but if not done you won't get the proper performance from it.
In my case when I got the airplane the autopilot would bank 35 degrees one way and only 20 the other way, I would descend at flutter speed and climb at 110 mph. With one flight with the front face removed and the limit pots adjusted properly it worked as new and 4 years later it still is rock solid.
If you have some time before dropping a lot of cash on new or overhauling, I would do a rough check on your connectors and cabling ( especially your auto trim sensor, there isn't much to go wrong with that unit except wiring connectors and dirty contacts)
Also if you have wandering headings or wing rocking this can be as simple as incorrect aileron cable tensions or the computer sensitivity set to high. There are 3 places where you can make adjustments to the altimatic 3, The computer installed behind the hatrack, the controller( pot switches behind the faceplate) and the radio coupler( adjusts the intercept angles).
If after you have exhausted the inexpensive things and you still have for instance wing rocking or heading problems then go for the servo overhaul or the new autopilot installation.
Good luck,
Gary

[Charles Schefer]

Thanks everyone for all these helpful comments. Looks like there is no one-clear answer. I am planning to put in an Aspen Pro PFD at some point (hopefully soon) so I'll get GPSS as it's built in the Aspen already - I agree it's great to have. I am not doing anything immediate (probably not in the next 6 months) so I'll keep researching and noodling this. In the mean time I'd continue to welcome any and all added comments on this topic...

- Charles

[Steven Lefferts]

[Charles Schefer]

[md11flyer]

[Charles Schefer]