Altimatic IIIB On/Off Switch

[Ted]

I've been blessed with a very capable autopilot that came with my Aztec. Unfortunately it's an Altimatic/Century IIIB that hasn't worked fully properly since three owners ago when it was on 135 operation. I ended up spending the first 250 hours or so hand flying the thing, finally getting the autopilot working in January of this year.

At Gaston's, Lance and I went flying and he diagnosed a good number of the problems with the unit, including teaching me where the on/off switch is (it's not actually labeled on/off). I then took it to Griffin Avionics in Hyannis, MA to get some work done on it and since then, the roll has worked well, the pitch has worked pretty well, and the altitude hold still doesn't work. I need to take it back again for some more work, it's just difficult considering the amount that the plane gets used.

Anyway, the most recent issue that's come up on it is that the on/off switch now only turns on when it feels like. It seems to be dependent on the flight. This weekend on two legs it refused to turn on or even try to catch. On one flight it caught, worked flawlessly for the entire flight, and if I turned it off it would turn right back on without issue.

This sounds simple to me, but I'm confused that it seems to vary with the flight. Any ideas for something that my A&P and I can do without taking it anywhere? His philosophy on autopilots is to rip them out entirely and send them off somewhere to get fixed. However that somewhere he sends them tends to not fix them (they come back broken), which defeats the purpose in my opinion.

In a few months I'm hoping to take it back to Hyannis for some additional work and hopefully get the altitude hold functioning, then I'll have a proper autopilot. The roll does 90% of what I want, though. The altitude hold I can live without, I'd just like for it to be all functioning properly.

Thanks in advance, I really don't feel like buying an S-Tec for the thing, even though that's probably what I should do.

 

[gismo]

I've been blessed with a very capable autopilot that came with my Aztec. Unfortunately it's an Altimatic/Century IIIB that hasn't worked fully properly since three owners ago when it was on 135 operation. I ended up spending the first 250 hours or so hand flying the thing, finally getting the autopilot working in January of this year.

At Gaston's, Lance and I went flying and he diagnosed a good number of the problems with the unit, including teaching me where the on/off switch is (it's not actually labeled on/off). I then took it to Griffin Avionics in Hyannis, MA to get some work done on it and since then, the roll has worked well, the pitch has worked pretty well, and the altitude hold still doesn't work. I need to take it back again for some more work, it's just difficult considering the amount that the plane gets used.

Anyway, the most recent issue that's come up on it is that the on/off switch now only turns on when it feels like. It seems to be dependent on the flight. This weekend on two legs it refused to turn on or even try to catch. On one flight it caught, worked flawlessly for the entire flight, and if I turned it off it would turn right back on without issue.

This sounds simple to me, but I'm confused that it seems to vary with the flight. Any ideas for something that my A&P and I can do without taking it anywhere? His philosophy on autopilots is to rip them out entirely and send them off somewhere to get fixed. However that somewhere he sends them tends to not fix them (they come back broken), which defeats the purpose in my opinion.

In a few months I'm hoping to take it back to Hyannis for some additional work and hopefully get the altitude hold functioning, then I'll have a proper autopilot. The roll does 90% of what I want, though. The altitude hold I can live without, I'd just like for it to be all functioning properly.

Thanks in advance, I really don't feel like buying an S-Tec for the thing, even though that's probably what I should do.

By "On/Off Switch" do you mean the rocker switch labeled "Roll" on the autopilot console? And if so is the problem that the switch won't stay in the on position or does the autopilot simply not work when the switch is on?

The Roll rocker switch is held on by a small electromagnet and there is an adjustment inside the console which affects the ability of the magnet to hold the switch on. AFaIK the only other issues that might prevent the switch from staying on would be the switch contacts that power the holding electromagnet and possibly a low voltage on the power source to the console. Also FWIW the console can probably be configured for 14 or 28v operation and if your plane is 14v and the console were incorrectly wired for 28 the strength of the holding magnet would be significantly compromised.

 

[Ted]

Yes, I meant the "roll" rocker switch, sorry for not clarifying.

Normally I can tell when the magnet that holds it in place is at least trying to hold it. In this case, either it works fine and the magnet holds it, or it feels like the magnet isn't receiving any power and not working at all. None of this "trying to work" stuff. So, it seems like it either has power or doesn't. A problem it used to have was that it would sometimes try to hold and take a couple tries (plus the magnet would "chatter" as it tried to hold it), but that went away after being serviced by Griffin.

My plane is 14V and I have no idea if the autopilot was wired for a 28V, but I think that is unlikely given what I said above.

 

[gismo]

Yes, I meant the "roll" rocker switch, sorry for not clarifying.

Normally I can tell when the magnet that holds it in place is at least trying to hold it. In this case, either it works fine and the magnet holds it, or it feels like the magnet isn't receiving any power and not working at all. None of this "trying to work" stuff. So, it seems like it either has power or doesn't. A problem it used to have was that it would sometimes try to hold and take a couple tries (plus the magnet would "chatter" as it tried to hold it), but that went away after being serviced by Griffin.

My plane is 14V and I have no idea if the autopilot was wired for a 28V, but I think that is unlikely given what I said above.

That sounds like the issue with the switch contacts I mentioned. A simple burnishing of the contacts might cure the problem, at least for a while. If you go that route, make sure a real burnishing tool is used, not a file or sandpaper. It's also possible that the coil itself has an intermittent open or that the contacts are misadjusted (bent). Any competent tech should be able to sort that out with the console (only) on the bench.

I've forgotten whether or not you have the pilot's guide for the Altimatic III. I found a downloadable one at:

www.acepilot.com/files/Piper Altimatic lll B-1 Autopilot Manual.pdf

I should also mention that the magnet is normally so weak that unless the magnetic gap is virtually zero there will be no perceivable holding force.