Sharing so that others who have similar issues can try this as an alternate approach to replacing with an all new autopilot.
When I bought my PA-32R I had an altitude hold Century III but it wasn't working.
* Turn it on and sometimes "lunges" up at an alarming attitude - well out of range of passenger comfort. (kinda scary).
* OR: Turn it on and climbs 200 fpm, ending up about 500 - 1000 feet higher than when set.
* Didn't hold for long in either case.
I figured it was worth a try to swap a few parts and see if it could be fixed. Lots of Century III systems are being pulled out these days.
The logic here is... it's already broke. So putting in a broke part doesn't change anything.
Disclaimer: I am planning to upgrade to either a Garmin or a TrueTrak at some point. The goal here was to "eek by" for a year or two, as cheaply as possible, until I have the budget to do something more. Meanwhile I am putting money into the radio stack, transponder, interior and speed mods. Once better autopilots are available for my airplane I'll buy a new one. For now the options are limited
Disclaimer 2: I don't recommend doing this without an avionics expert helping you. I decided to go this route because it's becoming nearly impossible to get service on these old units unless you are willing to wait months.
* Be aware that the airplane must be level and trimmed before engaging. Otherwise it can be erratic.
* Also be aware that the attitude indicator can cause this same problem. Mine was already serviced so there was only two remaining parts which could cause this problem: the altitude hold can and the head unit. (refer to the service manual for your specific unit)
Step 1: Got a new 1C725 altitude hold "can" assembly from eBay. It was listed as "working when removed" from a Seneca II and was tagged as serviceable. Made an offer which was accepted. I have a local guy who can swap avionics parts but not necessarily someone who does major installs or repairs. It took about an hour to swap this part. The part is attached to the rear of the glove box on PA-32 and PA-34. You can easily get to it through the front baggage compartment by removing the upper panel.
Step 2: Try it. The verdict: It was better, but not great.
* Turn it on and it climbs 200 fpm for about 300 feet, then leveled off. After that it pretty much held except maybe a 20 fpm climb.
Step 3: Read the shop manual again. There are in-flight alignments that can be done. On a Piper this is easy to do because it's all in the head unit.
3a. Remove faceplate.
3b. There are 6 adjustment screws for roll, pitch and altitude engage. I'm not going to say which is which because you need to carefully read the manual.
4. Went for a flight. Engage, observe, disengage, adjust. Repeat until it is within acceptable limits.
*** note: read the shop manual carefully. There are some specifics about trim on/off, heading on/off, roll on/off, etc. All of these things affect what happens when the altitude hold is turned engaged, if it is too far out of alignment.
Result:
* I can engage the autopilot now and it climbs no more than 20 - 40 feet.
* holds altitude within 50 feet when making 180 degree turn ( used to climb 400' )
* I can make power adjustments to change speed. Autopilot auto-trims correctly.
I suppose I could adjust it more but after turning the pots 3 - 4 full turns I was getting nervous that it might come all the way out (bad). I figured this enough and I call it a job done for now.
Knowing this forum, I'm sure there are people going to tell me not to trust a Century III. I AGREE! I use it to reduce fatigue on long flights. I fly approaches by hand. And my thumb is always near the kill switch.
Anyway, I hope this helps.
NOTE: There is a similar procedure for adjusting Century II "hunting" for heading. I didn't have that problem. But one of the adjustment pots sets the tolerance for this. Over time these need to be re-adjusted because they are simply an analog resister.
When I bought my PA-32R I had an altitude hold Century III but it wasn't working.
* Turn it on and sometimes "lunges" up at an alarming attitude - well out of range of passenger comfort. (kinda scary).
* OR: Turn it on and climbs 200 fpm, ending up about 500 - 1000 feet higher than when set.
* Didn't hold for long in either case.
I figured it was worth a try to swap a few parts and see if it could be fixed. Lots of Century III systems are being pulled out these days.
The logic here is... it's already broke. So putting in a broke part doesn't change anything.
Disclaimer: I am planning to upgrade to either a Garmin or a TrueTrak at some point. The goal here was to "eek by" for a year or two, as cheaply as possible, until I have the budget to do something more. Meanwhile I am putting money into the radio stack, transponder, interior and speed mods. Once better autopilots are available for my airplane I'll buy a new one. For now the options are limited
Disclaimer 2: I don't recommend doing this without an avionics expert helping you. I decided to go this route because it's becoming nearly impossible to get service on these old units unless you are willing to wait months.
* Be aware that the airplane must be level and trimmed before engaging. Otherwise it can be erratic.
* Also be aware that the attitude indicator can cause this same problem. Mine was already serviced so there was only two remaining parts which could cause this problem: the altitude hold can and the head unit. (refer to the service manual for your specific unit)
Step 1: Got a new 1C725 altitude hold "can" assembly from eBay. It was listed as "working when removed" from a Seneca II and was tagged as serviceable. Made an offer which was accepted. I have a local guy who can swap avionics parts but not necessarily someone who does major installs or repairs. It took about an hour to swap this part. The part is attached to the rear of the glove box on PA-32 and PA-34. You can easily get to it through the front baggage compartment by removing the upper panel.
Step 2: Try it. The verdict: It was better, but not great.
* Turn it on and it climbs 200 fpm for about 300 feet, then leveled off. After that it pretty much held except maybe a 20 fpm climb.
Step 3: Read the shop manual again. There are in-flight alignments that can be done. On a Piper this is easy to do because it's all in the head unit.
3a. Remove faceplate.
3b. There are 6 adjustment screws for roll, pitch and altitude engage. I'm not going to say which is which because you need to carefully read the manual.
4. Went for a flight. Engage, observe, disengage, adjust. Repeat until it is within acceptable limits.
*** note: read the shop manual carefully. There are some specifics about trim on/off, heading on/off, roll on/off, etc. All of these things affect what happens when the altitude hold is turned engaged, if it is too far out of alignment.
Result:
* I can engage the autopilot now and it climbs no more than 20 - 40 feet.
* holds altitude within 50 feet when making 180 degree turn ( used to climb 400' )
* I can make power adjustments to change speed. Autopilot auto-trims correctly.
I suppose I could adjust it more but after turning the pots 3 - 4 full turns I was getting nervous that it might come all the way out (bad). I figured this enough and I call it a job done for now.
Knowing this forum, I'm sure there are people going to tell me not to trust a Century III. I AGREE! I use it to reduce fatigue on long flights. I fly approaches by hand. And my thumb is always near the kill switch.
Anyway, I hope this helps.
NOTE: There is a similar procedure for adjusting Century II "hunting" for heading. I didn't have that problem. But one of the adjustment pots sets the tolerance for this. Over time these need to be re-adjusted because they are simply an analog resister.
