poloengr
Well-Known Member
I am posting a brief synopsis of my 10 month struggle to get the Altitude Hold and Pitch functions working on my Seneca II. I purchased my Seneca in late 2014 and started my Multi training in early 2015. The first time I attempted to engage Altitude Hold on the Altimatic IIIC autopilot, the airplane entered a dive and the trim started to run away. The Roll and Heading functions worked perfectly. Subsequent attempts to engage ALT or PITCH were unsuccessful so I first called the A&P I used for the prebuy and he said it worked as designed when he checked out the airplane. The previous owner also said the autopilot always worked for him and in fact, several years previously many of the components had been "serviced" to ensure the Autopilot would not fail him.
To make a long story short, after many calls to Autopilots Central and Century, I just bit the bullet and took the plane in to the local avionics shop. They replaced the hose going to the "altitude can" and removed the Attitude Gyro (Mitchell p/n 52D67) and sent it in for overhaul. When I got the plane back, the Altitude Hold and Pitch worked perfectly, but it was not until my first 3 hour flight (about 6 flight hours after the repairs) that I started having issues again. During cruise with Altitude Hold and Heading modes engaged, the airplane climbed 250' before I took control and descended back to my assigned altitude. The Attitude Indicator showed a 10 degree nose down attitude at level flight and continued to do so until after I landed safely and shut down. The next day we checked it out and the AI seemed to be working normally. I flew back home VFR and checked the autopilot functions and everything seemed to be working fine but I hand flew the plane just to be safe. It was not until 2 hours into the flight that the AI started showing erroneous pitch information but roll indications were correct. The avionics shop removed the AI and sent it back to the Instrument shop for warranty repair. They found the coil to be set too tight causing slight drag which resulted in problems with the pitch portion of the gyro. Repairs were made, it was bench tested, and sent back to be reinstalled. After reinstallation, the AI worked fine, even on long flights, but my autopilot's Alt and Pitch functions were not working so back to the shop a 3rd time where the Gyro was removed and sent back for warranty repairs. The shop (well known and respected) tested the Attitude Indicator and found no issues with function or autopilot signals so it was returned and installed again. The Alt and Pitch functions continued to be inoperable so we started checking for issues in the connectors and wiring harness. No issues were found. During my IFR Inspection at a different FBO, the glass on the Attitude Gyro was somehow nicked and a small blemish was left on the glass so the shop that performed the IFR Check agreed to remove the Gyro and send it in to have the glass replaced ($50) at no charge to me. Just by coincidence, they sent it to the same shop the Avionics shop used and I got a call from one of the technicians who said that they were sending me a factory refurbished AI because during testing they found a problem with the pitch signal and since they had already overhauled this unit (1st), repaired it (2nd), and tested it (3rd) on three previous occasions in the past 4 months without seeing this problem they figured it was not worth trying to repair.
When the shop installed the "new" Sigma-Tek Attitude Gyro (still p/n 52D67), all functions of my Altimatic IIIC autopilot started working and it has behaved flawlessly for the past 12 flight hours.
If you are having problems with the Altitude or Pitch modes on your autopilot and you have sent the Attitude Gyro in for testing and no problems were found, I suggest you recheck the AI and push the issue. My original AI passed testing on 2 occasions where the autopilot signals were shown to be working properly and in spec but ultimately it was the AI that caused my Autopilot to be unable to hold altitude or pitch. Had the other shop not scratched the glass on my Attitude Indicator and sent it back in for repair, I would still be searching for a solution and we would have ruled out the Attitude Gyro as a source of the problem.
To make a long story short, after many calls to Autopilots Central and Century, I just bit the bullet and took the plane in to the local avionics shop. They replaced the hose going to the "altitude can" and removed the Attitude Gyro (Mitchell p/n 52D67) and sent it in for overhaul. When I got the plane back, the Altitude Hold and Pitch worked perfectly, but it was not until my first 3 hour flight (about 6 flight hours after the repairs) that I started having issues again. During cruise with Altitude Hold and Heading modes engaged, the airplane climbed 250' before I took control and descended back to my assigned altitude. The Attitude Indicator showed a 10 degree nose down attitude at level flight and continued to do so until after I landed safely and shut down. The next day we checked it out and the AI seemed to be working normally. I flew back home VFR and checked the autopilot functions and everything seemed to be working fine but I hand flew the plane just to be safe. It was not until 2 hours into the flight that the AI started showing erroneous pitch information but roll indications were correct. The avionics shop removed the AI and sent it back to the Instrument shop for warranty repair. They found the coil to be set too tight causing slight drag which resulted in problems with the pitch portion of the gyro. Repairs were made, it was bench tested, and sent back to be reinstalled. After reinstallation, the AI worked fine, even on long flights, but my autopilot's Alt and Pitch functions were not working so back to the shop a 3rd time where the Gyro was removed and sent back for warranty repairs. The shop (well known and respected) tested the Attitude Indicator and found no issues with function or autopilot signals so it was returned and installed again. The Alt and Pitch functions continued to be inoperable so we started checking for issues in the connectors and wiring harness. No issues were found. During my IFR Inspection at a different FBO, the glass on the Attitude Gyro was somehow nicked and a small blemish was left on the glass so the shop that performed the IFR Check agreed to remove the Gyro and send it in to have the glass replaced ($50) at no charge to me. Just by coincidence, they sent it to the same shop the Avionics shop used and I got a call from one of the technicians who said that they were sending me a factory refurbished AI because during testing they found a problem with the pitch signal and since they had already overhauled this unit (1st), repaired it (2nd), and tested it (3rd) on three previous occasions in the past 4 months without seeing this problem they figured it was not worth trying to repair.
When the shop installed the "new" Sigma-Tek Attitude Gyro (still p/n 52D67), all functions of my Altimatic IIIC autopilot started working and it has behaved flawlessly for the past 12 flight hours.
If you are having problems with the Altitude or Pitch modes on your autopilot and you have sent the Attitude Gyro in for testing and no problems were found, I suggest you recheck the AI and push the issue. My original AI passed testing on 2 occasions where the autopilot signals were shown to be working properly and in spec but ultimately it was the AI that caused my Autopilot to be unable to hold altitude or pitch. Had the other shop not scratched the glass on my Attitude Indicator and sent it back in for repair, I would still be searching for a solution and we would have ruled out the Attitude Gyro as a source of the problem.
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