What am I doing wrong?

[Wildhorsetrail]

My airplane has a century 21 autopilot. I've used it several times but sometimes it will only command a left turn no matter what source or heading is selected. Am I missing something or is the old circuitry waving goodbye to me on the way to complete failure? Anyone else experience this failure mode?

 

Sent from my SM-A326U1 using Tapatalk

 

 

[201Mooniac]

I had an intermittent similar experience with the century 21 that was in my airplane when I bought it.  It died soon afterwards and I replaced it.  I hope you have better luck with yours but a nice GFC-500 would be a great replacement :-)

[LANCECASPER]

10 hours ago, Wildhorsetrail said:

 

My airplane has a century 21 autopilot. I've used it several times but sometimes it will only command a left turn no matter what source or heading is selected. Am I missing something or is the old circuitry waving goodbye to me on the way to complete failure? Anyone else experience this failure mode?

@Jake@BevanAviation Jake will know what's wrong

[Jake@BevanAviation]

My guess is part of the drive circuitry for the roll servo is faulty resulting in only drive in one direction.  Possibly a set of faulty drive transistors.  First try reseating the unit in the rack and see if that resolves the issue.  If the issue is intermittent it could be a issue with the flight computer, roll servo or possible wiring. Additional troubleshooting would be needed to isolate the problem.

If you are doing ground testing make sure the AI is spooled up and as close to level as possible.  If the AI is laid over the system will try and drive the servo to correct the laid over orientation of the AI.  For example, if the AI is laid over 30 deg to the right indicating a right bank, the system will want to drive to the left to return to wings level with the heading bug under the lubber and heading mode selected.  Heading drive will be limited in this scenario, you would be able to drive left but very limited drive to the right if any.

[N201MKTurbo]

12 minutes ago, Jake@BevanAviation said:

My guess is part of the drive circuitry for the roll servo is faulty resulting in only drive in one direction.  Possibly a set of faulty drive transistors.  First try reseating the unit in the rack and see if that resolves the issue.  If the issue is intermittent it could be a issue with the flight computer, roll servo or possible wiring. Additional troubleshooting would be needed to isolate the problem.

If you are doing ground testing make sure the AI is spooled up and as close to level as possible.  If the AI is laid over the system will try and drive the servo to correct the laid over orientation of the AI.  For example, if the AI is laid over 30 deg to the right indicating a right bank, the system will want to drive to the left to return to wings level with the heading bug under the lubber and heading mode selected.  Heading drive will be limited in this scenario, you would be able to drive left but very limited drive to the right if any.

Can't you zero the roll error with the roll knob even if it is laid over?

[Jake@BevanAviation]

@N201MKTurbo The Century 21 has no roll knob function.  This was a feature in older Century systems.  Depending on where the bank angle is set for the individual system, 30 deg bank is very close to the limit of the system.  Heading errors typically default to the around 20 deg limit for most legacy systems (book spec for the Century 21 is around 24 deg).  This can vary based on where the limit pots are set for the aircraft.

For the example given of 30 deg right bank, that is beyond the 24 deg heading limit (book spec).  In theory, when it is beyond the max bank limit no amount of right heading bug error would allow the unit to continue to drive to the right.  A good rule of thumb is when possible always try to null the AI when doing ground testing or simply unplug the AI for ground test.  When you unplug the AI the computer thinks it is getting nulled information back from the AI.  If the circuity is working properly you should be able to fully control the system on the ground.  Just don't forget to plug the AI back into the system or you will have a very interesting ride.

[N201MKTurbo]

52 minutes ago, Jake@BevanAviation said:

@N201MKTurbo The Century 21 has no roll knob function.  This was a feature in older Century systems.  Depending on where the bank angle is set for the individual system, 30 deg bank is very close to the limit of the system.  Heading errors typically default to the around 20 deg limit for most legacy systems (book spec for the Century 21 is around 24 deg).  This can vary based on where the limit pots are set for the aircraft.

For the example given of 30 deg right bank, that is beyond the 24 deg heading limit (book spec).  In theory, when it is beyond the max bank limit no amount of right heading bug error would allow the unit to continue to drive to the right.  A good rule of thumb is when possible always try to null the AI when doing ground testing or simply unplug the AI for ground test.  When you unplug the AI the computer thinks it is getting nulled information back from the AI.  If the circuity is working properly you should be able to fully control the system on the ground.  Just don't forget to plug the AI back into the system or you will have a very interesting ride.

Thanks for the info. I've never used a Century 21 and assumed it had a roll knob. I believe on the CIIB the roll error is the difference between the AI bank signal and the roll knob, but that might only apply with the heading mode off.

[whiskytango]

Before replacing it with a GFC500, I had a Century 31.  It is similar to the 21, but had altitude hold and attitude (pitch) hold.  It would fly GPS NAV courses very accurately, but every now and then would do a "Crazy Ivan" turn for no apparent reason. It was an intermittent problem, and none of the avionics shops that looked at it could resolve the problem. Good luck!

[Fly Boomer]

2 hours ago, Jake@BevanAviation said:

Just don't forget to plug the AI back into the system or you will have a very interesting ride.

The rest of your post displayed some amazingly detailed knowledge, but this last sentence made me laugh.  Thanks.

[Sue Bon]

My autopilot will work dandy until it suddenly starts turning left for no reason whatsoever. I used to get flustered and turn the whole AP off with the switch on the yoke. Now I just switch off HDG and use the roll knob to steer while keeping the altitude hold. I've ordered a GFC 500 to be installed this winter (assuming it ever arrives).

I would love to know why this happens. My avionics guy says that it's old and trying to die.

Edit: I have a Century IIB and S-TEC PSS ST-049 for altitude/VS/GS

[Wildhorsetrail]

4 hours ago, Jake@BevanAviation said:

My guess is part of the drive circuitry for the roll servo is faulty resulting in only drive in one direction.  Possibly a set of faulty drive transistors.  First try reseating the unit in the rack and see if that resolves the issue.  If the issue is intermittent it could be a issue with the flight computer, roll servo or possible wiring. Additional troubleshooting would be needed to isolate the problem.

If you are doing ground testing make sure the AI is spooled up and as close to level as possible.  If the AI is laid over the system will try and drive the servo to correct the laid over orientation of the AI.  For example, if the AI is laid over 30 deg to the right indicating a right bank, the system will want to drive to the left to return to wings level with the heading bug under the lubber and heading mode selected.  Heading drive will be limited in this scenario, you would be able to drive left but very limited drive to the right if any.

Thanks for the input, I much appreciate you sharing your expertise.

[201Mooniac]

2 hours ago, whiskytango said:

Before replacing it with a GFC500, I had a Century 31.  It is similar to the 21, but had altitude hold and attitude (pitch) hold.  It would fly GPS NAV courses very accurately, but every now and then would do a "Crazy Ivan" turn for no apparent reason. It was an intermittent problem, and none of the avionics shops that looked at it could resolve the problem. Good luck!

This sounds very much like the problem I had with my C21.  I like the "crazy Ivan" terminology to describe it.

[Jake@BevanAviation]

@Sue Bon The very early Century IIb units that specifically had the square white push button switches had griplet boards. The griplets connected traces from the top of the board to the bottom of the board and have a history of being intermittent and a pain to troubleshoot.  Also, the white push button switch had contacts that were prone to spreading over time that would cause intermittent signals.  When the system looses the heading error voltage, it will default to the roll centering pot.  If the roll centering pot happens to be set for left roll, that would be the direction the system would drive if it lost the heading error. When you turn heading mode off, it defaults to the roll turn knob and the roll centering pot for correction commands.

Later revisions of the Century IIb changed to through-hole circuit traces and did away with the griplets.  They also change the push button switch to a white rocker switch that controlled a micro switch to provide the heading error to the system.  A very common issue with any legacy Century system is the blue amphenol female split pin connector, it has a history of being intermittent.  They used this connector on a variety of different components.

[INA201]

I had similar issues.  I bought Stabilant 22 and reestablished connections on everything even the servo in the wing.  It is funky interesting stuff but worked for me.  If anything it’s a good first step.  Search Stabilant and you will find various posts on folks here who have used it.  

[Sue Bon]

10 hours ago, Jake@BevanAviation said:

The very early Century IIb units that specifically had the square white push button switches had griplet boards. The griplets connected traces from the top of the board to the bottom of the board and have a history of being intermittent and a pain to troubleshoot.  Also, the white push button switch had contacts that were prone to spreading over time that would cause intermittent signals.  When the system looses the heading error voltage, it will default to the roll centering pot.  If the roll centering pot happens to be set for left roll, that would be the direction the system would drive if it lost the heading error. When you turn heading mode off, it defaults to the roll turn knob and the roll centering pot for correction commands.

Later revisions of the Century IIb changed to through-hole circuit traces and did away with the griplets.  They also change the push button switch to a white rocker switch that controlled a micro switch to provide the heading error to the system.  A very common issue with any legacy Century system is the blue amphenol female split pin connector, it has a history of being intermittent.  They used this connector on a variety of different components.

Thank you very much!