Century IIB Autopilot

[wishboneash]

My 201 has a Century IIB Auto-pilot along with a Sys 60 PSS. Works OK intermittently so I can't depend on it for serious IFR flying. It performs OK sometimes, but it may suddenly lose its bearings and begin an oscillatory behaviour and deviate significantly from assigned Nav or Hdg and I have to disengage it. Same with the PSS, it may work fine for 15 minutes, then suddenly it will send the plane climbing or descending and I have to overrde it. Is it a matter of servicing or calibration? Any suggestions on shops in (Northern) California who might be experts on these systems? Thanks.

Sriram

[OR75]

it may be an easy fix:

the CIIB gets its input from the DG and the AI. Are they both working fine ? (recently overhauled ? - that may not mean much !, no precession ?)

if you are ok with the DG and the AI, next step could be the gain or sentivity of the console. It may need to be adjusted. ( by removing the controle knob and tuning via a tiny screws behind the faceplate). needs to be donein flight.

the issue with the PSS: do you have autotrim ?

[jetdriven]

They may be related. We have a similar setup with a CIIB and a STEC-30ALT. The 30ALT has no trim feature, so when the plane turns in HDG mode it banks 22 degrees and the nose drops slightly before the 30ALT recognizes an altitude deviation and corrects. in ROLL mode (turning the roll knob) it banks 30 degrees, which the 30ALT doesnt like. It has a larger (like 50ft) deviation followed by the trim beeper. It can't correct for all the loss of lift in that much of a bank.

Your CIIB may need adjustment, and the 60PSS is just tryign to correct for the altitude excursions because it is playing catchup.

There are 4 adjust ent screws behind the Century panel. The left and right bank adjustments should be adjusted ot 30 degree bank in roll mode. the centering adjustment ot to center the ehading bug, turn the screw the opposite direction of the heading bug displacement. the roll sensitivity needs adjustment in our plane once a year. back it out until a lazy wandering develops. then screw it in until it has a fast oscillation. Then split the differnece. They are 20-turn trimpots. Do it in flight.

[N201MKTurbo]

If it looses its bearings, there is most likely a loose connection in the radio coupler.

The CIIB autopilot is an analog autopilot that sums up to three error signals to get the final error signal to correct the roll angle of the plane. The lowest level error comes from AI, and with the heading switch off the error is the difference between the AI output and the roll knob. The first thing to check is performance of the autopilot with the heading switch off. This will eliminate the radio coupler and DG.

With the heading switch on, the error signal is the sum of the bank angle from the AI and the error signal from the radio coupler. The radio coupler has two levels of operation, the most basic is heading mode. In heading mode the error from the radio coupler is the error signal from the DG which is the difference between the heading bug position and straight up. Testing the the autopilot in heading mode will eliminate any problems caused by your radios (GPS, LOC, NAV).

The highest level of operation is when the autopilot is coupled to a radio navigation source. The radio coupler will sum the error from the DG and the radio to form the error signal sent to the amplifier (the box with the roll knob).

By testing these three modes seperatly you can find where the intermittent connection is that is causing your auropilot to loose its way.

[ToddDPT]

If you need to go somewhere in Northern California I've had good experience with Executive Autopilots at KSAC.

http://www.execap.com/

[wishboneash]

Hi all

Thanks all for the valuable feedback! I downloaded the CIIB maintenance manual so I have bit of reading to do. I will try to diagnose it first myself first before taking it to an avionics shop. OR75, I received your PM and have replied.

I do have autotrim so I should be able to try get more information out of the PSS with it turned off.

Sriram

[320KPH]

I have a Century 41 AP in a 201 that keeps thinking its a Stuka! Engage the AP and in about five seconds the nose keeps pitching down and down and down bubble!

Checked online for Service Manuals and cant seem to find any free.

The POH has no info about how to adjust the nose down gains, any tips?

[wishboneash]

I got mine repaired at Airtronics at San Andreas, CA. The servos needed cleaning, no parts were replaced. One of the vacuum hoses was disconnected for the Sys 60 PSS.

[Mcstealth]

After you doing the reading and adjusting, It was a vacume hose that was causing the problem?

 

David

[OR75]

the 60PSS does not have a vacuum hose.  It has a static air hose that goes into the pressure altitude sensor.

 

If the only issue was with the vacuum hose only, the 60PPS would work fine in ALT mode and in GS mode but would struggle a bit in VS mode (the sensor being in the tail cone)

[wishboneash]

the 60PSS does not have a vacuum hose.  It has a static air hose that goes into the pressure altitude sensor.

 

If the only issue was with the vacuum hose only, the 60PPS would work fine in ALT mode and in GS mode but would struggle a bit in VS mode (the sensor being in the tail cone)

 

You are right, it is the static line. I rarely use the VS mode.

[wishboneash]

I got mine repaired at Airtronics at San Andreas, CA. The servos needed cleaning, no parts were replaced. One of the vacuum hoses was disconnected for the Sys 60 PSS.

 

The AP problem was not reliably fixed the last time. I started to recently see the AP lose its way and turn in the wrong direction etc. I took it in again, and they found a loose capacitor in the left/right roll amplifier. After resoldering, the AP works fine now. Whew, rather a simple issue! Also the AP board was reshimmed properly so there is no play in the connector and won't vibrate now.

[golfpilot]

On 11/25/2012 at 5:19 AM, jetdriven said:

They may be related. We have a similar setup with a CIIB and a STEC-30ALT. The 30ALT has no trim feature, so when the plane turns in HDG mode it banks 22 degrees and the nose drops slightly before the 30ALT recognizes an altitude deviation and corrects. in ROLL mode (turning the roll knob) it banks 30 degrees, which the 30ALT doesnt like. It has a larger (like 50ft) deviation followed by the trim beeper. It can't correct for all the loss of lift in that much of a bank.

 

Your CIIB may need adjustment, and the 60PSS is just tryign to correct for the altitude excursions because it is playing catchup.

 

There are 4 adjust ent screws behind the Century panel. The left and right bank adjustments should be adjusted ot 30 degree bank in roll mode. the centering adjustment ot to center the ehading bug, turn the screw the opposite direction of the heading bug displacement. the roll sensitivity needs adjustment in our plane once a year. back it out until a lazy wandering develops. then screw it in until it has a fast oscillation. Then split the differnece. They are 20-turn trimpots. Do it in flight.

I know this is an old thread but maybe you are still around to help me. My autopilot is doing the "Lazy wondering" you mention to back out to. Which of the 4 adjustments do I need to turn in to fix that?

[jetdriven]

Threshold adjustment.  As described above.  Split difference between lazy wander and fast oscillation. Try a few turns CW. 

Often this is a aircraft rigging issue. Make sure the plane flies wings level ball centered and heading does not change. Sometimes the yaw is not trimmed right and the airplane skids in a wings level attitude. The autopilot looks at bank angle and at heading. It wants wings level constant heading. If the airplane changes heading wings level it banks slightly to recover heading. Then it’s not wings level. So it goes back and forth between wings level and assigned heading. Follow the SMM but it needs to be at zero yaw. Before bending the rudder trailing edge, verify the gear doors are closing on both sides and your ailerons and flaps and the roll trim is all dead on. 

Edited January 29, 2018 by jetdriven

[Piloto]

My C31 was having pitch oscillations. I raised the vacuum from 4.5"Hg to 5.5"Hg by adjusting the vac regulator and problem solved. Steady as rock now.

José 

[Bartman]

www.csobeech.com/files/CenturyII&IIIManual.pdf

Section 6 is what you are looking for.  Step by step instructions with figures. This fixed mine  

[INA201]

On 11/25/2012 at 8:19 AM, jetdriven said:

They may be related. We have a similar setup with a CIIB and a STEC-30ALT. The 30ALT has no trim feature, so when the plane turns in HDG mode it banks 22 degrees and the nose drops slightly before the 30ALT recognizes an altitude deviation and corrects. in ROLL mode (turning the roll knob) it banks 30 degrees, which the 30ALT doesnt like. It has a larger (like 50ft) deviation followed by the trim beeper. It can't correct for all the loss of lift in that much of a bank.

 

Your CIIB may need adjustment, and the 60PSS is just tryign to correct for the altitude excursions because it is playing catchup.

 

There are 4 adjust ent screws behind the Century panel. The left and right bank adjustments should be adjusted ot 30 degree bank in roll mode. the centering adjustment ot to center the ehading bug, turn the screw the opposite direction of the heading bug displacement. the roll sensitivity needs adjustment in our plane once a year. back it out until a lazy wandering develops. then screw it in until it has a fast oscillation. Then split the differnece. They are 20-turn trimpots. Do it in flight.

Ive been tweaking my IIB to remove oscillation and have probably done a counterclockwise turn from full oscillation of a couple of revolutions.  I still have a very slow oscillation so my question is about the "20 turn trimpot."  Should I be turning vigorously or just a 1/4 turn and wait, 1/4 turn and wait on the threshold adjustments.  Thanks in advance.

[jetdriven]

About one full turn wait about 20 seconds let it settle down. Maybe turn the hearing bug 10 or 20° left and note the behavior

[Bartman]

My Century IIb with STEC-30 ALT is perfectly level and straight with no oscillating in heading mode. When I go to NAV mode running off of the Avidyne 440 I have a slight oscillation that I was not able to adjust out.

The altitude hold works perfect if I manually trim it slightly up or down, but if I trim it perfect then turn on the STEC-30 ALT hold it hunts up and down and is uncomfortable. If it does that I turn the trim up or down slightly then is butter smooth. 

[Sandman993]

Does anyone know which inspection plate the roll servo is on the century IIB? How to clean? asking for a friend...hehe

[jetdriven]

It is under the inspection panel just outboard of the left either on pushrod where it goes to the aileron. There’s not really a way to clean it, the brushes are inside the servo motor and the gears may get some stuff on them but you have to take servo  out of the airplane and take the cover off of it to access those

[jetdriven]

Just measure the startup  voltage between C-D on the connector. If it’s over 1.2 or 1.5 V, run it on 12 V power supply for 20 minutes then try again. If that doesn’t work Ship for overhaul

Edited October 31, 2019 by jetdriven

[EricJ]

6 hours ago, Sandman993 said:

Does anyone know which inspection plate the roll servo is on the century IIB? How to clean? asking for a friend...hehe

As mentioned, just outboard of the pushrod for the left aileron.   Looks like this inside:

 

[N201MKTurbo]

26 minutes ago, EricJ said:

As mentioned, just outboard of the pushrod for the left aileron.   Looks like this inside:

 

Wow, they must have forgotten to put the inspection panel back on last time they painted your plane.

[EricJ]

25 minutes ago, N201MKTurbo said:

Wow, they must have forgotten to put the inspection panel back on last time they painted your plane.

I know, there are quite a few places where it's like that.   It's weird, I've seen a number of examples of airplanes painted with the panels off, and the panels painted separately.   I guess it's so that the entire panel get painted and there's not a paint seam or something?   And they did mask off the entire wheel well areas, as those are all still original paint, probably so that they didn't have to strip around the tanks.

The place in Wickenburg did it in the late 90s or so.