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If a member of the Cirrus Aircraft Owners Forum could post

a link to this thread on that forum I would appreciate it.


Cirrus SR-20/-22 Aircraft

Autopilot HDG/NAV

Offset Tracking





This document provides instructions for resolving Heading/Navigation Offset issues in Cirrus SR-20 and SR-22 Aircraft equipped with the Sandel EHSI and the S-Tec Sys-55X Autopilot systems.


If your Cirrus Aircraft is equipped with a Sandel SN3308 EHSI configured for Century 21/31/41 FCS Emulation, and an S-Tec 55X AP System strapped for use with an NSD-360 Heading System then this document will apply.


If your Sandel SN3308 EHSI is configured for KI-525A FCS Emulation and the S-Tec Sys 55X AP is strapped for KCS-55A Heading System then this document will not apply.




In the last three years Southern Avionics & Communications, Inc. (SAC) has had four (4) Cirrus aircraft (3ea SR-22 and 1ea SR-20) come in for evaluation and repair of Autopilot HDG and NAV Track offset and oscillation errors. With the first two SR-22 aircraft it took multiple component exchanges to accomplish a repair. Flight testing of these two aircraft showed proper, though marginal, operation of the Autopilot System in HDG and NAV Modes.


With permission from the last two SR-20 and SR-22 aircraft the SAC AP Technician took the time to perform deep level system evaluation and testing to provide a better resolution to the offset tracking faults. As with the previous two aircraft, it was determined that there were discrepancies with the control surface rigging and aileron trim tab adjustment  (Note 1). These discrepancies were resolved by an Authorized Cirrus Factory Service Center. Further testing showed no faults in the roll trim module and motor assembly. OEM and SAC bench testing of the Sandel SN3308 and S-Tec Sys55X Autopilot System showed both units to be free of operational defects. It was determined that the fault lay in the unit’s configuration and interface wiring. Neither unit meets the OEM recommended configuration of KI-525A FCS emulation in the Sandel SN3308 and KCS-55A HDG System strapping of the S-Tec Sys55X Autopilot computer (Note 2).


The Sandel SN3308 HDG/CRS FCS outputs were configured as Century 21/31/41. The Century 21/31/41 FCS emulation output is a non-referenced DC output, see Sandel SN3308 FCS Interconnect Installation Drawing 90106-10 sheet 20 Century IV Interconnect (DC Version). In this mode the SN3308 provides a DC HDG and CRS null output that is not referenced to any input source.


The S-Tec Sys55X Autopilot Computer was strapped for the NSD-360/ HSI System. This strapping configures the AP Computer to look for a DC HDG and CRS signal referenced to a +10vdc excitation provided by the AP computer to the HDG system, see S-Tec Schematic, Wiring Interconnect, System 55/55X Drawing 1094.


The SAC AP Technician surmises that this was an interim fix during the transition from NSD-360 HDG Systems to Sandel SN3308 EHSI Systems by Cirrus. By moving one connection, pin 6 of J/P-689, to an unused pin hole the

+10vdc reference was removed from the EHSI harness, leaving only the excitation ground and the HDG/CRS dc signal lines, see the appropriate Cirrus wiring diagrams per aircraft type and serial number. This excitation ground connection between the EHSI and the 55X is the ground connection preferred by the Sys55X for AC Excitation Reference, not DC Excitation.


Under ideal circumstances this would not cause any imbalance or offset in the signals being acted upon by the AP computer. However, the slightest degradation in signal wiring connectors or in the internal components of the AP computer will have the AP computer trying to buy off the nulls against the excitation reference. Since there is no excitation reference at the SN3308 to balance the DC HDG/CRS nulls, a HDG or NAV offset or oscillation will occur.




Cobham/S-Tec System 55/55X Installation Manual

Sandel SN3308 Installation Manual

Century 21/31/41 Installation Manual

Century NSD-360/-1000 HDG System Installation Manual

Cirrus SR-20 Maintenance and Wiring Diagram Manuals

Cirrus SR-22 Maintenance and Wiring Diagram Manuals


All specific diagrams and drawings are provided with this document in pdf format (see attachments).





It is estimated that approximately 16 man hours will be required to accomplish the wiring and unit configuration changes described in this document. If the Sys55X has to be returned to the OEM for restrapping your down time and costs will be greater.




These instructions present field upgrade procedures to reconfigure the current interface to meet the recommended respective OEM installation requirements. The person performing the upgrade must be familiar with the aircraft and have a working knowledge of the Sandel EHSI and S-Tec Sys55X systems.




2ea Connector Pin, Female

AMP P/N: 205090-1   or

Positronic P/N: M39029/63-368   or

Garmin P/N: 336-00022-00


1ea 3 inches of 22g aircraft grade coated wire


1ea UG88 Coax Connector

1ea UG89 Coax Connector




Gain access to and remove: Sandel EHSI, MFD screen, all plastic dash trim pieces including the lower inboard kick panels, all radio rack components. Reconfigure the Sandel EHSI for KI-525A FCS emulation (either in aircraft after reassembly or on the bench). Restrap Sys55X for KCS-55A HDG system or return to OEM for restrapping. Remove the radio rack back shell assembly in it entirety (see attached diagram). SAC found it expedient to cut the Transponder Antenna Coax near the top of the rack assembly and install UG88/UG89 Coax connectors to facilitate rack removal and reinstallation. Gain access to Sys55X P1 connector. Remove the wire from pin 43, cut off the current pin and discard. Solder connect two (2) wires to the end of the wire just removed from pin 43. Strip and crimp two (2) female pins on the 'Y' wire. Install the two pins in pin hole number 12 and 29. Reassemble all racks, components and trim pieces removed to facilitate repair.




Flight test and perform in flight Roll Centering Alignment of Sys55X AP System per OEM requirements. Perform in flight testing of AP, EHSI and all other affected systems and verify proper operation per their respective OEM requirements for return to service.


Note 1:


A quick way to determine if your aircrafts flight controls and aileron trim tab are properly aligned is to do the in flight roll rate balance check. Fly to a safe altitude in smooth air. Trim the aircraft as best as possible for hands off flight. Run the Roll Trim full RIGHT while holding the aircraft level. Then bank to 30 degrees LEFT bank and release the controls. With a stopwatch, start timing as the attitude indicator passes through 20 degrees and stop timing as the attitude indicator passes through 0 degrees. Record that time. Now run the trim full LEFT, bank the aircraft to 30 degrees RIGHT bank and release the controls. With a stopwatch, start timing as the attitude indicator passes through 20 degrees and stop timing as the attitude indicator passes through 0 degrees. Record that time. Retrim the aircraft for straight and level flight. If the two recorded times are within .5 seconds of each other your aileron trim tab is properly set. If the recorded time difference exceeds .5 seconds then SAC recommends you have your flight controls and trim tabs checked by a Cirrus Authorized Factory Service Center for proper rigging and tab alignment.


Note 2:


These are the proffered configurations as stated by the respective OEM Installation Manuals.


Attached Files
pdf 01_AP_Tracking_Offset_Resolution_for_Sandel_S-Tec_Equipped_Cirrus_Acft.pdf (46.45 KB, 152 views)
pdf 02_Cirrus_SR-20-22_Rack_and_Connector_Diagram.pdf (141.87 KB, 75 views)
pdf 03_Cirrus_SR-20_Acft_SN_1179-1287_Sys_55X_AP_Interface_22-12-01_2.pdf (162.94 KB, 69 views)
pdf 04_Cirrus_SR-20_Acft_SN_1288-1338_Sys_55X_AP_Interface_22-12-01_3.pdf (185.39 KB, 57 views)
pdf 05_Cirrus_SR-20_Acft_SN_1178-1287_NSD_HDG_Sys_Interface_34-50-03_3.pdf (194.83 KB, 60 views)
pdf 06_Cirrus_SR-20_Acft_SN_1178-1287_Sandel_HDG_Sys_Interface_34-50-03_7.pdf (222.10 KB, 33 views)
pdf 07_Cirrus_SR-22_Acft_SN_0002-0141_Sys-55X_Interface_22-11-01_1.pdf (188.63 KB, 69 views)
pdf 08_Cirrus_SR-22_Acft_SN_0142-0434_Sys-55X_Interface_22-11-01_2.pdf (154.55 KB, 70 views)
pdf 09_Cirrus_SR-22_Acft_SN_0002-0141_NSD_HDG_Sys_Interface_34-50-03_1.pdf (176.16 KB, 60 views)
pdf 10_Cirrus_SR-22_Acft_SN_0142-0434_NSD_HDG_Sys_Interface_34-50-03_3.pdf (154.61 KB, 44 views)
pdf 11_Cirrus_SR-22_Acft_SN_0002-0141_Sandel_HDG_Sys_Interface_34-50-03_5.pdf (178.38 KB, 35 views)
pdf 12_Cirrus_SR-22_Acft_SN_0142-0434_Sandel_HDG_Sys_Interface_34-50-03_6.pdf (148.20 KB, 35 views)
pdf 13_Sandel_3308_FCS_Interconnect_90106-10_Sht_20.pdf (21.13 KB, 59 views)
pdf 14_Sandel_3308_S-Tec_AP_Interconnect_90106-10_Sht_23.pdf (19.40 KB, 74 views)
pdf 15_S-Tec_Wiring_Interface_55-55X.pdf (108.59 KB, 158 views)
pdf 16_S-Tec_HDG_Interface_55-55X.pdf (75.76 KB, 103 views)
pdf 17_C41_AP_Wiring_Interface_63D595.pdf (146.04 KB, 78 views)
pdf 18_C41_HDG_Sys_Interface_63D585_Sht_1.pdf (185.47 KB, 70 views)
pdf 19_C41_HDG_Sys_Interface_63D585_Sht_2.pdf (72.49 KB, 62 views)

Jerry "Butch" Giles
Pilot, Autopilot Tech
So. Avionics
Mobile, AL.

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