Narrative:

The s-tec autoplt computer model #55X has a pitch porpoise problem in all PA32 series aircraft as well as localizer/navigation tracking issues. The pitch and roll gain resistors could be changed to a different value in order to eliminate the pitch porpoise and to track the localizer tighter eliminating the s-turning that is normal. It has been a common practice to replace the resistor to fix the problem. The s-tec manuals have not been updated since 2003 when s-tec sent a letter out that this procedure along with others could no longer be done in the field as they were not going to update their repair manuals. I was not aware of the memo until the manufacturer notified me. I have issued a stop work notice on all s-tec 55X computers for this work; that the units have to go to the factory. We are recalling all aircraft that had this work done to them from 2004 - 2007; and notifying the customer of the error. Furthermore; the aircraft needs to have the computer returned back to the manufacturer's specifications through this facility or another repair station that is an s-tec dealer with a logbook entry showing the return to service being completed and a copy sent to this repair station. All personnel have been notified that these computers must go back to the factory and all s-tec manuals reflect the latest changes.callback conversation with reporter revealed the following information: reporter stated s-tec company has merged with meggitt avionics. Meggitt has kept the s-tec product name. Reporter also states his repair facility had been repairing the s-tec 55X autopilot computers with resistors of different values that corrected the rapid porpoise oscillations of twenty feet up and down while in the autopilot mode. The different valued resistors also tightened up the localizer/navigation tracking s-turning issues. All piper PA32's; whether new or older aircraft; with this 55X autopilot computer; experienced the same flight issues. S-tec had recently notified his repair facility that s-tec alone; would do the repairs; that field repairs were not allowed. Reporter also noted that s-tecsouth repair on the model 55X units was to replace the original tso resistors with new resistors of exactly the same value as those being removed. And the same autopilot problems still surface after the mfg repair. Reporter has informed s-tec about his repair facility fix for the pitch porpoise and tracking problems but feels s-tec is not interested.

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Original NASA ASRS Text

Title: PIPER PA32 ACFT WITH S-TEC AUTOPLT COMPUTER MODEL 55X ARE SUBJECT TO PITCH PORPOISE PROBS AND LOCALIZER/NAV TRACKING ISSUES.

Narrative: THE S-TEC AUTOPLT COMPUTER MODEL #55X HAS A PITCH PORPOISE PROB IN ALL PA32 SERIES ACFT AS WELL AS LOCALIZER/NAV TRACKING ISSUES. THE PITCH AND ROLL GAIN RESISTORS COULD BE CHANGED TO A DIFFERENT VALUE IN ORDER TO ELIMINATE THE PITCH PORPOISE AND TO TRACK THE LOCALIZER TIGHTER ELIMINATING THE S-TURNING THAT IS NORMAL. IT HAS BEEN A COMMON PRACTICE TO REPLACE THE RESISTOR TO FIX THE PROB. THE S-TEC MANUALS HAVE NOT BEEN UPDATED SINCE 2003 WHEN S-TEC SENT A LETTER OUT THAT THIS PROC ALONG WITH OTHERS COULD NO LONGER BE DONE IN THE FIELD AS THEY WERE NOT GOING TO UPDATE THEIR REPAIR MANUALS. I WAS NOT AWARE OF THE MEMO UNTIL THE MANUFACTURER NOTIFIED ME. I HAVE ISSUED A STOP WORK NOTICE ON ALL S-TEC 55X COMPUTERS FOR THIS WORK; THAT THE UNITS HAVE TO GO TO THE FACTORY. WE ARE RECALLING ALL ACFT THAT HAD THIS WORK DONE TO THEM FROM 2004 - 2007; AND NOTIFYING THE CUSTOMER OF THE ERROR. FURTHERMORE; THE ACFT NEEDS TO HAVE THE COMPUTER RETURNED BACK TO THE MANUFACTURER'S SPECIFICATIONS THROUGH THIS FACILITY OR ANOTHER REPAIR STATION THAT IS AN S-TEC DEALER WITH A LOGBOOK ENTRY SHOWING THE RETURN TO SVC BEING COMPLETED AND A COPY SENT TO THIS REPAIR STATION. ALL PERSONNEL HAVE BEEN NOTIFIED THAT THESE COMPUTERS MUST GO BACK TO THE FACTORY AND ALL S-TEC MANUALS REFLECT THE LATEST CHANGES.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED S-TEC COMPANY HAS MERGED WITH MEGGITT AVIONICS. MEGGITT HAS KEPT THE S-TEC PRODUCT NAME. REPORTER ALSO STATES HIS REPAIR FACILITY HAD BEEN REPAIRING THE S-TEC 55X AUTOPILOT COMPUTERS WITH RESISTORS OF DIFFERENT VALUES THAT CORRECTED THE RAPID PORPOISE OSCILLATIONS OF TWENTY FEET UP AND DOWN WHILE IN THE AUTOPILOT MODE. THE DIFFERENT VALUED RESISTORS ALSO TIGHTENED UP THE LOCALIZER/NAV TRACKING S-TURNING ISSUES. ALL PIPER PA32'S; WHETHER NEW OR OLDER ACFT; WITH THIS 55X AUTOPILOT COMPUTER; EXPERIENCED THE SAME FLIGHT ISSUES. S-TEC HAD RECENTLY NOTIFIED HIS REPAIR FACILITY THAT S-TEC ALONE; WOULD DO THE REPAIRS; THAT FIELD REPAIRS WERE NOT ALLOWED. REPORTER ALSO NOTED THAT S-TECS REPAIR ON THE MODEL 55X UNITS WAS TO REPLACE THE ORIGINAL TSO RESISTORS WITH NEW RESISTORS OF EXACTLY THE SAME VALUE AS THOSE BEING REMOVED. AND THE SAME AUTOPILOT PROBLEMS STILL SURFACE AFTER THE MFG REPAIR. REPORTER HAS INFORMED S-TEC ABOUT HIS REPAIR FACILITY FIX FOR THE PITCH PORPOISE AND TRACKING PROBLEMS BUT FEELS S-TEC IS NOT INTERESTED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.