Narrative:

While leveling at FL350, I engaged the autoplt. Without warning, the airplane went into an uncommanded dive. I attempted to regain control by using the pitch trim on the autoplt. As we passed through FL345 I realized I had to do something else. I disengaged the autoplt completely and used the pitch trim on the yoke and power to stop the descent. By the time I was able to regain full control, the airplane was level at approximately FL343 to FL340. I immediately contacted center and advised them that we had a problem with the autoplt. ZDC replied 'ok, thanks.' center did not seem upset and there was not any other traffic around us. Center didn't inquire about the flight or crew information. This is not the first time we had problems with the autoplt. The system has been worked on in the past before. However, the system was working fine up to this point. We were subsequently cleared to FL410. Callback conversation with reporter revealed the following information: since he started working at this company 13 months ago, the captain has grounded this aircraft 4 times -- all in the last 3 months. The captain stated that he will no longer accept this aircraft for flight because the problem has not yet been resolved. Though the mechanics seem to try various fixes, the reporter stated that he felt as if he were a test pilot on each flight and that he is uncomfortable having patients on board when the aircraft is acting in such an unpredictable manner. He asserts that, on the ground, the autoplt test always works perfectly. When he moves the yoke forward and aft, the force bar for pitch moves down and up respectively. When he moves the yoke to the right or left, the force bar indicates a roll, and when he pushes on the rudder, the force bar indicates properly in the yaw axis. However, in the air, the force bars, which are located only on the captain's side behind the yoke, are often indicating uncommanded inputs which do not correspond to the actual position of the flight control surfaces. He has experienced activating the autoplt when the force bars are not visible and has unpleasantly discovered that the autoplt will follow the force bars rather than the yoke. In an attempt to engage the autoplt without any abrupt movements, the captain has experimented with coaxing the force bars to neutral by manipulating the yoke in such a fashion so as to cause the force bars to move. This generally has not been successful and so he has flown most of his flts by hand. He reported that, on one occasion the autoplt, which is lear factory equipment (jet fc-110A), popped a roll axis circuit breaker. He also noted that the heading bug does not align with the compass. When heading north, the bug indicates 008 degrees but he is not sure if that has anything to do with the autoplt malfunctions. Callback conversation with reporter acn 450749 revealed the following information: the first officer has been working at this company for 3 months. His first problem with this aircraft occurred when the autoplt was engaged and the aircraft suddenly banked 20 degrees to the right. This problem was written up and appeared to be resolved when the subsequent problem of diving upon autoplt activation began to recur. He found that if he tapped on the force bar indicators and wiggled the altitude button on the autoplt, the force bars centered, at which time the autoplt could be safely engaged. Both rptrs recommended that lear pilots check the force bars before engaging the autoplt.

Related reports:

Google
 

Original NASA ASRS Text

Title: A LEAR 25 CAPT AND FO BOTH SUBMITTED RPTS DESCRIBING AN UNCOMMANDED DIVE WHEN SETTING THE AUTOPLT AT FL350. THE CAPT RECOVERED AND HAND FLEW THE REST OF THE FLT.

Narrative: WHILE LEVELING AT FL350, I ENGAGED THE AUTOPLT. WITHOUT WARNING, THE AIRPLANE WENT INTO AN UNCOMMANDED DIVE. I ATTEMPTED TO REGAIN CTL BY USING THE PITCH TRIM ON THE AUTOPLT. AS WE PASSED THROUGH FL345 I REALIZED I HAD TO DO SOMETHING ELSE. I DISENGAGED THE AUTOPLT COMPLETELY AND USED THE PITCH TRIM ON THE YOKE AND PWR TO STOP THE DSCNT. BY THE TIME I WAS ABLE TO REGAIN FULL CTL, THE AIRPLANE WAS LEVEL AT APPROX FL343 TO FL340. I IMMEDIATELY CONTACTED CTR AND ADVISED THEM THAT WE HAD A PROB WITH THE AUTOPLT. ZDC REPLIED 'OK, THANKS.' CTR DID NOT SEEM UPSET AND THERE WAS NOT ANY OTHER TFC AROUND US. CTR DIDN'T INQUIRE ABOUT THE FLT OR CREW INFO. THIS IS NOT THE FIRST TIME WE HAD PROBS WITH THE AUTOPLT. THE SYS HAS BEEN WORKED ON IN THE PAST BEFORE. HOWEVER, THE SYS WAS WORKING FINE UP TO THIS POINT. WE WERE SUBSEQUENTLY CLRED TO FL410. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: SINCE HE STARTED WORKING AT THIS COMPANY 13 MONTHS AGO, THE CAPT HAS GNDED THIS ACFT 4 TIMES -- ALL IN THE LAST 3 MONTHS. THE CAPT STATED THAT HE WILL NO LONGER ACCEPT THIS ACFT FOR FLT BECAUSE THE PROB HAS NOT YET BEEN RESOLVED. THOUGH THE MECHS SEEM TO TRY VARIOUS FIXES, THE RPTR STATED THAT HE FELT AS IF HE WERE A TEST PLT ON EACH FLT AND THAT HE IS UNCOMFORTABLE HAVING PATIENTS ON BOARD WHEN THE ACFT IS ACTING IN SUCH AN UNPREDICTABLE MANNER. HE ASSERTS THAT, ON THE GND, THE AUTOPLT TEST ALWAYS WORKS PERFECTLY. WHEN HE MOVES THE YOKE FORWARD AND AFT, THE FORCE BAR FOR PITCH MOVES DOWN AND UP RESPECTIVELY. WHEN HE MOVES THE YOKE TO THE R OR L, THE FORCE BAR INDICATES A ROLL, AND WHEN HE PUSHES ON THE RUDDER, THE FORCE BAR INDICATES PROPERLY IN THE YAW AXIS. HOWEVER, IN THE AIR, THE FORCE BARS, WHICH ARE LOCATED ONLY ON THE CAPT'S SIDE BEHIND THE YOKE, ARE OFTEN INDICATING UNCOMMANDED INPUTS WHICH DO NOT CORRESPOND TO THE ACTUAL POS OF THE FLT CTL SURFACES. HE HAS EXPERIENCED ACTIVATING THE AUTOPLT WHEN THE FORCE BARS ARE NOT VISIBLE AND HAS UNPLEASANTLY DISCOVERED THAT THE AUTOPLT WILL FOLLOW THE FORCE BARS RATHER THAN THE YOKE. IN AN ATTEMPT TO ENGAGE THE AUTOPLT WITHOUT ANY ABRUPT MOVEMENTS, THE CAPT HAS EXPERIMENTED WITH COAXING THE FORCE BARS TO NEUTRAL BY MANIPULATING THE YOKE IN SUCH A FASHION SO AS TO CAUSE THE FORCE BARS TO MOVE. THIS GENERALLY HAS NOT BEEN SUCCESSFUL AND SO HE HAS FLOWN MOST OF HIS FLTS BY HAND. HE RPTED THAT, ON ONE OCCASION THE AUTOPLT, WHICH IS LEAR FACTORY EQUIP (JET FC-110A), POPPED A ROLL AXIS CIRCUIT BREAKER. HE ALSO NOTED THAT THE HDG BUG DOES NOT ALIGN WITH THE COMPASS. WHEN HDG N, THE BUG INDICATES 008 DEGS BUT HE IS NOT SURE IF THAT HAS ANYTHING TO DO WITH THE AUTOPLT MALFUNCTIONS. CALLBACK CONVERSATION WITH RPTR ACN 450749 REVEALED THE FOLLOWING INFO: THE FO HAS BEEN WORKING AT THIS COMPANY FOR 3 MONTHS. HIS FIRST PROB WITH THIS ACFT OCCURRED WHEN THE AUTOPLT WAS ENGAGED AND THE ACFT SUDDENLY BANKED 20 DEGS TO THE R. THIS PROB WAS WRITTEN UP AND APPEARED TO BE RESOLVED WHEN THE SUBSEQUENT PROB OF DIVING UPON AUTOPLT ACTIVATION BEGAN TO RECUR. HE FOUND THAT IF HE TAPPED ON THE FORCE BAR INDICATORS AND WIGGLED THE ALT BUTTON ON THE AUTOPLT, THE FORCE BARS CTRED, AT WHICH TIME THE AUTOPLT COULD BE SAFELY ENGAGED. BOTH RPTRS RECOMMENDED THAT LEAR PLTS CHK THE FORCE BARS BEFORE ENGAGING THE AUTOPLT.

Data retrieved from NASA's ASRS site as of July 2007 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.