Narrative:

I was conducting a stall test flight on a lear 60 between 15;000 and 17;000 ft MSL. This was a test flight with flight crew only on board following extensive maintenance prior to the aircraft being returned to service. At this time we were operating VFR with flight following from approach. During this time we encountered some issues with the autopilot. The lh side autopilot would not operate properly due to the altitudes (altitude select) indicator light being inoperative and therefore we were unable to verify its status. We also noted a red trim message would post occasionally on the autopilot panel with the autopilot engaged; indicating the autopilot servos may not be trimming properly. We did not see this anomaly on the rh side autopilot so we decided to continue with our plan to go to high altitude using only the rh autopilot. After completing the stalls; I activated our IFR flight plan and we climbed to FL410. Note also that we filed non-rvsm.after completing our systems observations at FL410; we requested a descent and return to [departure airport]. We had been using the autopilot while in class a airspace with no issues. While descending to an assigned altitude of FL260 with an autopilot connected descent in speed (speed) mode at an approximate airspeed of 280 KIAS; we observed the autopilot slowly pulling the nose upwards to begin leveling off; as expected. Suddenly; the autopilot disconnected and the nose went violently downward with a subsequent rapid increase in airspeed. The control yoke snapped almost full forward. The thrust levers were already at idle so I deployed the spoilers and pulled carefully on the control yoke while monitoring airspeed. It went into overspeed as I began leveling the aircraft. I retracted the spoilers at this point since they create more nose down force when above vmo/mmo. I continued to pull the nose upward carefully to avoid over stressing the aircraft. I got the airplane back to level flight and began assessing our situation when ATC called and instructed an immediate climb; which is when I first noted the altitude deviation...we were at about FL240 when we got the airplane back under control. We immediately climbed back to FL260 and did not engage the autopilot for the remainder of the flight. It is likely the red trim light had posted during the descent indicating the autopilot was having trouble with the pitch trim but we did not notice it. At this time I considered reporting the malfunction to the controller; but since he had already called us I knew we would be having a discussion back on the ground regarding the altitude deviation. Therefore; since we were back in control of the airplane I elected not to advise ATC. During the subsequent phone call; I provided them with details of the deviation.in retrospect; the autopilot anomalies observed at low altitude were more serious than originally thought even though the rh autopilot seemed to be functioning normally. In the future; I will give more consideration to possible system malfunctions and plan the remainder of the flight accordingly. I will also keep ATC advised whenever an issue or malfunction develops.

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Original NASA ASRS Text

Title: Lear 60 test pilot reported a 2000 ft altitude excursion due to an autopilot pitch malfunction.

Narrative: I was conducting a stall test flight on a Lear 60 between 15;000 and 17;000 ft MSL. This was a test flight with flight crew only on board following extensive maintenance prior to the aircraft being returned to service. At this time we were operating VFR with Flight Following from Approach. During this time we encountered some issues with the autopilot. The LH side autopilot would not operate properly due to the ALTS (Altitude Select) indicator light being inoperative and therefore we were unable to verify its status. We also noted a red trim message would post occasionally on the autopilot panel with the autopilot engaged; indicating the autopilot servos may not be trimming properly. We did not see this anomaly on the RH side autopilot so we decided to continue with our plan to go to high altitude using only the RH autopilot. After completing the stalls; I activated our IFR flight plan and we climbed to FL410. Note also that we filed non-RVSM.After completing our systems observations at FL410; we requested a descent and return to [departure airport]. We had been using the autopilot while in Class A airspace with no issues. While descending to an assigned altitude of FL260 with an autopilot connected descent in SPD (Speed) mode at an approximate airspeed of 280 KIAS; we observed the autopilot slowly pulling the nose upwards to begin leveling off; as expected. Suddenly; the autopilot disconnected and the nose went violently downward with a subsequent rapid increase in airspeed. The control yoke snapped almost full forward. The thrust levers were already at idle so I deployed the spoilers and pulled carefully on the control yoke while monitoring airspeed. It went into overspeed as I began leveling the aircraft. I retracted the spoilers at this point since they create more nose down force when above Vmo/Mmo. I continued to pull the nose upward carefully to avoid over stressing the aircraft. I got the airplane back to level flight and began assessing our situation when ATC called and instructed an immediate climb; which is when I first noted the altitude deviation...we were at about FL240 when we got the airplane back under control. We immediately climbed back to FL260 and did not engage the autopilot for the remainder of the flight. It is likely the red trim light had posted during the descent indicating the autopilot was having trouble with the pitch trim but we did not notice it. At this time I considered reporting the malfunction to the controller; but since he had already called us I knew we would be having a discussion back on the ground regarding the altitude deviation. Therefore; since we were back in control of the airplane I elected not to advise ATC. During the subsequent phone call; I provided them with details of the deviation.In retrospect; the autopilot anomalies observed at low altitude were more serious than originally thought even though the RH autopilot seemed to be functioning normally. In the future; I will give more consideration to possible system malfunctions and plan the remainder of the flight accordingly. I will also keep ATC advised whenever an issue or malfunction develops.

Data retrieved from NASA's ASRS site 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.