Narrative:

Flight conditions were light winds; light choppy air; and severe clear below a broken layer at 12;000. While on an approximately 8 to 10 mile final for runway 12R at sat (san antonio; tx); capturing (flight director indicated 'asel') ATC assigned altitude of 3;000 ft (though we had also just been cleared for a visual approach with the aerodrome in sight); the autopilot self-disconnected and the aircraft rapidly pitched up to approximately 5 degrees; which also caused us to climb 300 ft above last assigned altitude of 3;000. We were at approximately 185 KTS indicated airspeed and the flaps were in transit from zero degrees to the fifteen degree position. The pitch movement was abrupt and uncomfortable; though I believe it was within normal 'g' limits. The first officer was the pilot flying. He immediately grabbed the control wheel; arrested the increasing pitch and; with very heavy stick forward force; returned the aircraft to a nose low attitude and recovered the altitude. During this recovery he used 'a lot' of electric trim to obtain neutral pressure on the control wheel. Because we were unsure what caused the out of trim situation and high control forces which seemed to have caused the autopilot to disconnect; we left the flaps at the fifteen degree position for the landing and landed normally after quickly completing the no flap landing checklist in the abnormal procedures. Adequate runway was available with landing distance multiplier applied. No emergency was declared. No abnormal annunciator lights were noted other than the 'hydraulic press' coming on during the flap extension. However; both of us felt that the light was on (and therefore the hydraulic system being pressurized to run a system) for much less than the 23 seconds the hydraulic stabilizer needed to run through full transit. This was noted in the maintenance write-up; though after discussions with maintenance personnel; it is possible that we experienced time compression and the system was pressurized for the appropriate time. After landing we did not retract flaps so we could check the position of the horizontal stabilizer; which after shutdown and post-flight; it appeared to be in the correct position per the indicators on the left horizontal stabilizer. Maintenance write-up is as follows: 'during approach and level off at 3;000 ft from a descent; and stable airspeed of 185 KTS; flaps were selected to 15 degrees. During flap transit the autopilot abruptly kicked off and the aircraft pitched up causing a rapid 300 ft climb. The 'hydraulic press' light was on for only 10 to 12 seconds. When the first officer grabbed the controls; he needed strong stick forward pressure. He used a lot of electric trim to regain neutral stick forces. No other abnormal annunciations were noted.' maintenance corrective action is as follows: 'checked event codes. Noted code in air is code 221. This code is caused by pilot action resulting in back driving the servo. Per the P-1000 sys test and fault isolation manual. No maintenance action is required unless code continues to occur. Ops checked good in accordance with 560XL mmch 22-11-00.'it is both flight crewmembers contention that we acted in compliance with all aircraft limitations and air carrier approved operating procedures. I am concerned that the maintenance corrective action implies that our approved normal procedures can cause a pitch upset in this aircraft type. The only procedural change that I can think to do in the future would be to not select a flap change; perhaps especially from the zero degree position because of movement of the hydraulic stabilizer; during an autopilot altitude capture. Perhaps the autopilot trim could not keep up with trim change during both a level off and configuration change. Perhaps the trim momentarily hung up for an unknown reason.

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Original NASA ASRS Text

Title: CE560 flight crew experienced autopilot pitch up during glideslope capture and flap extension. First Officer took over manually needing a great deal of nose down pitch trim and when stable reengaged the autopilot.

Narrative: Flight conditions were light winds; light choppy air; and severe clear below a broken layer at 12;000. While on an approximately 8 to 10 mile final for runway 12R at SAT (San Antonio; TX); capturing (Flight Director indicated 'ASEL') ATC assigned altitude of 3;000 FT (though we had also just been cleared for a visual approach with the aerodrome in sight); the autopilot self-disconnected and the aircraft rapidly pitched up to approximately 5 degrees; which also caused us to climb 300 FT above last assigned altitude of 3;000. We were at approximately 185 KTS indicated airspeed and the flaps were in transit from zero degrees to the fifteen degree position. The pitch movement was abrupt and uncomfortable; though I believe it was within normal 'g' limits. The First Officer was the pilot flying. He immediately grabbed the control wheel; arrested the increasing pitch and; with very heavy stick forward force; returned the aircraft to a nose low attitude and recovered the altitude. During this recovery he used 'a lot' of electric trim to obtain neutral pressure on the control wheel. Because we were unsure what caused the out of trim situation and high control forces which seemed to have caused the autopilot to disconnect; we left the flaps at the fifteen degree position for the landing and landed normally after quickly completing the No Flap Landing checklist in the abnormal procedures. Adequate runway was available with landing distance multiplier applied. No emergency was declared. No abnormal annunciator lights were noted other than the 'HYD PRESS' coming on during the flap extension. However; both of us felt that the light was on (and therefore the hydraulic system being pressurized to run a system) for much less than the 23 seconds the hydraulic stabilizer needed to run through full transit. This was noted in the maintenance write-up; though after discussions with maintenance personnel; it is possible that we experienced time compression and the system was pressurized for the appropriate time. After landing we did not retract flaps so we could check the position of the horizontal stabilizer; which after shutdown and post-flight; it appeared to be in the correct position per the indicators on the left horizontal stabilizer. Maintenance write-up is as follows: 'During approach and level off at 3;000 FT from a descent; and stable airspeed of 185 KTS; flaps were selected to 15 degrees. During flap transit the autopilot abruptly kicked off and the aircraft pitched up causing a rapid 300 FT climb. The 'HYD PRESS' light was on for only 10 to 12 seconds. When the First Officer grabbed the controls; he needed strong stick forward pressure. He used a lot of electric trim to regain neutral stick forces. No other abnormal annunciations were noted.' Maintenance Corrective Action is as follows: 'Checked event codes. Noted code in air is code 221. This code is caused by pilot action resulting in back driving the servo. Per the P-1000 sys test and fault isolation manual. No maintenance action is required unless code continues to occur. Ops checked good in accordance with 560XL MMCH 22-11-00.'It is both flight crewmembers contention that we acted in compliance with all aircraft limitations and air carrier approved operating procedures. I am concerned that the maintenance corrective action implies that our approved normal procedures can cause a pitch upset in this aircraft type. The only procedural change that I can think to do in the future would be to not select a flap change; perhaps especially from the zero degree position because of movement of the hydraulic stabilizer; during an autopilot altitude capture. Perhaps the autopilot trim could not keep up with trim change during both a level off and configuration change. Perhaps the trim momentarily hung up for an unknown reason.

Data retrieved from NASA's ASRS site as of April 2012 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.