A C560 flight crew failed to level at their cleared altitude when the altitude capture function of t...

Narrative:

During descent into las I was cleared to descend to 11;000 MSL. The autopilot was coupled and the altitude preselector was also set to 11;000 MSL. The autopilot did not capture the selected altitude. I'm certain this is due to a change in pitch of the autopilot (ap) pitch trim control wheel located on the ap MCP. If this trim wheel is moved within the 'capture' algorithm of this autopilot the 'capture' disengages causing the altitude to be flown through by the autopilot.at that time I was looking out the window to identify the correct runway so as to be ready for the visual approach so my attention was not on the altitude during those few moments of planned level off. When the aircraft did not level off at the preselected altitude the preselector chimed at 300 ft below the selected level off altitude (as it should). This chime got my attention; as it would not have been expected; causing me to notice that we had descended through 11;000 MSL. At this time we were at 10;700 MSL. I disengaged the autopilot and arrested the descent at approximately 10;500 MSL and immediately brought the aircraft altitude back to 11;000. The entire occurrence did not exceed 10 seconds. There was no radio or telephone communication between us and ATC about this deviation. I don't really even know whether it was observed by the controller as it was a small deviation which was corrected rapidly.as a professional pilot; I believe these occurrences are both inevitable and avoidable. However; in order for the inevitable to be avoided; a plan must be developed to ensure these occurrences are minimized or eliminated.I view this as an opportunity to better our CRM. To prevent an occurrence like this from happening again; I've elected to enact a new policy to our standard operating procedures. We will now require the flying pilot to make a standard call of: 'altitude captured' when the autopilot begins its capture algorithm. If for some reason the flying pilot's eyes are outside; he/she will indicate that by saying; 'I'm outside'. This will transfer the monitoring of the altitude and various other instruments to the non flying pilot who will then be responsible for the 'altitude captured' call out in the event a level off is planned.

Original NASA ASRS Text

Title: A C560 Flight Crew failed to level at their cleared altitude when the altitude capture function of the autoflight system malfunctioned.

Narrative: During descent into LAS I was cleared to descend to 11;000 MSL. The autopilot was coupled and the altitude preselector was also set to 11;000 MSL. The autopilot did not capture the selected altitude. I'm certain this is due to a change in pitch of the autopilot (AP) pitch trim control wheel located on the AP MCP. If this trim wheel is moved within the 'capture' algorithm of this autopilot the 'capture' disengages causing the altitude to be flown through by the autopilot.At that time I was looking out the window to identify the correct runway so as to be ready for the visual approach so my attention was not on the altitude during those few moments of planned level off. When the aircraft did not level off at the preselected altitude the preselector chimed at 300 FT below the selected level off altitude (as it should). This chime got my attention; as it would not have been expected; causing me to notice that we had descended through 11;000 MSL. At this time we were at 10;700 MSL. I disengaged the autopilot and arrested the descent at approximately 10;500 MSL and immediately brought the aircraft altitude back to 11;000. The entire occurrence did not exceed 10 seconds. There was no radio or telephone communication between us and ATC about this deviation. I don't really even know whether it was observed by the Controller as it was a small deviation which was corrected rapidly.As a professional pilot; I believe these occurrences are both inevitable and avoidable. However; in order for the inevitable to be avoided; a plan must be developed to ensure these occurrences are minimized or eliminated.I view this as an opportunity to better our CRM. To prevent an occurrence like this from happening again; I've elected to enact a new policy to our standard operating procedures. We will now require the flying pilot to make a standard call of: 'Altitude Captured' when the autopilot begins its capture algorithm. If for some reason the flying pilot's eyes are outside; he/she will indicate that by saying; 'I'm Outside'. This will transfer the monitoring of the altitude and various other instruments to the non flying pilot who will then be responsible for the 'Altitude Captured' call out in the event a level off is planned.

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.

Attributes
ACN	912385
Time
Date	201010
Local Time Of Day	1201-1800
Place
Locale Reference	LAS.Airport
State Reference	NV
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Citation III VI VII (C650)
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 55 Flight Crew Total 8000 Flight Crew Type 1000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

A C560 flight crew failed to level at their cleared altitude when the altitude capture function of the autoflight system malfunctioned.