King air pilot at 16;000 feet on autopilot experiences a pitch over losing 250 ft before manually re...

Attributes
ACN	1152259
Time
Date	201402
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	King Air C90 E90
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 255 Flight Crew Total 1605 Flight Crew Type 305
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

About one hour into the flight at 16;000 ft the aircraft pitched down approximately 10 degrees below the horizon. Immediate action was taken; but 250 ft of altitude was lost before correcting back to 16;000. The center controller gave me the current altimeter setting 30.11 and my assigned altitude of 16;000. After correcting; I determined the flight could be continued without the autopilot with any safety issues. Additionally; there is a number 2 autopilot that was operational. Both autopilots were tested and found to be functioning properly on the ground prior to this flight and had no recent history of adverse issues; but the aircraft will be sent to maintenance for further inspection of the autopilot. I plan to have the issue diagnosed before the next flight in the airplane. From my experience this could possibly be a pitch servo failure. To prevent recurrence of this situation I plan to continue my pre-flight checks of the autopilot and to closely monitor altitude and headings at all times to be ready to manually correct issues that may arise as quickly as possible.

Original NASA ASRS Text

Title: King Air pilot at 16;000 feet on autopilot experiences a pitch over losing 250 FT before manually recovering. ATC takes notice.

Narrative: About one hour into the flight at 16;000 FT the aircraft pitched down approximately 10 degrees below the horizon. Immediate action was taken; but 250 FT of altitude was lost before correcting back to 16;000. The Center Controller gave me the current altimeter setting 30.11 and my assigned altitude of 16;000. After correcting; I determined the flight could be continued without the autopilot with any safety issues. Additionally; there is a number 2 autopilot that was operational. Both autopilots were tested and found to be functioning properly on the ground prior to this flight and had no recent history of adverse issues; but the aircraft will be sent to maintenance for further inspection of the autopilot. I plan to have the issue diagnosed before the next flight in the airplane. From my experience this could possibly be a pitch servo failure. To prevent recurrence of this situation I plan to continue my pre-flight checks of the autopilot and to closely monitor altitude and headings at all times to be ready to manually correct issues that may arise as quickly as possible.

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.

King air pilot at 16;000 feet on autopilot experiences a pitch over losing 250 ft before manually recovering. ATC takes notice.