Hawker 4000 captain reported an altitude deviation while troubleshooting a yaw dampener failure.

Narrative:

While enroute at FL400 and mach .82 on an IFR flight; aircraft X; a hawker 4000; experienced a yaw dampener failure. With the failure comes a dutch roll tendency of the aircraft that can be controlled by deploying speed brakes. The autopilot and auto throttles are also not operational once the yaw dampener failure occurs. Since the aircraft no longer meets rvsm requirements a request for immediate decent was requested and approved to FL280. ATC also requested an immediate turn to the south. While controlling the aircraft and the copilot troubleshooting the system the decent reached approximately 3-4000 per minute. While leveling off with the auto pilot and auto throttle not operational and speed brakes deployed I failed to start decreasing the descent rate prior to the clearance level resulting in over shooting the altitude approximately 450 feet lower. An immediate correction was initiated and the correct flight level was achieved in less than 15 seconds. The flight continued at FL280 and autopilot and autothrottle systems were brought back on line. A rudder fault cas message remained until the aircraft computers were power cycled. The failures could not be replicated and the maintenance software download was sent to maintenance for the review. Emergency procedures for this failure will be reviewed as well as a safety brief to other pilots of this aircraft.

Original NASA ASRS Text

Title: Hawker 4000 Captain reported an altitude deviation while troubleshooting a yaw dampener failure.

Narrative: While enroute at FL400 and Mach .82 on an IFR flight; Aircraft X; a Hawker 4000; experienced a Yaw dampener failure. With the failure comes a Dutch roll tendency of the aircraft that can be controlled by deploying speed brakes. The autopilot and auto throttles are also not operational once the yaw dampener failure occurs. Since the aircraft no longer meets RVSM requirements a request for immediate decent was requested and approved to FL280. ATC also requested an immediate turn to the south. While controlling the aircraft and the copilot troubleshooting the system the decent reached approximately 3-4000 per minute. While leveling off with the auto pilot and auto throttle not operational and speed brakes deployed I failed to start decreasing the descent rate prior to the clearance level resulting in over shooting the altitude approximately 450 feet lower. An immediate correction was initiated and the correct flight level was achieved in less than 15 seconds. The flight continued at FL280 and autopilot and autothrottle systems were brought back on line. A rudder Fault CAS message remained until the aircraft computers were power cycled. The failures could not be replicated and the maintenance software download was sent to maintenance for the review. Emergency procedures for this failure will be reviewed as well as a safety brief to other pilots of this aircraft.

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.

Attributes
ACN	1669398
Time
Date	201907
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Make Model Name	Hawker 4000
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Autoflight Yaw Damper
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Flight Instructor Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 25 Flight Crew Total 3900 Flight Crew Type 125
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance