During climb MD80 flight crew experienced 'yaw damp off' and 'mach trim inop' lights on the MCP. Pro...

Narrative:

Climbing out through FL260 the copilot was flying and felt a slight yaw of the aircraft. At the same time the autopilot and autothrottles disengaged and would not reconnect in any position. We leveled off at FL310. The 'yaw damp off' and 'mach trim inop' lights illuminated on the MCP. On the fgcp the altitude window was blank and the vertical speed and speed/mach switches were inop. We were below maximum landing weight so I decided to hand fly the plane in VFR conditions to phx. I made an uneventful landing at phx. As we taxied to the gate the FMA read; 'fail/power/tst/Box2' over '26/vac/lft'. Maintenance found the '28 vac dfg system switch a' circuit breaker popped (it wasn't out when checked in flight). Maintenance reset the circuit breaker and ran several tests which all passed. They then decided to replace the circuit breaker because it was deemed 'weak'. It was replaced and we flew the aircraft later that day with no problems.

Original NASA ASRS Text

Title: During climb MD80 flight crew experienced 'Yaw Damp Off' and 'Mach Trim Inop' lights on the MCP. Problem could not be corrected and crew elects to divert to enroute alternate.

Narrative: Climbing out through FL260 the copilot was flying and felt a slight yaw of the aircraft. At the same time the autopilot and autothrottles disengaged and would not reconnect in any position. We leveled off at FL310. The 'Yaw Damp Off' and 'Mach Trim Inop' lights illuminated on the MCP. On the FGCP the altitude window was blank and the vertical speed and speed/mach switches were inop. We were below maximum landing weight so I decided to hand fly the plane in VFR conditions to PHX. I made an uneventful landing at PHX. As we taxied to the gate the FMA read; 'Fail/Power/TST/Box2' over '26/VAC/LFT'. Maintenance found the '28 VAC DFG System Switch A' circuit breaker popped (it wasn't out when checked in flight). Maintenance reset the circuit breaker and ran several tests which all passed. They then decided to replace the circuit breaker because it was deemed 'weak'. It was replaced and we flew the aircraft later that day with no problems.

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	854484
Time
Date	200909
Local Time Of Day	0601-1200
Place
Locale Reference	ZLA.ARTCC
State Reference	CA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Circuit Breaker / Fuse / Thermocouple
Person 1
Function	Captain Pilot Not Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe

During climb MD80 flight crew experienced 'yaw damp off' and 'mach trim inop' lights on the MCP. Problem could not be corrected and crew elects to divert to enroute alternate.