A mu-2 pilot flying an autopilot functional test flight inadvertently changed the garmin 600 baromet...

Narrative:

I was trouble shooting an autopilot issue and had been given a maintain 3;000 ft command. The autopilot was not functioning correctly and I was trying to trouble shoot when I looked up and saw a 4;000 ft altitude indicated on the garmin 600. I immediately descended which got ATC to ask me why I was descending. I descended back to 3;000 which really was 2;000. During the wrestling with the autopilot I must have inadvertently bumped the baro switch that is right next to the knob that is used to select all functions of the garmin 600 instrument. The baro was set to the correct setting on departure and I had not made a change. Upon the query from ATC on the altitude I cross checked the altimeters that were not tied to the garmin 600 and recognized I had a much higher baro set in the garmin. Way beyond what was set on departure. My feedback is that the garmin 600 position makes you reach through the yoke with the non flying hand and it is awkward at best. Also; with the autopilot that is in this airplane the autopilot is not tied to the garmin 600 and is tied to the co-pilot altimeter. This requires you set altitude pre-select in two places which can be cumbersome in single pilot operations. In addition; because this was an autopilot functional test flight and it was not functioning properly; the cockpit was abnormally busy between operationally checking the autopilot and complying with ATC. The case could be made that it would be prudent to have two pilots on any maintenance check flight. The autopilot ground check was fine but was having interface problems with the garmin. Hence I was recording values while flying so ground adjustments could be made. There was no conflict and the flight was conducted VFR.

Original NASA ASRS Text

Title: A MU-2 pilot flying an autopilot functional test flight inadvertently changed the Garmin 600 barometric setting so it indicated 4;000 FT and; thinking he was a thousand feet high; descended to 2;000 FT while assigned 3;000 FT.

Narrative: I was trouble shooting an autopilot issue and had been given a maintain 3;000 FT command. The autopilot was not functioning correctly and I was trying to trouble shoot when I looked up and saw a 4;000 FT altitude indicated on the Garmin 600. I immediately descended which got ATC to ask me why I was descending. I descended back to 3;000 which really was 2;000. During the wrestling with the autopilot I must have inadvertently bumped the BARO switch that is right next to the knob that is used to select all functions of the Garmin 600 instrument. The BARO was set to the correct setting on departure and I had not made a change. Upon the query from ATC on the altitude I cross checked the altimeters that were not tied to the Garmin 600 and recognized I had a much higher BARO set in the Garmin. Way beyond what was set on departure. My feedback is that the Garmin 600 position makes you reach through the yoke with the non flying hand and it is awkward at best. Also; with the autopilot that is in this airplane the autopilot is not tied to the Garmin 600 and is tied to the co-pilot altimeter. This requires you set altitude pre-select in two places which can be cumbersome in single pilot operations. In addition; because this was an autopilot functional test flight and it was not functioning properly; the cockpit was abnormally busy between operationally checking the autopilot and complying with ATC. The case could be made that it would be prudent to have two pilots on any maintenance check flight. The autopilot ground check was fine but was having interface problems with the Garmin. Hence I was recording values while flying so ground adjustments could be made. There was no conflict and the flight was conducted VFR.

Data retrieved from NASA's ASRS site as of July 2013 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	1068108
Time
Date	201302
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	MU-2 Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	VFR
Component
Aircraft Component	Altimeter
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 60 Flight Crew Total 5000 Flight Crew Type 3000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

A mu-2 pilot flying an autopilot functional test flight inadvertently changed the garmin 600 barometric setting so it indicated 4;000 ft and; thinking he was a thousand feet high; descended to 2;000 ft while assigned 3;000 ft.