C210 pilot reported gpss unreliability due to speaker magnet interference with magnetometer.

Attributes
ACN	1671167
Time
Date	201907
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Cessna 210 Centurion / Turbo Centurion 210C 210D
Operating Under FAR Part	Part 91
Flight Phase	Climb
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Positional / Directional Sensing
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Instrument
Experience	Flight Crew Last 90 Days 10 Flight Crew Total 750 Flight Crew Type 250
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

Departing ZZZ; navigating via GPS and aspen pfd HSI (primary flight display; horizontal situation indicator). Inadvertent off course route up to 45 degrees due to incorrect heading display on the aspen pfd; the primary navigation source for the gpss steering on the autopilot. ATC informed of incorrect route. Corrected course and rebooted aspen; with no resolution. Elected to return to departure airport for further troubleshooting on the ground. ATC provided no-gyro turn assistance. Visual approach to ZZZ with uneventful landing. Aspen was indicating correctly on run up and pre-takeoff checks. Problem with the displayed heading on the aspen pfd continued on the ground; although to a lesser degree; while taxiing. During consultation with avionics shop; realized a pair of speakers being transported had been inadvertently placed in the passenger cabin below the roof mounted magnetometer for the aspen. Confirmed heading interference via aspen configuration screens with speaker movement. After removal of the speakers (large magnets); confirmed proper aspen indications on the ground. Refiled IFR flight plan and departed on 3.5 hour flight with no further abnormalities.

Original NASA ASRS Text

Title: C210 Pilot reported GPSS unreliability due to speaker magnet interference with magnetometer.

Narrative: Departing ZZZ; navigating via GPS and Aspen PFD HSI (Primary Flight Display; Horizontal Situation Indicator). Inadvertent off course route up to 45 degrees due to incorrect heading display on the Aspen PFD; the primary navigation source for the GPSS steering on the autopilot. ATC informed of incorrect route. Corrected course and rebooted Aspen; with no resolution. Elected to return to departure airport for further troubleshooting on the ground. ATC provided no-gyro turn assistance. Visual approach to ZZZ with uneventful landing. Aspen was indicating correctly on run up and pre-takeoff checks. Problem with the displayed heading on the Aspen PFD continued on the ground; although to a lesser degree; while taxiing. During consultation with avionics shop; realized a pair of speakers being transported had been inadvertently placed in the passenger cabin below the roof mounted magnetometer for the Aspen. Confirmed heading interference via Aspen configuration screens with speaker movement. After removal of the speakers (large magnets); confirmed proper Aspen indications on the ground. Refiled IFR flight plan and departed on 3.5 hour flight with no further abnormalities.

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.