Bonanza 35
Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial
Flight Crew Last 90 Days 144; Flight Crew Total 4791; Flight Crew Type 2000
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Aircraft; Human Factors
Columbus approach put me on vectors for the ILS 24L. Vectors were intended to align aircraft with ZAMBO IAF. Before handing me over to Dayton Tower; Columbus Approach mentioned that I should check my compass for deviations as he noticed my compass headings being off by about 30 degrees; requiring him to make several adjustments. On the ground I investigated the cause of the deviation and found that the Garmin 496 GPS unit (on glareshield with bean-bag mount) I had used on this flight was close enough to the compass to cause this deviation. When removed; the compass worked accurately as per my compass deviation card. There was no hazard in this instance but I am making this report so that others can benefit from this observation that a glareshield mounted portable GPS unit may in fact cause the compass to show a deviated heading.
BE35 pilot is advised by ATC to have his compass checked after landing. It is found that the glareshield mounted Garmin 496 causes a 30 degree compass deviation.