A CL300 compass comparator alerted as the aircraft approached runway 31L approach end on taxiway F. ...

Narrative:

While taking runway 31C for takeoff we experienced a heading mis-compare flag. I reset my directional gyro and the flag went out until right after takeoff. We were given an initial heading of 220 after takeoff. Then we were given another left turn to 180. The heading flag returned while we were in these turns; so we never knew exactly what our real heading was. When we leveled out our heading was 40 degrees off. The controller then gave us direct to our first fix intersection. When we finally got our headings corrected after the heading flag went out; we were 40 degrees off our assigned heading. I advised the controller of our heading problem and he gave us a vector. There was no traffic conflict and the flight continued without incident. Due to the fact that we were in a constant left turn; we were never able to establish a known heading. We could have advised ATC earlier and aligned our heading to determine which one was off. The magnetic anomalies around mdw are a known fact among pilots; maybe the airport can find what causes this and correct it.

Original NASA ASRS Text

Title: A CL300 compass Comparator alerted as the aircraft approached Runway 31L approach end on Taxiway F. The compass was reset on Runway 31C but alerted a second time during takeoff and resulted in a 40 degree heading error requiring ATC vectors.

Narrative: While taking Runway 31C for takeoff we experienced a HDG mis-compare Flag. I reset my directional gyro and the flag went out until right after takeoff. We were given an initial heading of 220 after takeoff. Then we were given another left turn to 180. The HDG flag returned while we were in these turns; so we never knew exactly what our real heading was. When we leveled out our heading was 40 degrees off. The Controller then gave us direct to our first fix intersection. When we finally got our headings corrected after the HDG flag went out; we were 40 degrees off our assigned heading. I advised the Controller of our heading problem and he gave us a vector. There was no traffic conflict and the flight continued without incident. Due to the fact that we were in a constant left turn; we were never able to establish a known heading. We could have advised ATC earlier and aligned our heading to determine which one was off. The magnetic anomalies around MDW are a known fact among pilots; maybe the airport can find what causes this and correct it.

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	917513
Time
Date	201011
Local Time Of Day	1201-1800
Place
Locale Reference	MDW.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Challenger 300
Flight Phase	Initial Climb
Flight Plan	IFR
Component
Aircraft Component	Positional / Directional Sensing
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Track / Heading All Types

A CL300 compass comparator alerted as the aircraft approached runway 31L approach end on taxiway F. The compass was reset on runway 31C but alerted a second time during takeoff and resulted in a 40 degree heading error requiring ATC vectors.