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.
Synopsis
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.
NASA callback
The Reporter stated that the aircraft was taxied down Taxiway F; across the 22 Runways and as it approached Runway 31L the first Comparator light alerted. The compass was reset after the aircraft was aligned on the runway and the Comparator light alerted a second time. The Reporter's Company has put a note in the pilot's MDW airport briefing describing the compass anomalies. The reporter has seen this before at MDW during taxi but has never seen it occur during takeoff and inflight.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.