Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 10; Flight Crew Total 13000; Flight Crew Type 3000
Confusion
Aircraft Equipment Problem Critical; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented; Flight Crew Returned To Clearance; General Maintenance Action
Aircraft; Human Factors
This was an IFR flight.  I departed doing the FRG 4 Departure and was told to make a left turn to a HDG of 030; which I did; so I thought.  The Controller came back and told be I was on a HDG of 330.  I double checked my Compass and it was 030.  I looked at the Magnetic compass and it showed around 310 and at that time I realized something was wrong. I asked him to give me a turn because my compass was giving me an incorrect reading.  He turned me back towards 030 degrees. I tried several times to reslave my compass and kept asking him to check that I was heading in the right direction.  He asked me if I wanted to go back to FRG.  I did not want to do an instrument approach with the way it was working so I asked to continue into VFR conditions on top and head west because it was VFR to the west where I was going. As I headed west in level flight; the compass started to correct itself and I kept checking with ATC. For some reason the compass or gyros spooled up slowly so when I stopped turning on departure at 030 as cleared; I was really at 330. It is [a holiday week] and there not much I could do; but the aircraft is going to the avionics shop to be checked on why the heading lagged so much in my turn on climbout.
BE58 pilot is informed of a heading error on departure by ATC which is due to a malfunctioning compass system.  In level VMC flight the compass eventually corrects itself and the flight continues to destination.