An AS350 helicopter pilot reported that during ATC vectors he discovered the Directional Gyro had approximately a thirty five degree heading error. Maintenance later reported that corroded electrical connectors caused the equipment error.

Narrative

After dropping off a patient during a reposition flight to base approximately ten minutes into the flight I was given an instruction by ATC to turn to heading 180 vectors for traffic. I immediately turned to a heading of 180 as referenced by the Directional Gyro. Within one minute I was re-contacted by ATC and asked for current heading. I informed him; 179 degrees. He advised that I must have hellacious winds because he had me at 145 degrees; turn left heading 120. While receiving the new course instruction I discovered that the Directional Gyro was in error by as much as 35 degrees as verified with a cross check of the GPS and Magnetic compass. I complied with new course and was within a few minutes given an instruction to resume own navigation; frequency change approved. No further communication was given by ATC. Upon the continued flight to base I confirmed that the Directional Gyro was unreliable in slave mode; but operative in free mode. After landing I wrote up the discrepancy in the aircraft log book and deferred the item per the MEL and contacted the mechanic. The mechanic responded and discovered corroded/contaminated terminal ends. He cleaned them and the aircraft deferral was cleared by the mechanic. A more attentive cross check with the magnetic compass and GPS would have discovered the Directional Gyro error earlier; preventing the apparent failure to comply with ATC instruction.