EMB145 flight crew has a heading comparator alert during takeoff from ord.

Attributes
ACN	700553
Time
Date	200606
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	agl single value : 0
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	EMB ERJ 145 ER&LR
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	climbout : takeoff ground : takeoff roll
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 11800 flight time type : 2500
ASRS Report	700553
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	other controllerb other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended or assigned course
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

During takeoff roll; our heading comparator alert came on. This is a fairly common occurrence at ord. Usually 1 of our 2 directional gyros are off by 5-7 degrees. We continued as our speed was close to V1. ATC had told us to fly a 330 degree heading after takeoff but apparently my directional gyro precessed to the left about 30 degrees; so when I turned to 330 degrees my actual heading was about 300 degrees. Both my first officer and myself noticed the discrepancy. Unfortunately; due to acceleration forces; our magnetic compass would not give accurate heading information. ATC told us to turn 30 degrees right. We leveled at 4000 ft. When magnetic compass settled down; we determined that captain's side directional gyro was in error. Xferred control to first officer. Captain's directional gyro 'corrected' itself about 5 mins later.

Original NASA ASRS Text

Title: EMB145 FLT CREW HAS A HDG COMPARATOR ALERT DURING TKOF FROM ORD.

Narrative: DURING TKOF ROLL; OUR HDG COMPARATOR ALERT CAME ON. THIS IS A FAIRLY COMMON OCCURRENCE AT ORD. USUALLY 1 OF OUR 2 DIRECTIONAL GYROS ARE OFF BY 5-7 DEGS. WE CONTINUED AS OUR SPD WAS CLOSE TO V1. ATC HAD TOLD US TO FLY A 330 DEG HDG AFTER TKOF BUT APPARENTLY MY DIRECTIONAL GYRO PRECESSED TO THE L ABOUT 30 DEGS; SO WHEN I TURNED TO 330 DEGS MY ACTUAL HDG WAS ABOUT 300 DEGS. BOTH MY FO AND MYSELF NOTICED THE DISCREPANCY. UNFORTUNATELY; DUE TO ACCELERATION FORCES; OUR MAGNETIC COMPASS WOULD NOT GIVE ACCURATE HDG INFO. ATC TOLD US TO TURN 30 DEGS R. WE LEVELED AT 4000 FT. WHEN MAGNETIC COMPASS SETTLED DOWN; WE DETERMINED THAT CAPT'S SIDE DIRECTIONAL GYRO WAS IN ERROR. XFERRED CTL TO FO. CAPT'S DIRECTIONAL GYRO 'CORRECTED' ITSELF ABOUT 5 MINS LATER.

Data retrieved from NASA's ASRS site as of January 2009 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.