Narrative:

All compasses were checked for correct headings at the gate and during early taxi. They were ok. We had a 15 min wait before taking the runway due to north atlantic clearance. During this time a widebody transport and another widebody transport taxied within close proximity. We took off on runway 26L, and didn't check the compasses on the runway heading. After takeoff, at 2.5 DME, we turned right to 083 degrees, and contacted london radar. Radar told us that we were headed north, not east. Both RMI's and both CDI's indicated 083 degrees. The wet compass was reading north. We took radar steers until we could unslave and fast erect both gyros. This corrected the problem. This aircraft has a dual INS installation. The 2 INS gyros are used as attitude and compass gyros. We later learned that if the INS's have not been turned back on for a long enough time, the heading indications may be unreliable. With the dual INS installation aircraft, one should always: a) fast erect the gyros, and B) check the compass headings on the runway. Callback conversation with reporter revealed the following: when we checked the compasses at the gate we apparently miscalculated our heading and only thought the compasses had erected on the proper heading. Fast erecting in the air on a stable heading caused the compasses to slave to the INS on the proper headings. INS was working properly. When picking up an aircraft with dual INS installation, the gyros are not normally erected until about 20-30 mins after the INS is switched on. Therefore, the compasses (both RMI's and both CDI's) are not reliable. You should always go to the fast erect mode by switching both overhead compass switches from 'slaved' to 'free dg' for 35 seconds and back to slaved. This will prevent the possibility of incorrect heading after takeoff.

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Original NASA ASRS Text

Title: FLT CREW OF WDB TOOK UP ERRONEOUS HEADING ACCOUNT BOTH COMPASSES HAD SLAVED TO INS ON IMPROPER HEADING.

Narrative: ALL COMPASSES WERE CHKED FOR CORRECT HDGS AT THE GATE AND DURING EARLY TAXI. THEY WERE OK. WE HAD A 15 MIN WAIT BEFORE TAKING THE RWY DUE TO N ATLANTIC CLRNC. DURING THIS TIME A WDB AND ANOTHER WDB TAXIED WITHIN CLOSE PROX. WE TOOK OFF ON RWY 26L, AND DIDN'T CHK THE COMPASSES ON THE RWY HDG. AFTER TKOF, AT 2.5 DME, WE TURNED RIGHT TO 083 DEGS, AND CONTACTED LONDON RADAR. RADAR TOLD US THAT WE WERE HEADED N, NOT E. BOTH RMI'S AND BOTH CDI'S INDICATED 083 DEGS. THE WET COMPASS WAS READING N. WE TOOK RADAR STEERS UNTIL WE COULD UNSLAVE AND FAST ERECT BOTH GYROS. THIS CORRECTED THE PROB. THIS ACFT HAS A DUAL INS INSTALLATION. THE 2 INS GYROS ARE USED AS ATTITUDE AND COMPASS GYROS. WE LATER LEARNED THAT IF THE INS'S HAVE NOT BEEN TURNED BACK ON FOR A LONG ENOUGH TIME, THE HDG INDICATIONS MAY BE UNRELIABLE. WITH THE DUAL INS INSTALLATION ACFT, ONE SHOULD ALWAYS: A) FAST ERECT THE GYROS, AND B) CHK THE COMPASS HDGS ON THE RWY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: WHEN WE CHKED THE COMPASSES AT THE GATE WE APPARENTLY MISCALCULATED OUR HDG AND ONLY THOUGHT THE COMPASSES HAD ERECTED ON THE PROPER HDG. FAST ERECTING IN THE AIR ON A STABLE HDG CAUSED THE COMPASSES TO SLAVE TO THE INS ON THE PROPER HDGS. INS WAS WORKING PROPERLY. WHEN PICKING UP AN ACFT WITH DUAL INS INSTALLATION, THE GYROS ARE NOT NORMALLY ERECTED UNTIL ABOUT 20-30 MINS AFTER THE INS IS SWITCHED ON. THEREFORE, THE COMPASSES (BOTH RMI'S AND BOTH CDI'S) ARE NOT RELIABLE. YOU SHOULD ALWAYS GO TO THE FAST ERECT MODE BY SWITCHING BOTH OVERHEAD COMPASS SWITCHES FROM 'SLAVED' TO 'FREE DG' FOR 35 SECS AND BACK TO SLAVED. THIS WILL PREVENT THE POSSIBILITY OF INCORRECT HDG AFTER TKOF.

Data retrieved from NASA's ASRS site as of August 2007 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.