Narrative:

I had obtained an ATC clearance to depart sfo class B airspace under VFR. Just prior to being cleared for takeoff on runway 28R, my clearance was changed to 'left downwind departure.' upon release, I complied by making a continuous wide 180 degree climbing turn to the left, well across runway 28L. As I rolled out on heading 100 degrees, tower instructed me to proceed direct to the sfo VORTAC and depart on a heading of 050 degrees. This required me to turn further to the left, which I did at standard rate. I estimate that the total turn was very nearly 150 degrees which meant that I should have crossed the VORTAC and basically maintained my heading outbound. The whole thing went very smoothly, and I was given a 'good job' by the tower controller as I crossed the VORTAC. However, during the 150 degree left turn maneuver, my primary gyrocompass developed a lagging error of at least 30 degrees without any warning flag. Therefore, when I crossed the VORTAC (on a heading of about 050 degrees), my compass read at least 080 degrees. The airport was under my wings and behind me at that point, so I had no obvious visual reference to the runways. Upon crossing the VORTAC, I turned left to 050 degrees indicated, which was a lot closer to 020 degrees, only 10 degrees off of their departure runways 1L/right. I recall having a brief 'sa alert' thinking I was turning toward the runways 1 departure path, but that was it. Then ATC (tower, I think) asked me what my assigned heading was. That should have been enough to wake me up, but I answered '050 degrees' and they gave me 'turn right heading 090 degrees.' I still didn't wake up. I was then handed off to departure control and again they asked me what tower had given me for an assigned heading. Finally I woke up, checked the magnetic compass and discovered the difference. I immediately called departure control back and reported the problem. That seemed to make them a bit less apprehensive. I also noted that the gyro vacuum indication was at the high end of green, and both vacuum pumps were operating normally. Upon arriving at my destination, I had the vacuum and compass system checked. Both appeared to be functioning normally. In retrospect, there were 2 events that should have alerted me early: 1) my sa alert about turning toward runways 1, and 2) the assigned heading check by tower as I departed the VORTAC. My gyrocompass is not a slaved unit, therefore, I should xchk it regularly with the magnetic compass, particularly after large turns, and particularly early in the flight, when cold WX could make the compass sluggish.

Google
 

Original NASA ASRS Text

Title: BE55 ACFT DEPARTING VFR IN CLASS B AIRSPACE HAD PRIMARY GYROCOMPASS LAG WHICH RESULTED IN ACFT ACTUALLY BEING 30 DEGS OFF ASSIGNED HEADING. AFTER BEING QUERIED BY 2 CTLRS REGARDING HEADING, RPTR CHKED MAGNETIC COMPASS AND DISCOVERED THE LAG ERROR.

Narrative: I HAD OBTAINED AN ATC CLRNC TO DEPART SFO CLASS B AIRSPACE UNDER VFR. JUST PRIOR TO BEING CLRED FOR TKOF ON RWY 28R, MY CLRNC WAS CHANGED TO 'L DOWNWIND DEP.' UPON RELEASE, I COMPLIED BY MAKING A CONTINUOUS WIDE 180 DEG CLBING TURN TO THE L, WELL ACROSS RWY 28L. AS I ROLLED OUT ON HDG 100 DEGS, TWR INSTRUCTED ME TO PROCEED DIRECT TO THE SFO VORTAC AND DEPART ON A HDG OF 050 DEGS. THIS REQUIRED ME TO TURN FURTHER TO THE L, WHICH I DID AT STANDARD RATE. I ESTIMATE THAT THE TOTAL TURN WAS VERY NEARLY 150 DEGS WHICH MEANT THAT I SHOULD HAVE CROSSED THE VORTAC AND BASICALLY MAINTAINED MY HEADING OUTBOUND. THE WHOLE THING WENT VERY SMOOTHLY, AND I WAS GIVEN A 'GOOD JOB' BY THE TWR CTLR AS I CROSSED THE VORTAC. HOWEVER, DURING THE 150 DEG L TURN MANEUVER, MY PRIMARY GYROCOMPASS DEVELOPED A LAGGING ERROR OF AT LEAST 30 DEGS WITHOUT ANY WARNING FLAG. THEREFORE, WHEN I CROSSED THE VORTAC (ON A HDG OF ABOUT 050 DEGS), MY COMPASS READ AT LEAST 080 DEGS. THE ARPT WAS UNDER MY WINGS AND BEHIND ME AT THAT POINT, SO I HAD NO OBVIOUS VISUAL REF TO THE RWYS. UPON XING THE VORTAC, I TURNED L TO 050 DEGS INDICATED, WHICH WAS A LOT CLOSER TO 020 DEGS, ONLY 10 DEGS OFF OF THEIR DEP RWYS 1L/R. I RECALL HAVING A BRIEF 'SA ALERT' THINKING I WAS TURNING TOWARD THE RWYS 1 DEP PATH, BUT THAT WAS IT. THEN ATC (TWR, I THINK) ASKED ME WHAT MY ASSIGNED HEADING WAS. THAT SHOULD HAVE BEEN ENOUGH TO WAKE ME UP, BUT I ANSWERED '050 DEGS' AND THEY GAVE ME 'TURN R HDG 090 DEGS.' I STILL DIDN'T WAKE UP. I WAS THEN HANDED OFF TO DEP CTL AND AGAIN THEY ASKED ME WHAT TWR HAD GIVEN ME FOR AN ASSIGNED HEADING. FINALLY I WOKE UP, CHKED THE MAGNETIC COMPASS AND DISCOVERED THE DIFFERENCE. I IMMEDIATELY CALLED DEP CTL BACK AND RPTED THE PROB. THAT SEEMED TO MAKE THEM A BIT LESS APPREHENSIVE. I ALSO NOTED THAT THE GYRO VACUUM INDICATION WAS AT THE HIGH END OF GREEN, AND BOTH VACUUM PUMPS WERE OPERATING NORMALLY. UPON ARRIVING AT MY DEST, I HAD THE VACUUM AND COMPASS SYS CHKED. BOTH APPEARED TO BE FUNCTIONING NORMALLY. IN RETROSPECT, THERE WERE 2 EVENTS THAT SHOULD HAVE ALERTED ME EARLY: 1) MY SA ALERT ABOUT TURNING TOWARD RWYS 1, AND 2) THE ASSIGNED HEADING CHK BY TWR AS I DEPARTED THE VORTAC. MY GYROCOMPASS IS NOT A SLAVED UNIT, THEREFORE, I SHOULD XCHK IT REGULARLY WITH THE MAGNETIC COMPASS, PARTICULARLY AFTER LARGE TURNS, AND PARTICULARLY EARLY IN THE FLT, WHEN COLD WX COULD MAKE THE COMPASS SLUGGISH.

Data retrieved from NASA's ASRS site as of July 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.