A CFI RPT REGARDING THE EXCESSIVE PRECESSION OF HIS DIRECTIONAL GYRO AND THE NEAR CFTT NW OF SQL; CA.

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THE SIT OCCURRED DURING AN IFR TRAINING FLT FROM SQL-SAC SEP/XA/99 AT ABOUT AB20. WX CONDITIONS AT SQL WERE 600 FT OVCST WITH THE TOPS OF THE OVCST AT 1500 FT. THE ATC CLRNC WAS; AT 400 FT BEGIN A R TURN TO 120 DEGS. AS WE ENTERED THE CLOUDS; THE STUDENT IMMEDIATELY EXPERIENCED SPATIAL DISORIENTATION AND BEGAN OVERBANKING. I BEGAN INSTRUCTING THE STUDENT TO SHALLOW THE BANK AT ABOUT 30-35 OF BANK. THE STUDENT CONTINUED TO OVERBANK AND I TOOK CTL OF THE ACFT WITH A BANK ANGLE OF APPROX 50 DEGS. DURING THE STEEP TURN; THE DIRECTIONAL GYRO HAD PRECESSED ABOUT 150 DEGS; SO WHEN I THOUGHT WE WERE ON A HDG OF 120 DEGS WE WERE REALLY HDG ABOUT 270 DEGS. THE DEV WAS REALIZED WHEN ATC BEGAN QUERYING US AS TO OUR HDG. IT WAS THEN I CROSS-REFERENCED THE MAGNETIC COMPASS AND REALIZED THE ERROR IN OUR HDG. I WAS THEN GIVEN A NEW HDG TO FLY; AND MADE THAT TURN USING THE MAGNETIC COMPASS. THEN; AFTER I WAS ESTABLISHED ON THE NEW HDG I RESET THE DIRECTIONAL GYRO. THERE WERE NO MORE PROBS DURING THE REST OF THE FLT. AS THE BANK OF THE ACFT BEGAN TO INCREASE AND I BEGAN TALKING TO THE STUDENT; TOO MUCH OF MY ATTN WAS FOCUSED ON THE ATTITUDE INDICATOR AND TRYING TO HELP THE STUDENT THROUGH THE SPATIAL DISORIENTATION. I SHOULD HAVE FOCUSED MORE ON THE AIRPLANE AND NOT THE INSTRUCTIONAL LESSON. ALTHOUGH THE DIRECTIONAL GYRO WAS WITHIN TOLERANCES ON THE GND; I HAVE NOTICED MORE PRECESSION OF THE HDG INDICATOR DURING LATER FLTS.