LOW FLT THAT IS CONSIDERED TO BE CTLED FLT INTO TERRAIN IN A NIGHT OP.

Narrative

THE SEA CLASS B AIRSPACE CONTAINS A FLOOR AT 1100 FT MSL THAT OVERLAYS THE S 3/4 OF BFI RWY 13R. 2 NIGHTS AGO I WAS GIVING INSTRUCTION TO A PVT PLT. HE HAD COMPLETED A PRACTICE ILS 13R APCH AT BFI AND WAS CLRED FOR AN EARLY L XWIND FOR A VFR TRANSITION INTO RNT. HE CLBED AT VY TO ABOUT 1500 MSL AND MISSED THE FLOOR; I.E. DID NOT PENETRATE THE CLASS B AIRSPACE; BUT DURING THE POSTFLT DISCUSSION I REALIZED HE WAS NOT AWARE OF THE 1100 FT FLOOR AND DISCUSSED IT WITH HIM; STRESSING THE IMPORTANCE OF STAYING WELL BELOW IT ON STRAIGHT DEPS FROM 13R (STAY AT 900 FT TILL S OF THE ARPT BOUNDARY). TONIGHT I WAS FLYING A PRACTICE APCH TO THE SAME RWY. I REQUESTED THE OPTION; AND THEN AN EARLY XWIND TO RNT. I WAS CLRED FOR THE OPTION AND TOLD TO REMAIN OUTSIDE THE SEATTLE CLASS B AIRSPACE. BECAUSE OF THIS; I EXPECTED A STRAIGHT OUT DEP AND PLANNED TO CLB TO NO MORE THAN 900 FT; TO REMAIN BELOW THE 1100 FT FLOOR. ANOTHER ACFT WAS NEGOTIATING AN INTXN DEP; AND I WAS THEN CLRED FOR A FULL STOP AND TO HOLD POS THE RWY. I ASKED IF I WAS CLRED FOR A TOUCH AND GO; AND WAS CLRED FOR IT; WHILE THE OTHER ACFT HELD SHORT. I WAS THEN CLRED FOR AN EARLY XWIND. THERE IS A RIDGE TO THE E OF BFI. I CLBED AT ABOUT 500 FPM TARGETING THE 900 FT ALT FOR A STRAIGHT OUT AND CROSSED THE RIDGE AT ABOUT 800 FT. THIS IS TOO LOW; BECAUSE THE RIDGE IS A CONGESTED AREA. ALTHOUGH I WAS DISTRACTED BY THE TWR CTLR I SHOULD HAVE REVISED MY PLANNED ALT TO ACCOMMODATE THE XWIND DEP. I ALSO THINK I FIXATED ON THE 1110 FT FLOOR BECAUSE OF MY DISCUSSION 2 NIGHTS EARLIER.