PLT OF AN SMA SKYDIVING ACFT FAILED TO CONTACT ATC AS REQUIRED AND PENETRATED CLASS B AIRSPACE.

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ON OCT/XX/94 WE HAD FAA APPROVAL TO DROP SKYDIVERS APPROX 8 MI NNE OF MEM ARPT AT 4500 FT MSL. AFTER TKOF FROM OLIVE BRANCH (OLV) I WAS UNABLE TO CONTACT MEM APCH. I CLBED TO 2900 FT MSL THINKING THAT THE CLASS B AIRSPACE STARTED AT 3000 FT MSL. I XMITTED ON 125.8 THAT I WAS RECEIVING NO RESPONSE FROM MEM APCH AND THAT I WAS DROPPING A SKYDIVER. AFTER THE SKYDIVER DROPPED I ANNOUNCED 'JUMPERS AWAY' ON 125.8 AND IMMEDIATELY STARTED MY DSCNT AND RETURNED TO OLIVE BRANCH. AFTER LNDG AT OLV I MADE A CLOSER INSPECTION OF MY SECTIONAL AND DISCOVERED THAT THE DROP SITE WAS UNDER THE 1800 FT CLASS B AIRSPACE; NOT THE 3000 FT CEILING. I CALLED THE MEM CTL TWR TO FIND OUT WHAT WAS REQUIRED OF ME. THEY TOLD ME TO FILL OUT THIS RPT AND SEND IT IN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE DID NOT REVIEW THE FAA WAIVER SINCE IT WAS OBTAINED BY THE LEAD PARACHUTIST/SKYDIVER AND THAT NO CALL WAS MADE TO ATC PRIOR TO THE FLT. THE RADIO FREQ USED WAS THAT SUPPLIED BY THE PARACHUTIST FOR MEM ARPT. HE FOUND AFTER THE FLC THAT THE RADIO FREQ SELECTION DIAL MALFUNCTIONED AND SLIPPED WHICH ALLOWED THE RADIO TO NOT BE TUNED TO THE FREQ SHOWN ON THE DIAL. HE FURTHER STATED THAT THE REASON THAT HE USED 2900 FT MSL; WAS AS HE INDICATED IN HIS RPT THAT HE THOUGHT THAT HE WAS STILL UNDER THE FLOOR OF THE CLASS B AIRSPACE. THEREFORE; SINCE HE WAS UNABLE TO CONTACT ATC; HE ADVISED THE SKYDIVERS THAT THEY HAD TO 'JUMP AT 2900 FT OR RIDE BACK DOWN WITH HIM!' HE SUBSEQUENTLY LEARNED THAT HE WAS ACTUALLY IN THE CLASS B AIRSPACE SINCE THE FLOOR WAS 1800 FT MSL AT THE JUMP SITE. IT WAS RECOMMENDED THAT HE REVIEW THE WAIVER AND PLAN WHERE IN THE AIRSPACE THE JUMP WOULD ACTUALLY TAKE PLACE PRIOR TO FUTURE FLTS.