PA-23 EXPERIENCING RADIO PROBS STARTS DSCNT FORM ASSIGNED ALT IN ZME AIRSPACE.

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WE WERE ON THE SECOND HOP OF THE DAY FLYING AN AERIAL SURVEY MISSION OVER WESTERN KENTUCKY. WE WERE ON AN IFR FLT PLAN THAT TOOK US FROM PAH TO CCT VOR TO DELAY AT FL200 NEAR M25 AND THEN BACK TO PAH. WE DEPARTED PAH AND CLBED AS WE PROCEEDED TO CCT. BEFORE REACHING CCT WE WERE APPROVED TO REVERSE COURSE TOWARDS OUR SURVEY AREA. WHEN WE REACHED FL180 WE WERE THEN CLRED TO FL200. THE CTLR COMPLAINED THAT I WAS DIFFICULT TO UNDERSTAND. I THOUGHT IT WAS BECAUSE OF THE OXYGEN MASK EVEN THOUGH INTERCOM COMS WERE NOT BEING A PROB. OUR CLRNC TO FL200 CAME WELL BEFORE WE TURNED ONTO OUR GRID BUT THERE WAS NO FURTHER COM WITH THE CTLR. WE WERE ON THE GRID FOR OVER AN HR AND DURING THAT TIME IT WAS QUITE OBVIOUS THAT CTR WAS VERY BUSY. THEY WERE REFUSING TO ACCEPT ACFT FOR FLT FOLLOWING AND THE FREQ WAS SO BUSY THAT THEY HAD TO ASK ALL ACFT TO STOP XMITTING EXCEPT FOR ONE THAT THEY WOULD DESIGNATE. OTHER TIMES THEY TOLD ACFT TO CALL BACK LATER. THERE WAS ANOTHER AERIAL SURVEY ACFT ON THE FREQ THAT ASKED THAT THEY BE ALLOWED TO MAKE AN UNANNOUNCED DSCNT FROM A FLT LEVEL WHEN THEY RAN OUT OF OXYGEN. OUR TIME ON THE SURVEY WAS LIMITED BY THE AMOUNT OF OXYGEN WE HAD ON BOARD. WE PLANNED TO END THE SURVEY WITH AN ACCEPTABLE AMOUNT OF RESERVE. WHEN WE DECIDED IT WAS TIME TO DSND; I ATTEMPTED TO CONTACT ATC FOR THE DSCNT. OVER A PERIOD OF 10 MINS I MADE SEVERAL CALLS WITH NO RESPONSE. THERE HAD BEEN SEVERAL CTLR CHANGES WHILE WE WORKED. I WAS NOT SURE IF THEY HAD LOST TRACK OF US; IF WE WERE OUT OF RANGE OF THEIR RECEIVERS; IF OUR XMITTERS WERE MALFUNCTIONING OR MY OXYGEN MASK WAS A FACTOR. IN ANY EVENT I CONSIDERED US IN A LOST COMS SIT AND; IN ACCORDANCE WITH THE PROVISIONS OF FAR 91.185 (LOST COMS); I BEGAN A DSCNT USING THE SAME PROFILE THAT I HAD BEEN GIVEN WHEN WE DSNDED FOR PAH FROM THIS ALT EARLIER IN THE DAY. IT WAS A TOTALLY CLR DAY AND THE RADIO TFC INDICATED THAT THERE WOULD BE NO CONFLICTING TFC. I FELT THAT A NORMAL DSCNT TO AVOID OXYGEN DEPLETION WAS BETTER THAN AN EMER DSCNT AFTER RUNNING OUT OF OXYGEN. AFTER DSNDING ABOUT 1500 FT MY CONTINUED CALLS WERE ANSWERED AND I WAS CLRED TO A LOWER ALT. AFTER PASSING THROUGH FL180 I ATTEMPTED TO CANCEL IFR. IT TOOK 6 TRIES TO GET A RESPONSE AND THEN I WAS GIVEN THE STANDARD 'SQUAWK 1200 FREQ CHANGE APPROVED' AND WE WENT ON OUR WAY. WE DID NOT ALLOW THE SIT TO DETERIORATE INTO AN EMER. THE MAIN PROB SEEMS TO BE THAT THE CTR WAS SATURATED. I KNOW THAT THE CTLRS WERE DOING THE BEST THAT THEY COULD BUT THEY NEEDED MORE RESOURCES TO GET THE JOB DONE. IT WAS A BEAUTIFUL VFR SATURDAY AFTERNOON THAT BROUGHT OUT ALL TYPES OF AIRPLANES AND PLTS OF ALL LEVELS OF SKILL.