A G-IV AT FL400 WAS UNABLE TO MAINTAIN THAT ALT BECAUSE OF TURB AND HIGH ISA. WHILE WAITING FOR DSCNT CLRNC ACFT DESCENDED TO FL355 AND WAS GIVEN A VIOLATION.

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THE TRIP ORIGINATED IN ZZZ1 ENRTE TO ZZZ2 WITH A FULL LOAD OF FUEL; PAX AND BAGGAGE. DURING THE FLT LEVEL PHASE OF OUR CLB; THE OUTSIDE AIR TEMPS INDICATED HIGHER THAN EXPECTED; AND WE CONTINUED WITH A SLOW CLB. AS WE PASSED FL390; WE ENCOUNTERED A POCKET OF TURB; THAT BTWN 35500 FT TO 40000 FT THE AIRSPD BEGAN TO DROP OFF CONSIDERABLY. THE INDICATED OUTSIDE AIR TEMP AT THIS TIME WAS BTWN ISA +13 TO +15. AT FL400 IT BECAME EVIDENT THAT WE WERE UNABLE TO MAINTAIN THIS ALT; AND I ASKED THE PNF TO REQUEST A LOWER ALT. WE WERE INSTRUCTED TO MAINTAIN FL400 WHILE ABC CTR WORKED ON A LOWER ALT. I DISENGAGED ALT HOLD IN ORDER NOT TO SLOW ACFT ANY FURTHER; INTENDING TO STAY WITHIN THE 200 FT BUFFER; BUT RIDE CONDITIONS MADE THAT DIFFICULT. BY THE TIME WE RECEIVED OUR DSCNT CLRNC DOWN TO FL380; WE WERE LATER INFORMED THAT OUR ALT HAD DROPPED TO 35500 FT. THERE WAS NO INCIDENT WITH THE ABC CTR; BUT AT THIS PARTICULAR TIME WE WERE IN THE OVERLAP WITH XYZ CTR'S AIRSPACE; WITH WHOM THE DEV WAS NOTICED. ONCE HANDED OFF TO XYZ CTR; WE WERE INSTRUCTED TO TELEPHONE THE CTR SUPVR TO DISCUSS THE SITUATION. NO INCIDENT HAD OCCURRED; BUT I WAS INFORMED THAT WE HAD VACATED AN ASSIGNED ALT WITHOUT RECEIVING PROPER CLRNC; CLRLY A VIOLATION. I EXPLAINED THE EXTENUATING CIRCUMSTANCES; IT WAS DULY NOTED; AND I WAS THEN PROVIDED WITH A TELEPHONE NUMBER TO CONTACT AN INDIVIDUAL IN THE QUALITY ASSURANCE SECTION TO EXPLAIN THE MATTER FURTHER. CTR SHOULD HAVE BEEN NOTIFIED RIGHT AWAY THAT WE WOULD BE UNABLE TO MAINTAIN FL400. WHILE WE WERE AWAITING THE DSCNT CLRNC; THE TURB AND DIMINISHING AIRSPD REQUIRED THAT SOMETHING BE DONE IMMEDIATELY.