FLC OF AN CPR JET OVERSHOT ASSIGNED CLB ALT DUE TO THE PIC THINKING THE ALT RESTR WAS AIRSPD.

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WAS OPERATING AS PART 91 WITH COMPANY PRESIDENT WHO IS QUALIFIED IN ACFT; OCCUPYING THE PLT SEAT. SUBSEQUENT TO DEP WAS RE-ROUTED TO HOU DUE TO ARPT CLOSURE AT NEW. DURING CLB THROUGH 16000 FT TO THE ASSIGNED ALT OF FL310; WAS INSTRUCTED BY MERIDA CTR: 'NOT ABOVE 250 UNTIL FURTHER ADVISED.' I SUBSEQUENTLY READBACK 'ROGER MAINTAIN 250 KTS UNTIL ADVISED.' AS WE PASSED FL259 WAS INFORMED BY ATC TO INITIATE AN IMMEDIATE DSCNT TO FL250. AS WE LEVELED BACK AT FL250; A B727 PASSED OFF OF OUR L AT FL260. THE FOLLOWING ATTRIBUTED TO THIS SIT: THE CEO'S HURRY TO LEAVE; HIS WANTING TO FLY A COMPLICATED CORPORATE TURBOJET ACFT; ATC DELAYS AT CANCUN COUPLED WITH EXTREME HEAT AND DISCOMFORT; ICOA REFUSAL TO POLICE THE SPEAKING OF OTHER NON-ENGLISH LANGUAGES ON FREQ; USAGE OF NON-STANDARD RADIO TERMINOLOGY 'NOT ABOVE 250 UNTIL FURTHER ADVISED;' COMPLACENCY WITH REGARD TO THE ASSUMPTION THAT '250' MEANT AIRSPD AND NOT THE INTENDED ALT RESTR; ADDITIONAL DUTIES OF INTL OPS COUPLED WITH THE PF LIMITATIONS; DUE TO HIS NON-PROFESSIONAL STATUS.