ATR at FL290? Must've been -500, 42-300 needs ISA-10 to get to FL240, our current ceiling due to being non-RVSM. In the groundschool I've been taught that FL250 limit is due to pressurization. At that FL differential will reach 6.35 PSI (limitation) and cabin alt 6740 ft. If you go higher, outflow valves will maintain that differential and cabin will start climbing with the a/c. On ATRs that I fly excessive cabin altitude warning goes off at 10 000 ft cabin altitude. Silencing the bell and just moving along wouldn't be most appropriate course of action.