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I know that I can make the altimeter alert sing, but that's just annoying and won't do any warm,
I know for sure on the 777 and 744, the aircraft will maintain the FL based on the QNH if you change it from STD, in other word, will climb or descend depending upon the QNH and which autopilot engaged. Not sure how the TCAS will react when two aircraft flying in proximity one on STD and the other on local QNH.