Jan 6

FLT ALT 00000

In the event that you do not set a FLT ALT prior to take-off, how would the pressurisation system perform? In theory you should be able to climb until reaching max diff at which point any further climb will result in the cabin altitude climbing at the same rate as the aircraft. At what altitude will max diff be reached though? Could you climb to FL410 for example and still not require oxygen masks?