atco749, I stand corrected, but I understand that 2 main altimeters should be within 25 feet of each other (at QNH) on the ground, within 20 feet of airport elevation on the ground, and within 200 feet of each other prior to entry into RVSM / MNPS airspace. The last also at each WPT within RVSM / MNPS airspace , or on change of flight level.
