On 7/22/2015 1:03 PM, Bob Pasker wrote:
regarding altitude differences, 91.217 requires that the altimeter report pressure
altitude correctly within 125 feet. So two aircraft, one reporting
FL050 but 125ft
high at 5,125 MSL and another reporting FL053, but 125ft low at 5,175
MSL will pass
very close to each other, especially if one of them might be an ASH-30
in a level turn.

https://www.law.cornell.edu/cfr/text/14/91.217


I believe the typical situation is much worse than that. 91.217 only
seems to require that the equipment meet that standard on the day it was
calibrated, and then 95% probability of meeting that standard is
accepted. After that, the equipment goes out to meet the real world
and all bets are off until the next calibration!

Also, remember that a transponder only reports altitude in 100 foot
increments, so 5,125 MSL might report the same as 5,199 MSL.