Originally Posted by
Lead Balloon
That's assuming perfect equipment accuracy. My understanding is that the tolerance of e.g. an IFR altimeter in the USA is 75'. If that's correct, one IF aircraft with a 'legal' altimeter indicating 325' could in fact be at 250' and another IFR aircraft with a 'legal' altimeter indicating 175' could in fact be at ... 250'. I'm hoping that the avionics in the aircraft involved in this terrible tragedy were more accurate than that, but I always exercise caution in taking numbers out of avionics and ATC systems as 'gospel truth' to the foot. The altitudes on RADAR displays don't increase and decrease in 1 foot increments; nor do the outputs of aircraft transponders. RADALT is different.
The maximum allowable altimeter error between sea level and 1000 feet is +/- 20 feet, and even a simple stand-alone barometric altimeter has to be able to meet that requirement. The RVSM-certified Air Data Computers on something like a CRJ are typically much more precise than that - more like +/- 5 feet at almost all altitudes. I would assume the air data system on a Blackhawk would be equally precise at low levels.