In days gone by – passing another aircraft on an airway was an event that may or may not have brought it relatively close. In these days of GPS accuracy being within feet of stated position, it’s not uncommon to see other aircraft directly overhead or below.
This is UPS 88 just west of OMLOK intersection in Western Canada – 6 April 2008.
There was approximately 1000 knots of closure here. Total time from being able to recognize another aircraft in the area to passage is about 35 seconds. We knew he was there a good bit longer thanks to TCAS – Traffic Collision Avoidance System.