In this "incident", sufficient separation was achieved because:
ATC, who were in communication with both aircraft, ensured that there was 500' nominal vertical separation by instructing the 737 to arrest his descent at FL180 (atop the Class E space) while the C421 was known to be level at FL175;
Firstly, if Class E procedures had been adhered to, the C421 would NOT have been in communication. His clearance request was for the adjacent Class C airspace, not Class E.
Secondy, the instruction to maintain FL180 was NOT a class E procedure. It was a controller seeing Class E potentially going bad and intervening.
TCAS worked - it provided a traffic advisory (TA) to the 737 crew when the a/c were 5NM apart;
TCAS only worked because - luckily - the C421 pilot had called up requesting a clearance for the
adjacent Class C airspace, and the lack of a mode C read-out was detected. If he had followed class E procedures and kept quiet - the lack of mode C would not have been detected, and the TCAS defence would have been useless.
and this enabled the 737 to visually see and avoid the C421, even if the C421 did not see the 737...
So, with directed trafic information and radar assistance, ONE pilot sees the other aircraft at a range of 5NM. A study I cited in another post suggests that being alerted to traffic increases your chances of seeing it by a factor of eight.
So, if class E airspace procedures had been followed to the letter:
1) The C421 would have been silent, and invisible as far as mode C is concerened.
2) TCAS would have been no use whatsover.
3) In the absence of any information on the C421, no traffic or an instruction to maintain FL 180 would have been issued.
4) The ONLY way that a collision would have been avoided is if the B737 pilot had achieved an unalerted see and avoid, whelie descending at 3000fpm and 300KIAS.
Once again I ask - why are these procedures worth the decrease in safety and increase in cost?