Golden Eagle 421
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors; Procedure
A C421 was approaching towards TOP from the southwest for a visual approach to Runway 36 at TOP; and a KC135 was approaching FOE from the northwest for a GPS13 approach into FOE. I descended the KC135 to get him down below the C421 and get him into FOE so that the C421 could overfly the KC135 and approach TOP for a visual approach to Runway 36 at TOP. I cleared the KC135 to maintain 3;600 until established on the GPS and cleared him for approach at FOE. The C421 would fly over the KC135 at 5;000. Because I saw KC135 leave 4;000; I thought I could then descend the C421 to 4;000 for his visual approach into TOP. The C421 and the KC135 were already diverging at this point; but I forgot that KC135 was going to stop at 3;600 until established and then continue descent. The KC135 still had about a mile or so to go before commencing the approach and heading down. It wasn't until the conflict alert went off that I realized that the KC135 was going to stop at 3;600 for a short time and therefore I should have waited to give the C421 4;000 for his approach into TOP. Anyone watching this event up front probably wouldn't have realized that a technical separation error had occurred because had they been alerted to it and looked they would've seen the KC135 leaving 4;000; thinking then that I could descend the C421 down to 4;000; out of 5;000. It was a mistake on my part not remembering that the KC135 wasn't going to continue descending right away. I guess the only thing I recommend would be a reminder to controllers that just because we issue a minimum descent altitude for an IFR approach does not mean that; that aircraft is going to continue descending until on the approach itself. Like I said; I thought I was totally following the rules at the time and did not realize I had made an error in judgment until several flashes of the conflict alert and thought about it for a bit.
ZKC Controller working approach for FOE and TOP reports a loss of separation between a KC135 and a C421 on a visual to Runway 36 at TOP. The KC135 was cleared to 3;600 until established on the GPS 13 at FOE and the C421 was cleared to 4;000; forgetting that the KC135 would stop at 3;600 FT.