ZKC controller working approach for foe and top reports a loss of separation between a KC135 and a C...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1151072
Time
Date	201402
Local Time Of Day	0001-0600
Place
Locale Reference	ZKC.ARTCC
State Reference	KS
Aircraft 1
Make Model Name	Golden Eagle 421
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Stratolifter (C-135 / 717)
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

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.