ZTL controller reports of an incident where he is trying to separate two aircraft in a conflict due ...

Narrative:

I was working aircraft out of a holding pattern for atl. I thought I had used proper vertical separation. I was certain aircraft X was out of 150 and assigned aircraft Y 160 and a reverse heading of 340 to sequence them behind aircraft X. There was a jet that had just diverted to hsv; and I was in a conversation with them about going to atl since we had just started running aircraft into atl. The next thing I knew; conflict alert was going off and aircraft Y was asking me to verify their assigned altitude. I saw aircraft Y leaving descending through 169 and aircraft X at 165 right on top of each other. I told aircraft Y to level off at whatever altitude they were at; and turned them 40 degrees left. The next update showed aircraft Y at 163 and aircraft X's altitude was xed out. At that point I just wanted to make sure everyone was separated. Pilots were stepping on transmissions and I could not get replies right away. I then saw aircraft X out of 150. I think I may have told them to expedite their descent at some point; but I don't remember for certain. I could not think straight at this point and asked the controller in charge to get me out. I have since heard it was a mode C swap. I don't know if anything can be done about them; but it was an extremely upsetting experience to go through. Anything that can be done about mode C swaps.

Original NASA ASRS Text

Title: ZTL Controller reports of an incident where he is trying to separate two aircraft in a conflict due to altitude separation. The Mode C's of the aircraft swap and cause the Controller problems.

Narrative: I was working aircraft out of a holding pattern for ATL. I thought I had used proper vertical separation. I was certain Aircraft X was out of 150 and assigned Aircraft Y 160 and a reverse heading of 340 to sequence them behind Aircraft X. There was a jet that had just diverted to HSV; and I was in a conversation with them about going to ATL since we had just started running aircraft into ATL. The next thing I knew; conflict alert was going off and Aircraft Y was asking me to verify their assigned altitude. I saw Aircraft Y leaving descending through 169 and Aircraft X at 165 right on top of each other. I told Aircraft Y to level off at whatever altitude they were at; and turned them 40 degrees left. The next update showed Aircraft Y at 163 and Aircraft X's altitude was Xed out. At that point I just wanted to make sure everyone was separated. Pilots were stepping on transmissions and I could not get replies right away. I then saw Aircraft X out of 150. I think I may have told them to expedite their descent at some point; but I don't remember for certain. I could not think straight at this point and asked the CIC to get me out. I have since heard it was a Mode C swap. I don't know if anything can be done about them; but it was an extremely upsetting experience to go through. Anything that can be done about Mode C swaps.

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	1200520
Time
Date	201409
Local Time Of Day	1801-2400
Place
Locale Reference	ATL.Airport
State Reference	GA
Environment
Light	Daylight
Aircraft 1
Make Model Name	Medium Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 20.2
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy

ZTL controller reports of an incident where he is trying to separate two aircraft in a conflict due to altitude separation. The mode C's of the aircraft swap and cause the controller problems.