ZDC controller described a loss of separation event when stepping down aircraft in close proximity; ...

Narrative:

An A320 was at FL200 with a B737 underneath at FL180. The B737 was on a vector to gain spacing. The B737 was then cleared to 17;000; and a few second later the A320 was cleared to FL180 with anticipation of the B737 aircraft leaving FL180 on that update. The next update showed the B737 leaving FL180 but at his altimeter was adjusted; it showed the aircraft back at FL180 on the next update. The A320 had a rapid decent rate; and I thought there was a mode C error in the update so I didn't react quickly enough to stop the A320 before entering the altitude stratum of the B737. No recommendation; just a better understanding of the effect of an altimeter change when pressure is high; and a strict adherence to the rules with aircraft leaving and altitude before assigning it to another aircraft.

Original NASA ASRS Text

Title: ZDC Controller described a loss of separation event when stepping down aircraft in close proximity; assigning an altitude immediately upon received a vacating notification.

Narrative: An A320 was at FL200 with a B737 underneath at FL180. The B737 was on a vector to gain spacing. The B737 was then cleared to 17;000; and a few second later the A320 was cleared to FL180 with anticipation of the B737 aircraft leaving FL180 on that update. The next update showed the B737 leaving FL180 but at his altimeter was adjusted; it showed the aircraft back at FL180 on the next update. The A320 had a rapid decent rate; and I thought there was a Mode C error in the update so I didn't react quickly enough to stop the A320 before entering the altitude stratum of the B737. No recommendation; just a better understanding of the effect of an altimeter change when pressure is high; and a strict adherence to the rules with aircraft leaving and altitude before assigning it to another aircraft.

Data retrieved from NASA's ASRS site as of April 2012 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	983272
Time
Date	201112
Local Time Of Day	1201-1800
Place
Locale Reference	ZDC.ARTCC
State Reference	VA
Aircraft 1
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

ZDC controller described a loss of separation event when stepping down aircraft in close proximity; assigning an altitude immediately upon received a vacating notification.