Medium Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Confusion
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic
Procedure
I have two aircraft descending to land SFO. The first is Aircraft Y who I issued a descent to FL200 and got a good read back and then shipped to NCT for further descent. The second aircraft was Aircraft X also descending for SFO but converging from the right and higher and faster. I saw Aircraft Y leave FL240 and issued Aircraft X a descent to FL240. I then saw Aircraft Y leaving FL227 and issued Aircraft X FL230.My understanding of the rule is that when an aircraft has reported or is visual seen leaving that altitude that altitude can then be assigned to another aircraft. After issuing the descent to Aircraft X there was an overlap of a few seconds when Aircraft Y was at FL221 and Aircraft X had leveled off at FL230. I quoted the traffic and felt confident there was no safety risk.I will in the future allow the aircraft to completely vacate the altitude before issuing a new clearance.
Newly certified ZOA Controller describes situation using the vacated altitude rule; where the Controller decides he will never use this rule again.