Medium Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 22
Confusion; Distraction; Situational Awareness; Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic
Aircraft; Procedure
I was the trainer at sector 38.  TMU restriction of 15 MIT to DEN.  Departure push out of DSM and we were handed maybe 5 aircraft needing to be spaced.  Aircraft X checked on and was given climb and maintain I believe FL280. Just behind Aircraft X was Aircraft Y who was initially given a altitude below Aircraft X.  My Trainee was starting spacing and assigning speeds as well as altitudes and requested Aircraft Y to expedite his climb through FL240 for traffic inbound (a air carrier who was turned out).  When my trainee observed Aircraft X out of FL260; he assigned Aircraft Y FL260 and gave him normal rate of climb.  I told my developmental I did not like that plan and to change it so he revised the clearance to FL250 again but the pilot read back;  'Ok; we will maintain 2-5-0 on the speed.'  Neither of us heard the speed part because of the activity and trainer/trainee communication going on.  As I observed the Aircraft X go through the altitude I attempted to correct the mistake but was too late.  Not sure if separation was lost but the plan did not work either way. I don't use it and neither should anyone.  Just because a similar aircraft is out of an altitude; does not mean or should it mean that you can assign that altitude to another aircraft.  There are just too many factors that can make an aircraft climb at different rates.
I climbed Aircraft X to FL320 and told to maintain mach .77 or greater for spacing to Denver.  I climbed Aircraft Y to FL260 and expedited the aircraft up to miss another air carrier jet.  Both me and my trainer tried to stop the aircraft at FL250; and the pilot read back speed 250.  Aircraft X was originally out of FL260 when the Aircraft Y was assigned FL260.None-trainee involved.
ZMP instructor and Developmental describe a situation where the Developmental used vacated altitude rule and the second aircraft climbed faster than the first aircraft leading to a loss of separation.