Narrative:

We were cleared for descent to FL240. The first officer was flying the aircraft. I left VHF #1 to receive the dtw ATIS on #2 radio. When I came back on frequency the first officer was climbing back to FL250 from FL246 ft. I asked what's up and he stated ATC changed altitude from FL240 to level at FL250. I asked the first officer if he told ATC he was unable to level at FL250 on such short notice. He answered no. At that time ATC asked our altitude. I answered FL247 and climbing back to FL250. ATC gave us a heading of 340 degrees and asked about our altitude miss. I explained that we needed more advanced warning to level at a new and different altitude during descent. ATC explained that his equipment is old and slow and he showed us at FL257 when he gave us the new leveloff. When his equipment showed FL257 we were actually descending through FL252. Apparently there is a lag time on the controller's scope of approximately 500 ft. I believe the problem can be fixed by more controller training (recurrent training) or more updated ATC equipment.

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Original NASA ASRS Text

Title: FLC OF MLG IS GIVEN AN ALT CHANGE DURING DSCNT AND THEY ARE UNABLE TO LEVEL DUE TO LATENESS OF THE CHANGE. ATC QUERIES AND THEN INDICATES THEY THOUGHT THEY WERE HIGHER THAN THEY WERE.

Narrative: WE WERE CLRED FOR DSCNT TO FL240. THE FO WAS FLYING THE ACFT. I LEFT VHF #1 TO RECEIVE THE DTW ATIS ON #2 RADIO. WHEN I CAME BACK ON FREQ THE FO WAS CLBING BACK TO FL250 FROM FL246 FT. I ASKED WHAT'S UP AND HE STATED ATC CHANGED ALT FROM FL240 TO LEVEL AT FL250. I ASKED THE FO IF HE TOLD ATC HE WAS UNABLE TO LEVEL AT FL250 ON SUCH SHORT NOTICE. HE ANSWERED NO. AT THAT TIME ATC ASKED OUR ALT. I ANSWERED FL247 AND CLBING BACK TO FL250. ATC GAVE US A HDG OF 340 DEGS AND ASKED ABOUT OUR ALT MISS. I EXPLAINED THAT WE NEEDED MORE ADVANCED WARNING TO LEVEL AT A NEW AND DIFFERENT ALT DURING DSCNT. ATC EXPLAINED THAT HIS EQUIP IS OLD AND SLOW AND HE SHOWED US AT FL257 WHEN HE GAVE US THE NEW LEVELOFF. WHEN HIS EQUIP SHOWED FL257 WE WERE ACTUALLY DSNDING THROUGH FL252. APPARENTLY THERE IS A LAG TIME ON THE CTLR'S SCOPE OF APPROX 500 FT. I BELIEVE THE PROB CAN BE FIXED BY MORE CTLR TRAINING (RECURRENT TRAINING) OR MORE UPDATED ATC EQUIP.

Data retrieved from NASA's ASRS site as of July 2007 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.