Narrative:

ZJX advised us to expect to cross tabir intersection on the darbs one arrival at 250 KTS and 13000 ft. However we had been instructed to maintain 15000 ft. When our FMC gave us a warning to reset our MCP altitude alert for 13000 ft, I asked ATC if they were still going to want us to cross tabir at 250 KTS and 13000 ft. They said 'yes, cross tabir at 250 KTS and 13000 ft.' so I interpreted this as a clearance. So as I prepared to start the descent, ATC said, 'but for now maintain 15000 ft.' I then had to quickly stop the descent. This was not good ATC terminology. To give us clearance that sounds like a clearance but is not a clearance. By the time we did get a clearance to cross tabir at 250 KTS and 13000 ft we were less than 6 mi from tabir at 310 KTS. I simply was too close to tabir to make that restr. I would have had to make an emergency dive to make that restr. I elected not to and so advised ATC. ATC was clearly irritated and said I was too fast. My position was that we were not required to be at 250 KTS until crossing tabir. A clearance we were not given until it was too late. ATC did not take into account that jet aircraft do not and cannot be expected to make instantaneous responses to an ATC clearance. There has to be time lag understood for an aircraft to change its direction horizontally or vertically in order to comply with a last min clearance.

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Original NASA ASRS Text

Title: PREPARING FOR A DSCNT TO TPA ARPT, PLT AND CTLR MISCOM ON ALT AND SPD RESTR ON ARR RTE.

Narrative: ZJX ADVISED US TO EXPECT TO CROSS TABIR INTXN ON THE DARBS ONE ARR AT 250 KTS AND 13000 FT. HOWEVER WE HAD BEEN INSTRUCTED TO MAINTAIN 15000 FT. WHEN OUR FMC GAVE US A WARNING TO RESET OUR MCP ALT ALERT FOR 13000 FT, I ASKED ATC IF THEY WERE STILL GOING TO WANT US TO CROSS TABIR AT 250 KTS AND 13000 FT. THEY SAID 'YES, CROSS TABIR AT 250 KTS AND 13000 FT.' SO I INTERPRETED THIS AS A CLRNC. SO AS I PREPARED TO START THE DSCNT, ATC SAID, 'BUT FOR NOW MAINTAIN 15000 FT.' I THEN HAD TO QUICKLY STOP THE DSCNT. THIS WAS NOT GOOD ATC TERMINOLOGY. TO GIVE US CLRNC THAT SOUNDS LIKE A CLRNC BUT IS NOT A CLRNC. BY THE TIME WE DID GET A CLRNC TO CROSS TABIR AT 250 KTS AND 13000 FT WE WERE LESS THAN 6 MI FROM TABIR AT 310 KTS. I SIMPLY WAS TOO CLOSE TO TABIR TO MAKE THAT RESTR. I WOULD HAVE HAD TO MAKE AN EMER DIVE TO MAKE THAT RESTR. I ELECTED NOT TO AND SO ADVISED ATC. ATC WAS CLRLY IRRITATED AND SAID I WAS TOO FAST. MY POS WAS THAT WE WERE NOT REQUIRED TO BE AT 250 KTS UNTIL XING TABIR. A CLRNC WE WERE NOT GIVEN UNTIL IT WAS TOO LATE. ATC DID NOT TAKE INTO ACCOUNT THAT JET ACFT DO NOT AND CANNOT BE EXPECTED TO MAKE INSTANTANEOUS RESPONSES TO AN ATC CLRNC. THERE HAS TO BE TIME LAG UNDERSTOOD FOR AN ACFT TO CHANGE ITS DIRECTION HORIZLY OR VERTLY IN ORDER TO COMPLY WITH A LAST MIN CLRNC.

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.