Narrative:

This [report is to advise of one of] ever more frequent occurrences on new airport arrivals across the us. ATC is making unrealistic demands for descents and speeds by altering the vertical [navigation constraints] on charted arrivals. These changes are being made with no warning or expect clearances. As per SOP we as pilots are adhering to our 3 to 1 profiles and the expected altitudes on the charts and then with no warning ATC is changing the vertical and/or airspeed constraints with no regard to the ability of the aircraft to complete the task. This event was one such occurrence.while descending on the twstd one RNAV arrival into iah we were given such a [clearance]. It was apparent that the controller was completely unaware that the clearance issued was not possible given the position of the aircraft on the arrival. Outside of mentl at FL360 we were given a descend and maintain FL240 clearance with which the flying pilot complied by selecting the descent mode of VNAV. The aircraft was below the 3 to 1 path at that time so the aircraft began a 1;000 FPM descent until reaching the desired power off VNAV path and 280 KTS for the assigned arrival. This path was planning to cross mentl as charted above FL240 while planning to cross the next waypoint constraint; whack; at 280 KTS between 17;000 and FL200. The VNAV path was planning to cross whack at approximately FL190 within the window and on speed.at approximately FL260 and 26 NM from whack the controller issued a clearance to cross whack at 12;000. This clearance came with no prior 'expect' clearance and was just over 7;000 ft below the VNAV path planned crossing. That left us only increased airspeed and flight spoilers in order to increase our descent rate. Immediately we questioned the clearance altitude as well as the planned arrival with the controller and he confirmed both as correct. The flying pilot immediately began an open descent and extended the spoilers to assist in losing altitude. I explained [to ATC] that the issued clearance was outside of the aircraft's capabilities. With the new and unexpected much lower altitude clearance programmed into the FMGC; it was again clear that the controller had put us in bad position. Using all measures at our disposal we were not going to be able to give the controller both the altitude and the airspeed. I made the controller aware of this and explained to him that the planned crossing was much higher than planned by the assigned arrival. He seemed confused as to why it would not work. When I reminded him of the charted altitudes he said that they were not using those altitudes today. I explained that we would need a heads up if that were to be the case in the future. There were no NOTAMS to this effect it was just another controller change to a charted arrival at the last minute. It is obvious that the aircraft would not be able to lose 14;000 ft in 26 NM while maintaining 280 KTS. It was clear that the altitude was most important to the controller so the captain used all available tools to make the 12;000 foot crossing although doing so required crossing at a higher airspeed than planned. After [the first controller] stated that they were not using the altitudes on the arrival the next controller issued a 'descend via' clearance! It appears that the controllers need some education on what the aircraft performance limits are and also need to be on the same page with each other. This type of issue has become a problem across our system with the many newer arrivals. The newer arrivals work; for the most part; as they are written and designed but; when the controller alters either altitude and/or speed constraints; it becomes extremely difficult or even--as in this case--impossible to comply.

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Original NASA ASRS Text

Title: An A320 flight crew took exception to ATC's frequent modifications to complex STAR crossing restriction; in this case the TWSTD RNAV STAR to IAH. Reporter believe controllers must be informed/trained with respect to aircraft capabilities in climb and descent and the impact that high or low speed constraints make on possible rates of climb.

Narrative: This [report is to advise of one of] ever more frequent occurrences on new airport arrivals across the US. ATC is making unrealistic demands for descents and speeds by altering the vertical [navigation constraints] on charted arrivals. These changes are being made with no warning or expect clearances. As per SOP we as pilots are adhering to our 3 to 1 profiles and the expected altitudes on the charts and then with no warning ATC is changing the vertical and/or airspeed constraints with no regard to the ability of the aircraft to complete the task. This event was one such occurrence.While descending on the TWSTD ONE RNAV Arrival into IAH we were given such a [clearance]. It was apparent that the controller was completely unaware that the clearance issued was not possible given the position of the aircraft on the arrival. Outside of MENTL at FL360 we were given a descend and maintain FL240 clearance with which the flying pilot complied by selecting the descent mode of VNAV. The aircraft was below the 3 to 1 path at that time so the aircraft began a 1;000 FPM descent until reaching the desired power off VNAV path and 280 KTS for the assigned arrival. This path was planning to cross MENTL as charted above FL240 while planning to cross the next waypoint constraint; WHACK; at 280 KTS between 17;000 and FL200. The VNAV path was planning to cross WHACK at approximately FL190 within the window and on speed.At approximately FL260 and 26 NM from WHACK the Controller issued a clearance to cross WHACK at 12;000. This clearance came with no prior 'expect' clearance and was just over 7;000 FT below the VNAV path planned crossing. That left us only increased airspeed and flight spoilers in order to increase our descent rate. Immediately we questioned the clearance altitude as well as the planned arrival with the controller and he confirmed both as correct. The flying pilot immediately began an open descent and extended the spoilers to assist in losing altitude. I explained [to ATC] that the issued clearance was outside of the aircraft's capabilities. With the new and unexpected much lower altitude clearance programmed into the FMGC; it was again clear that the Controller had put us in bad position. Using all measures at our disposal we were not going to be able to give the Controller both the altitude and the airspeed. I made the Controller aware of this and explained to him that the planned crossing was much higher than planned by the assigned arrival. He seemed confused as to why it would not work. When I reminded him of the charted altitudes he said that they were not using those altitudes today. I explained that we would need a heads up if that were to be the case in the future. There were no NOTAMS to this effect it was just another controller change to a charted arrival at the last minute. It is obvious that the aircraft would not be able to lose 14;000 FT in 26 NM while maintaining 280 KTS. It was clear that the altitude was most important to the Controller so the Captain used all available tools to make the 12;000 foot crossing although doing so required crossing at a higher airspeed than planned. After [the first controller] stated that they were not using the altitudes on the arrival the next controller issued a 'descend via' clearance! It appears that the controllers need some education on what the aircraft performance limits are and also need to be on the same page with each other. This type of issue has become a problem across our system with the many newer arrivals. The newer arrivals work; for the most part; as they are written and designed but; when the controller alters either altitude and/or speed constraints; it becomes extremely difficult or even--as in this case--impossible to comply.

Data retrieved from NASA's ASRS site 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.