Narrative:

This is being written because of a possible altitude deviation and because I believe this arrival into iad may need to be looked at. We were on the GIBBZ2 and were direct to kilmr at FL220 less than 30 miles out. We were close to the fix when we were cleared to 'descend via' which meant I needed to be at kilmr by 11000@250k. I entered in 11000 into the altitude [window] and knowing we were close started to descend. I used the FMS for guidance as all the fixes/altitudes/speeds/to[D] had been confirmed by us earlier. The carrot was showing me positioned low (it was pegged at the top ) and the vn data also showed me low by 4000 feet but finally started trending down. I did the mental math and my numbers came up with me being very high. So I checked the FMS again thinking I must have done the math wrong because the FMS has always been correct before. Unfortunately; this time it was off; by a lot; and I don't know why. I immediately put out the speed brakes and started a very steep descent to catch up. I believe we would have made it but ATC then asked us to slow down early (to 250K) because traffic ahead prematurely slowed down. We told him 'unable' and we could give him altitude or speed. He said 'speed' so I slowed down and we ended up high over kilmr. This caused me to be high for the rest of the arrival and ATC vectored us until we could finally catch up.I am confused on why the FMS had given me false information. All preliminary data was correctly input; checked; and we reached to[D]. I believe if I had two FMS; I would have been able to catch the error since I typically use the vn data and perf west/T data pages simultaneously to see the planned altitude to verify the altitude the FMS is planning to have me out. But; with one FMS not only was I looking behind my right shoulder;I also had only one screen to use and so many key strokes to navigate through the pages was impossible to do quickly. The non pilot flying was busy on the radio because he was trying to figure out the traffic position and what exactly ATC wanted from us. In relation to the arrival itself; I was also alarmed by the fact when we returned to iad later that day ; we ran into a similar issue with traffic ahead missing the altitude and then because we were so close to them; ATC vectored us zig zag and then cleared us to 'descend via' GIBBZ2 arrival too close to kilmr. It was impossible to make it down and we told them so. They gave us a speed restriction and a higher altitude. We were able to complete the rest of the arrival.the [cause of this event appears to have been]: the aircraft was too high when cleared to descend via.the FMS descent data was incorrect. Traffic ahead could not comply causing us to not comply as well.I think there is something not working on the arrival because similar issues occurred twice to us.

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Original NASA ASRS Text

Title: Twice in the same day the flight crew of an EMB-145 found themselves flummoxed by the demands of the GIBBZ RNAV STAR to IAD with the flight crew; the FMC and ATC unable to resolve the complexities of altitudes restrictions and traffic separation.

Narrative: This is being written because of a possible altitude deviation and because I believe this arrival into IAD may need to be looked at. We were on the GIBBZ2 and were direct to KILMR at FL220 less than 30 miles out. We were close to the fix when we were cleared to 'descend via' which meant I needed to be at KILMR by 11000@250k. I entered in 11000 into the altitude [window] and knowing we were close started to descend. I used the FMS for guidance as all the fixes/altitudes/speeds/TO[D] had been confirmed by us earlier. The carrot was showing me positioned low (it was pegged at the top ) and the VN data also showed me low by 4000 feet but finally started trending down. I did the mental math and my numbers came up with me being very high. So I checked the FMS again thinking I must have done the math wrong because the FMS has always been correct before. Unfortunately; this time it was off; by a lot; and I don't know why. I immediately put out the speed brakes and started a very steep descent to catch up. I believe we would have made it but ATC then asked us to slow down early (to 250K) because traffic ahead prematurely slowed down. We told him 'unable' and we could give him altitude OR speed. He said 'speed' so I slowed down and we ended up high over KILMR. This caused me to be high for the rest of the arrival and ATC vectored us until we could finally catch up.I am confused on why the FMS had given me false information. All preliminary data was correctly input; checked; and we reached TO[D]. I believe if I had two FMS; I would have been able to catch the error since I typically use the VN data and Perf W/T data pages simultaneously to see the planned altitude to verify the altitude the FMS is planning to have me out. But; with one FMS not only was I looking behind my right shoulder;I also had only one screen to use and so many key strokes to navigate through the pages was impossible to do quickly. The non pilot flying was busy on the radio because he was trying to figure out the traffic position and what exactly ATC wanted from us. In relation to the arrival itself; I was also alarmed by the fact when we returned to IAD later that day ; we ran into a similar issue with traffic ahead missing the altitude and then because we were so close to them; ATC vectored us zig zag and then cleared us to 'descend via' GIBBZ2 arrival too close to KILMR. It was impossible to make it down and we told them so. They gave us a speed restriction and a higher altitude. We were able to complete the rest of the arrival.The [cause of this event appears to have been]: the aircraft was too high when cleared to descend via.The FMS descent data was incorrect. Traffic ahead could not comply causing us to not comply as well.I think there is something not working on the arrival because similar issues occurred twice to us.

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.