Narrative:

We were level at 4000 feet MSL direct to stere for the RNAV Z 22L. We planned on doing the approach in visual conditions utilizing cdfa (continuous descent final approach). We were issued a clearance of maintain at or above 3000 feet over stere cleared for RNAV 22L approach. The RNAV Z calls for a mandatory altitude of 3000 feet over sailz but no defined altitude for the previous 2 fixes of tactt and stere; and the FMS lists those fixes as at or above 3000. Since we were at 4000 and issued an 'at or above' clearance; with the VNAV armed; we set 2400 feet in the altitude selector (which is the altitude for the FAF of keeel) and allowed the autopilot to fly the planned descent profile. About over tactt; ATC issued a statement that we missed a mandatory crossing restriction of 3000 feet over tactt and to descend immediately to avoid ord traffic. Both the pilot in command (PIC) and myself were confused because we could not see a crossing restriction on the approach; nor was the FMS instructing us to descend; so the PIC disconnected the autopilot and immediately initiated a descent. This in fact put us under the planned VNAV profile; but we wanted to comply with ATC's instructions. Since we were planning on flying cdfa we had 2400 feet in the altitude alerter; in the confusion of the moment we descended to 2400; which then put us under the mandatory altitude of 3000 feet over sailz. We were then given a low altitude alert; but given that we had the runway in sight; they cleared us for the visual approach at that point and we were able to accomplish a stabilized approach to an otherwise uneventful landing. I did query ATC once we were reestablished on the visual that I did not see a mandatory crossing restriction on the chart over tactt; and his response was that he was looking at the chart and it was published; but don't worry about it. After we were parked at the gate and the passengers were deplaned; I looked at the approach plate for the RNAV 22L Z and it is not published; however when I looked at the chart for the RNAV (rnp) Y 22L it has tactt as a mandatory 3000 feet. It is my opinion that the controller was looking at the RNAV Y rather than the RNAV Z; also note that the ATIS was calling for RNAV Y in use; but we cannot do the RNAV Y in [our] aircraft; so we were given the RNAV Z. Suggestions: had we not been given an incorrect instruction by ATC we would not have deviated from our original plan and what we had setup in the FMS and automation. Once we did initiate a manual descent we should have caught the fact that we had 2400 feet in the altitude alerter and we needed to stop at 3000 feet until passing sailz. I think extra care on ATC's part needs to be used when these two approaches are being used interchangeably; yet have a very significant altitude requirement. As a flight crew; we need to remain more conscientious of mandatory altitudes regardless of what instructions we are being issued.

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Original NASA ASRS Text

Title: Flight crew reported ATC commented on a missed crossing altitude that may have been a due to clearance confusion.

Narrative: We were level at 4000 feet MSL direct to STERE for the RNAV Z 22L. We planned on doing the approach in visual conditions utilizing CDFA (Continuous Descent Final Approach). We were issued a clearance of maintain at or above 3000 feet over STERE cleared for RNAV 22L Approach. The RNAV Z calls for a mandatory altitude of 3000 feet over SAILZ but no defined altitude for the previous 2 fixes of TACTT and STERE; and the FMS lists those fixes as at or above 3000. Since we were at 4000 and issued an 'at or above' clearance; with the VNAV armed; we set 2400 feet in the altitude selector (which is the altitude for the FAF of KEEEL) and allowed the autopilot to fly the planned descent profile. About over TACTT; ATC issued a statement that we missed a mandatory crossing restriction of 3000 feet over TACTT and to descend immediately to avoid ORD traffic. Both the Pilot in Command (PIC) and myself were confused because we could not see a crossing restriction on the approach; nor was the FMS instructing us to descend; so the PIC disconnected the autopilot and immediately initiated a descent. This in fact put us under the planned VNAV profile; but we wanted to comply with ATC's instructions. Since we were planning on flying CDFA we had 2400 feet in the altitude alerter; in the confusion of the moment we descended to 2400; which then put us under the mandatory altitude of 3000 feet over SAILZ. We were then given a low altitude alert; but given that we had the runway in sight; they cleared us for the visual approach at that point and we were able to accomplish a stabilized approach to an otherwise uneventful landing. I did query ATC once we were reestablished on the visual that I did not see a mandatory crossing restriction on the chart over TACTT; and his response was that he was looking at the chart and it was published; but don't worry about it. After we were parked at the gate and the passengers were deplaned; I looked at the approach plate for the RNAV 22L Z and it is not published; however when I looked at the chart for the RNAV (RNP) Y 22L it has TACTT as a mandatory 3000 feet. It is my opinion that the controller was looking at the RNAV Y rather than the RNAV Z; also note that the ATIS was calling for RNAV Y in use; but we cannot do the RNAV Y in [our] aircraft; so we were given the RNAV Z. Suggestions: Had we not been given an incorrect instruction by ATC we would not have deviated from our original plan and what we had setup in the FMS and automation. Once we did initiate a manual descent we should have caught the fact that we had 2400 feet in the altitude alerter and we needed to stop at 3000 feet until passing SAILZ. I think extra care on ATC's part needs to be used when these two approaches are being used interchangeably; yet have a very significant altitude requirement. As a flight crew; we need to remain more conscientious of mandatory altitudes regardless of what instructions we are being issued.

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.