Narrative:

On arrival to bwi; advised at first point on RAVNN6 that FL270 crossing didn't coincide with arrival restriction of FL270 at yeagr; we were 5 miles past and exactly on FMC path; both flight directors showing no deviation from path; FMC and CDU examined; all parameters entered on VNAV descent forecast page; winds; isa dev; qnh; ATC advised similarly at akell which had a FL240 restriction that aircraft was below path according to their computer; we still showed dead on FMC path with no deviation; I intervened and reset altitude to next crossing altitude and in fact FMC wanted to descend 1.5 miles prior to coach [caash?] which had a FL220 restriction; likewise I continued to intervene at echoz to stop at FL200; like before FMC wanted to descend 1.5 miles prior. Handed off to approach I let FMC remain on path at dnkey which had a FL190 restriction; aircraft crossed 450 feet low. At FL180 standard was switched on all 3 altimeters to [the airport altimeter setting] and then all remaining crossing coincided with arrival. It appears the FMC was using the qnh from the forecast page prior to FL180; I discussed the absolute certainty that prior to FL180 all altimeters were in standard; confirmed. We continued flight without incident; no traffic confliction during arrival noted or mentioned by ATC. We are taking aircraft on the next leg and will watch very close and report. Again all altitudes above FL180 wanted to cross 400-500 feet low on the FMC path all altitudes below FL180 coincided with the arrival.

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Original NASA ASRS Text

Title: B737 flight crew was informed by ATC that they were 400 to 500 feet low as they crossed each waypoint on the RAVNN6 arrival to BWI. However; the FMC showed the crossings being made correctly. After descending below FL180 and setting the altimeters to the airport setting; the crossings were made correctly.

Narrative: On arrival to BWI; advised at first point on RAVNN6 that FL270 crossing didn't coincide with arrival restriction of FL270 at YEAGR; we were 5 miles past and exactly on FMC path; both flight directors showing no deviation from path; FMC and CDU examined; all parameters entered on VNAV descent forecast page; winds; ISA dev; QNH; ATC advised similarly at AKELL which had a FL240 restriction that aircraft was below path according to their computer; we still showed dead on FMC path with no deviation; I intervened and reset altitude to next crossing altitude and in fact FMC wanted to descend 1.5 miles prior to COACH [CAASH?] which had a FL220 restriction; likewise I continued to intervene at ECHOZ to stop at FL200; like before FMC wanted to descend 1.5 miles prior. Handed off to approach I let FMC remain on path at DNKEY which had a FL190 restriction; aircraft crossed 450 feet low. At FL180 standard was switched on all 3 altimeters to [the airport altimeter setting] and then all remaining crossing coincided with arrival. It appears the FMC was using the QNH from the forecast page prior to FL180; I discussed the absolute certainty that prior to FL180 all altimeters were in standard; confirmed. We continued flight without incident; no traffic confliction during arrival noted or mentioned by ATC. We are taking aircraft on the next leg and will watch very close and report. Again all altitudes above FL180 wanted to cross 400-500 feet low on the FMC path all altitudes below FL180 coincided with the arrival.

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.