B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Total 16500; Flight Crew Type 4000
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Aircraft
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.
On arrival BWI on RAVNN 6; we were cleared direct to YEAGR descend via RAVNN6; we were landing 33L BWI; bottom of arrival 6000 at RAVNN; LNAV VNAV all wired 0 deviation; all info on VNAV descent forecast page loaded; winds; temp dev; QNH. Slightly past YEAGR (FL270) Washington called and said hey; you were supposed to cross YEAGR FL270; I was flying and said we just did; we were on the path; perfect; speed window closed on speed; VNAV PTH displayed on FMA VTI centered; dead on path? Washington replied disregard. Next point was AKELL FL240; got another call from Washington; now I'm thinking what is going on; I set the altitude window up to the next altitude ECHOZ FL200 and intervened as the FMC wanted to descend 1.5 miles prior to ECHOZ after ECHOZ we started back on path and switched to approach control I let AC fly path again; it crossed DNKEY 400 feet low; again wired on the path; VTI centered? We reset altimeters at transition altitude FL180 and all remaining points on arrival (FIMBO 160; UDUDE 130; REXEE 120; CAPKO 90; RAVNN 60) were coincidental with proper altitude. Altimeter setting was [the airport setting]; which was loaded on VNAV Descent Forecast page! It seemed FMC somehow was trying to cross higher points using this altimeter setting although aircraft was still STD; on all three altimeters. Not sure what was going on; very strange! I've been on this airplane 7 years 4000+ hours; never seen non-coincidental crossings with VNAV PTH WIRED; 0 feet deviation! The fact that all crossings below 180 were perfect and all crossings above 180 would have been off 400 feet had we not intervened; well go figure; we both confirmed aircraft was in STD above 180 and properly set to QNH below FL 180. ATC said no more; never used a 'value' of feet off crossings and because they called after we passed we weren't sure what our exact altitude was initially. Flight landed without further incident.
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.