Narrative:

I was the relief pilot (international relief officer) seated in the middle jump seat. The captain was the pilot monitoring (pm) and the first officer was the pilot flying (PF). Our arrival was the hyper 7 RNAV arrival; expecting assignment to rwy 19L at kiad. Approximately 15 miles prior to the hyper fix; ATC cleared our aircraft to descend via the hyper 7 arrival. The PF asked for; and the pm set; 4;000 ft in the MCP. Passing through 14;000 MSL; the first officer seemed to believe we would be unable to make the crossing altitude/speed over the hyper fix of 10;000 MSL and 250 KIAS. The FMC scratch pad displayed the 'drag required' message; and the entire crew was aware of this FMC message. The PF decided to initiate a flch descent to make the hyper crossing restriction of 10;000 ft and asked for 10;000 ft to be set in the MCP. The pm changed the MCP from 4;000 to 10;000. The PF then selected flch and extended the speed brake. Our aircraft descended to rejoin the descent path and the PF selected VNAV approaching 10;000 ft. The pm then reset 4;000 ft in the MCP. The 'preset' FMC speed for passing through 10;000 ft was 240 KIAS; which did display on the airspeed bug. The pm asked the PF if he wanted 240 KIAS (vs 250 KIAS). The PF initiated speed intervention on the MCP and selected 250 KIAS approximately 8 miles prior to crossing the hyper fix. The FMA changed from VNAV path to VNAV speed and the aircraft started a slow descent below 10;000. I believe this FMA change from VNAV path to VNAV speed initially went undetected by the entire crew. 2-3 miles prior to the hyper fix; we (collectively as a crew) noticed our altitude of 9;600 ft. The correct crossing altitude for the hyper fix is 10;000 ft. The PF disengaged the autopilot to prevent any further descent but remained at 9;600 as we were less than 1 NM from crossing the hyper fix. After passing the hyper fix; the PF re-engaged the autopilot and initiated a VNAV path descent without further deviation during the arrival and approach. ATC did not indicate any awareness of our altitude deviation crossing the hyper fix. No radio calls about this deviation were made or received.

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Original NASA ASRS Text

Title: Air Carrier flight crew reported that an RNAV arrival crossing altitude was not met due to an undetected FMC Mode change.

Narrative: I was the Relief Pilot (IRO) seated in the middle jump seat. The Captain was the Pilot Monitoring (PM) and the FO was the Pilot Flying (PF). Our arrival was the HYPER 7 RNAV arrival; expecting assignment to Rwy 19L at KIAD. Approximately 15 miles prior to the HYPER fix; ATC cleared our aircraft to descend via the HYPER 7 arrival. The PF asked for; and the PM set; 4;000 ft in the MCP. Passing through 14;000 MSL; the FO seemed to believe we would be unable to make the crossing altitude/speed over the HYPER fix of 10;000 MSL and 250 KIAS. The FMC scratch pad displayed the 'drag required' message; and the entire crew was aware of this FMC message. The PF decided to initiate a FLCH descent to make the HYPER crossing restriction of 10;000 ft and asked for 10;000 ft to be set in the MCP. The PM changed the MCP from 4;000 to 10;000. The PF then selected FLCH and extended the speed brake. Our aircraft descended to rejoin the descent path and the PF selected VNAV approaching 10;000 ft. The PM then reset 4;000 ft in the MCP. The 'preset' FMC speed for passing through 10;000 ft was 240 KIAS; which did display on the airspeed bug. The PM asked the PF if he wanted 240 KIAS (vs 250 KIAS). The PF initiated speed intervention on the MCP and selected 250 KIAS approximately 8 miles prior to crossing the HYPER fix. The FMA changed from VNAV Path to VNAV Speed and the aircraft started a slow descent below 10;000. I believe this FMA change from VNAV Path to VNAV Speed initially went undetected by the entire crew. 2-3 miles prior to the HYPER fix; we (collectively as a crew) noticed our altitude of 9;600 ft. The correct crossing altitude for the HYPER fix is 10;000 ft. The PF disengaged the autopilot to prevent any further descent but remained at 9;600 as we were less than 1 NM from crossing the HYPER fix. After passing the HYPER fix; the PF re-engaged the autopilot and initiated a VNAV Path descent without further deviation during the arrival and approach. ATC did not indicate any awareness of our altitude deviation crossing the HYPER fix. No radio calls about this deviation were made or received.

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.