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A B777-200 flight crew described anomalous behavior of the autoflight system when programming the FMS in order to comply with ATC alterations to the crossing window at bhawk on the fytte RNAV STAR to ord. In addition; ATC assigned an airspeed well in excess of those charted. The FMS inexplicably altered the active waypoint from bhawk to stnle; which was the first fix beyond bhawk. The aircraft began an abrupt right turn when to track to stnle would have only required a modest left turn. The flight crew abandoned VNAV operations in order to regain the assigned lateral track and while engaged in doing so descended below their cleared altitude at bhawk. The first officer's report was appended with detailed (and commonly reported) complaints with respect to the effects on flight crew workload and situational awareness imposed by ATC modifications to complex RNAV terminal area procedures.

Narrative:

On [the ord] fytte RNAV STAR; zzipr transition; chicago center's last clearance was to descend and maintain FL240 by bhawk. (Bhawk profile altitude constraints are between FL260 & fl 220) at approx. 10 NM prior to bhawk and level at FL240; ATC re-cleared us to descend via the FYTTE2; except maintain 300 knots until further advised.at that point we were already in LNAV; 11000 feet was selected in the FMC window; the captain (ca) selected VNAV and we started a descent in VNAV path. When the ca changed the speeds on the legs page to 300 knots; the altitude constraints on the legs page disappeared; bhawk turned white on the navigation display and the aircraft immediately began a 90 degree right turn; though the next fix ahead; stnle was now the active waypoint on the legs page. The ca engaged heading select and rotated the heading bug back to the left to rejoin the lateral arrival path; but the aircraft was very; very slow to reverse course in the auto-pilot (a/P) mode. The aircraft continued to descend as the ca disconnected the a/P; increased the left turn rate to rejoin the lateral path and leveled the aircraft now below FL220 just prior to bhawk. The controller queried us and I acknowledged that we had descended below the bhawk altitude limit. The controller indicated that there was no traffic conflict at that point. After bhawk; the ca re-selected the a/P; LNAV & VNAV and continued the arrival.it continues to be my experience that controller directed altitude and/or airspeed changes to arrival profiles require significant pilot attention allocation at dynamic moments to execute the changes in the FMC at multiple waypoints. Then additional heads-down time is required to verify that the executed changes result in the desired profile adjustments. See and avoid time out the front windows is negatively impacted by FMC profile adjustments and monitoring.

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Original NASA ASRS Text

Title: A B777-200 flight crew described anomalous behavior of the autoflight system when programming the FMS in order to comply with ATC alterations to the crossing window at BHAWK on the FYTTE RNAV STAR to ORD. In addition; ATC assigned an airspeed well in excess of those charted. The FMS Inexplicably altered the active waypoint from BHAWK to STNLE; which was the first fix beyond BHAWK. The aircraft began an abrupt right turn when to track to STNLE would have only required a modest left turn. The flight crew abandoned VNAV operations in order to regain the assigned lateral track and while engaged in doing so descended below their cleared altitude at BHAWK. The First Officer's report was appended with detailed (and commonly reported) complaints with respect to the effects on flight crew workload and situational awareness imposed by ATC modifications to complex RNAV terminal area procedures.

Narrative: On [the ORD] FYTTE RNAV STAR; ZZIPR transition; Chicago Center's last clearance was to descend and maintain FL240 by BHAWK. (BHAWK profile altitude constraints are between FL260 & FL 220) At approx. 10 NM prior to BHAWK and level at FL240; ATC re-cleared us to descend via the FYTTE2; except maintain 300 knots until further advised.At that point we were already in LNAV; 11000 feet was selected in the FMC window; the Captain (CA) selected VNAV and we started a descent in VNAV PATH. When the CA changed the speeds on the legs page to 300 knots; the altitude constraints on the legs page disappeared; BHAWK turned white on the NAV display and the aircraft immediately began a 90 degree right turn; though the next fix ahead; STNLE was now the active waypoint on the legs page. The CA engaged Heading Select and rotated the heading bug back to the left to rejoin the lateral arrival path; but the aircraft was very; very slow to reverse course in the Auto-Pilot (A/P) mode. The aircraft continued to descend as the CA disconnected the A/P; increased the left turn rate to rejoin the lateral path and leveled the aircraft now below FL220 just prior to BHAWK. The controller queried us and I acknowledged that we had descended below the BHAWK altitude limit. The controller indicated that there was no traffic conflict at that point. After BHAWK; the CA re-selected the A/P; LNAV & VNAV and continued the arrival.It continues to be my experience that controller directed altitude and/or airspeed changes to arrival profiles require significant pilot attention allocation at dynamic moments to execute the changes in the FMC at multiple waypoints. Then additional heads-down time is required to verify that the executed changes result in the desired profile adjustments. See and avoid time out the front windows is negatively impacted by FMC profile adjustments and monitoring.

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.