Narrative:

This was a flight from ZZZ to dtw. We were originally filed and cleared to descend via the lectr 3 arrival into dtw. During the descent; ATC amended our clearance to descend via the hanbl 3 arrival. Any time a new arrival is executed during a VNAV descent on the boeing/honeywell pegasus FMS; it takes up to a minute for the flight management computer to recalculate the new path. During that time period; the aircraft is descending unrestricted at idle and there is no path published for the pilots or autopilot to follow until it recalculates and presents itself.originally; on the lectr 3 the FMS had planned to cross hanbl at or above 11;000. When ATC switched us to the hanbl 3; the new crossing altitude at hanbl is at or above 17;000 and at or below FL210. The aircraft was descending during this period of recalculation and the new path did not present itself until the aircraft was at 16;800 just outside hanbl; at which time it leveled off. We were asked by cleveland center to say our altitude and we reported 16;800. Center asked us if we were descending and I answered negative.this is a recurring anomaly with the honeywell pegasus FMS and I don't know if ATC is even aware of what this does to the FMS when they issue a new descend via clearance while the aircraft is already descending via a different arrival. Virtually every airline places the bottom altitude into the mode control panel when cleared to descend via a STAR; so the autopilot will not level off to prevent busting a crossing restriction that is above the bottom altitude if the FMS is in the process of recalculating the new path.

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Original NASA ASRS Text

Title: Air carrier Captain reported an altitude overshoot during descent to DTW while on the HANBL 3 arrival. Captain stated they had been given a clearance change and a recurring recalculation delay on their Honeywell Pegasus FMS contributed to the incident.

Narrative: This was a flight from ZZZ to DTW. We were originally filed and cleared to descend via the LECTR 3 Arrival into DTW. During the descent; ATC amended our clearance to descend via the HANBL 3 Arrival. Any time a new arrival is executed during a VNAV descent on the Boeing/Honeywell Pegasus FMS; it takes up to a minute for the flight management computer to recalculate the new path. During that time period; the aircraft is descending unrestricted at idle and there is no path published for the pilots or autopilot to follow until it recalculates and presents itself.Originally; on the LECTR 3 the FMS had planned to cross HANBL at or above 11;000. When ATC switched us to the HANBL 3; the new crossing altitude at HANBL is at or above 17;000 and at or below FL210. The aircraft was descending during this period of recalculation and the new path did not present itself until the aircraft was at 16;800 just outside HANBL; at which time it leveled off. We were asked by Cleveland Center to say our altitude and we reported 16;800. Center asked us if we were descending and I answered negative.This is a recurring anomaly with the Honeywell Pegasus FMS and I don't know if ATC is even aware of what this does to the FMS when they issue a new descend via clearance while the aircraft is already descending via a different arrival. Virtually every airline places the bottom altitude into the Mode Control Panel when cleared to descend via a STAR; so the autopilot will not level off to prevent busting a crossing restriction that is above the bottom altitude if the FMS is in the process of recalculating the new path.

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.