Narrative:

This as soon as possible documents the fourth time an aircraft I was flying failed to cross chavo at 9000 ft /210 knots; while descending via the hydrr 1 RNAV arrival in phx. The autopilot and autothrottles were engaged; we were in LNAV/VNAV path; and we had 7000 ft in the MCP altitude window. The first officer (first officer) was flying this leg and had to deploy the speedbrakes prior to chavo as the speed was increasing to 225 knots. About two to three NM west of chavo; the aircraft appeared to begin a level off at 9000 ft; but then pitched over with increasing vvi (vertical velocity indicator); clearly indicating an impending altitude deviation. We both grabbed the yoke; and manually leveled the aircraft at 9000 ft. Without intervention; the aircraft would have descended below 9000 ft prior to chavo. Again; the only reason we caught this failure was our pre-brief; I've seen it before and we were ready. The speed during the event was in excess of 220 knots; and never went slower than 210 knots. Anecdotally; I've talked about these incidents with other pilots. Two of them told me they've seen it; too. The other three events I reported prior to this were in [different B737] series aircraft. Since this one happened in a -700; I thought another data point via this as soon as possible is appropriate.

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Original NASA ASRS Text

Title: B737-700 Captain reported he has seen multiple instances of the B737 autoflight system deviating from the HYDRR1 RNAV arrival into PHX.

Narrative: This ASAP documents the fourth time an aircraft I was flying failed to cross CHAVO at 9000 ft /210 knots; while descending via the HYDRR 1 RNAV Arrival in PHX. The autopilot and autothrottles were engaged; we were in LNAV/VNAV PATH; and we had 7000 ft in the MCP Altitude window. The FO (First Officer) was flying this leg and had to deploy the speedbrakes prior to CHAVO as the speed was increasing to 225 knots. About two to three NM west of CHAVO; the aircraft appeared to begin a level off at 9000 ft; but then pitched over with increasing VVI (Vertical Velocity Indicator); clearly indicating an impending altitude deviation. We both grabbed the yoke; and manually leveled the aircraft at 9000 ft. Without intervention; the aircraft would have descended below 9000 ft prior to CHAVO. Again; the only reason we caught this failure was our pre-brief; I've seen it before and we were ready. The speed during the event was in excess of 220 knots; and never went slower than 210 knots. Anecdotally; I've talked about these incidents with other pilots. Two of them told me they've seen it; too. The other three events I reported prior to this were in [different B737] series aircraft. Since this one happened in a -700; I thought another data point via this ASAP is appropriate.

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.