B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 253; Flight Crew Type 10804
Confusion
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Aircraft
This report documents the third time; with LNAV and VNAV PATH engaged; my aircraft failed to cross CHAVO at 9000 FT while descending via the HYDRR One RNAV Arrival into PHX. In each of my events; we were flying a -[Classic] series; there was a tailwind with turbulence; the Pilot Flying had to deploy the speedbrakes to arrest IAS in excess of 20 knots fast; and the VVI dropped out significantly between two and three NM west of CHAVO.The FO and I briefed this possibility since I had already seen it happen two times in July; we were ready. When it happened this time; we intervened; and manually arrested the impending altitude deviation; but just barely. The failure is subtle because the aircraft first appears to begin a level off approaching 9000 FT. Then it smoothly pitches over prior to CHAVO and leaves little time to intervene. I also believe it's happening to other Pilots because it is such a subtle failure; you might not see it unless watching very closely. A jumpseat Pilot who was with us said he never saw anything like it.Be prepared to quickly intervene prior to CHAVO; or an altitude deviation will likely occur. Consider some kind of warning to other Pilots.
B737-Classic Captain reports that he has seen the -Classic aircraft miss the crossing restriction at CHAVO on the HYDRR ONE RNAV Arrival into PHX on two previous occasions with LNAV and VNAV engaged. This time he is ready and prevents the aircraft from descending below the 9000 foot crossing restriction.