Date: 201507
Local time of day: 1201-1800
Location: PHX.Airport, AZ
Flight conditions: VMC
Weather elements / visibility: Turbulence
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent; Initial Approach
Airspace: Class B PHX
ATC / advisory: TRACON P50
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Captain; Pilot Not Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Aircraft

Narrative: While conducting the HYDRR1 RNAV Arrival into PHX in a B737; we were directed by ATC to keep our speed at 300 knots until GEELA; and then comply with speeds at GEELA. We complied with this; and as we approached GEELA we were given a speed reduction; which we complied with.  At .3 DME prior to CHAVO with the aircraft in LNAV/VNAV PATH; the aircraft was leveling at 9000 feet at 210 knots as depicted on the arrival.  It was mid-afternoon in PHX with temperature in the low 100s and occasional moderate turbulence.  At this point I received a VNAV disconnect message; which I cleared.  In the few seconds it took to assess what the aircraft was doing; and reengage a mode on the MCP; the aircraft may have crossed CHAVO up to 200 feet low since I had the bottom altitude of 7000 feet in the Altitude window while the airplane was VNAV PATH.  I had applied a little power while determining whether I needed to level or continue descent.  Now; (maybe five seconds after the disconnect) we were past CHAVO. I decided to idle the power; dial 1000 FPM into Vertical Speed and continue the descent to comply with LEMOE at 7000 feet.  With the turbulence and the disconnect; the speed had accelerated to near 230 knots.  I deployed speed boards to slow the aircraft back to 210 knots while continuing to comply with the LEMOE crossing restriction.


Second narrative: We were descending on the HYDRR 1 RNAV Arrival; Runway 25L transition; into PHX. We just received the new ATIS; which included a windshear advisory. The arrival was normal until AGGLA; with LNAV and VNAV PATH engaged. As we were approaching CHAVO; 210 knots/9000 FT; the FO had to deploy the speedbrakes as the airspeed was increasing. Simultaneously; we encountered moderate chop. The Controller asked us what the winds were; and I reported they were 350/10. At that moment; I think the VNAV PATH disconnected. It appeared the aircraft was not going to level at 9000 FT because the Vertical Speed was also increasing! The FO immediately tried to level the aircraft and arrest the increasing speed; but I believe we went below 9000 FT at CHAVO. Our maximum airspeed through the event was 220 knots. We were back in VNAV PATH after CHAVO; and the rest of the arrival and landing were uneventful. The FO thought we made the CHAVO restriction; but I'm not so sure. There were no other aircraft involved; and the Controller did not say anything else to us.Although I don't think this was a full-blown windshear event; I suspect a combination of the shifting tailwind and turbulence drove the VNAV to disconnect. Also; the thrust levers were at idle; and the speedbrakes were deployed throughout. Therefore; I don't think the FO could have done much more to slow down. Finally; I think the FO did a good job of recognizing this dynamic event; and responding about as quickly as he could.Although I don't think the FPA went to VNAV SPD; an additional signal to alert us when the FPA comes out of PATH; or switches to SPD; would be very helpful. I'm sure many other Pilots would agree.


Synopsis: B737 flight crew experience a VNAV disconnect during the HYDRR1 RNAV Arrival into PHX approaching CHAVO. CHAVO may have been crossed 200 feet low and the speed was allowed to increase above the CHAVO limit of 210 knots.