Date: 201409
Local time of day: 0601-1200
Location: IAH.Airport, TX
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B IAH
ATC / advisory: TRACON I90
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Distraction; Human-Machine Interface
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation
Contributing factors: Procedure; Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Departing on the PITZZ RNAV SID; we initially were assigned 4000 FT per the SID. The Captain was flying with the autopilot engaged and I was the pilot monitoring. The aircraft was exceptionally light for the short flight and was climbing quickly. We were in VNAV for the climb and not quite leveling at 4000 FT when atc assigned us to 'climb via' the SID except maintain 14000 FT. We heard the clearance correctly; I dialed in 14000 FT and the Captain acknowledged. At that point I began to verify in the FMS that the altitude restriction of 5000 FT at JAYLO and the other restrictions would be complied with. I verbalized that everything was correct in the FMS and looked back at the PFD/ND and I noticed we were quickly approaching 5000 FT and nearing JAYLO. Nearing the altitude I verbalized; 'I wonder why its not leveling off' and the Captain quickly disengaged the autopilot and manually leveled the aircraft. The autopilot was disengaged at about 5000 FT and it took the captain about 300 FT to level off. By that time we were already at JAYLO and 300 FT high. It wasn't until we passed JAYLO and were ready to begin the climb again that the Captain verbalized that VNAV had disengaged. We re-engaged VNAV and continued the climb with the autopilot on. It took us a second to figure out why VNAV had disengaged but after thinking things through we determined that the aircraft must have switched in to the Altitude capture mode when I selected the new altitude which would have kicked the autopilot out of VNAV. Normally when I know the aircraft is leveling off I always verify that VNAV is re-engaged after selecting a new altitude; but because I thought I HAD selected the new altitude well before the aircraft had begun the level off I assumed that the aircraft had remained in VNAV and because of the fast climb rate I decided to turn my attention to the FMS to double check it was programmed to level off correctly. Had there been more time I am pretty sure I would have noticed that the aircraft had been bumped out of VNAV but the lack of time to verify was a threat that lead to this error. the whole event seemed like it last 30 seconds it happened quickly.  Note: looking back on it now I realize the autopilot switched to altitude capture earlier than normal because we were climbing out so quickly because of the light aircraft weight.




Synopsis: A B737 flight crew departing IAH on the PITZZ RNAV SID as cleared to 'climb via' the SID 'except maintain 14;000 feet' as they approached the charted 4;000 initial top of climb.  The FMS apparently dropped out of VNAV mode as the new altitude was inserted in the ALT ALERT window and the crew manually intervened as they approached JAYLO climbing through the required 'at or below 5;000 feet.'