Date: 201306
Local time of day: 0601-1200
Location: L30.TRACON, NV
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
ATC / advisory: TRACON L30
Aircraft component: Autoflight System
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Time Pressure; Workload; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Environment - Non Weather Related; Procedure
Primary problem: Ambiguous

Narrative: Holding at FUZZY Intersection at 16;000 feet. Runway 19R already NOTAMed closed so LAS was now on two runways. Fuel on board 6;600 [LBS] and arrival fuel with expected EFC 4.5. Attempted to contact Dispatch via ACARS. Unsuccessful and made very difficult by repeated interruptions. First Officer was pilot flying. Finally contacted Dispatch but still did not receive a diversion message. Were advised that Nellis and Laughlin were available if need be. Started searching offline airport kit for planning purposes and then switched to LAS Approach.Received new clearance direct FUZZY and descend via arrival but now for Runway 25R. Previous arrival brief was predicated on landing Runway 19L. Autopilot engaged in LNAV/VNAV modes so exit hold selected and turned inbound. As we had slowed to approximately 205 knots for holding; arrival indicated 250 knots at FUZZY. Queried Approach as to their speed requirements and were told 250 knots. Entered 250 in descent page; verified LNAV/VNAV and First Officer began reprogramming arrival for Runway 25R. As I was verifying the arrival route; I noticed the aircraft had descended to 15;400 feet even though we had not reached FUZZY. MCP mode and now Level Change. A very similar occurrence the very next day on a RNAV arrival with ATC change entered. Autopilot once again changed to Level Change. If this is normal; maybe something in training to point it out. Have had three RNAV incidents similar to this all.


Second narrative: As I was verifying the arrival route; I noticed the aircraft had descended to 15;400 feet even though we had not reached FUZZY. MCP mode and now Level Change. Advised ATC of error. Primarily; crews must ensure that aircraft flight performance is monitored at all times by at least one crew member. Both of us going 'heads down' to quickly prepare for the approach changes permitted the flight altitude deviation to go unrecognized for too long. Also; it would be helpful to understand why the SUNST ARRIVAL hard altitude at FUZZY; 16;000 feet; coincident with our holding altitude; suddenly changed to a soft altitude upon exiting the hold through the FMC. This was unexpected.


Synopsis: A B737NG autoflight system transitioned to LEVEL CHANGE (LVL CHG) and began a descent after the crew began programming the FMS's to exit holding and continue a RNAV arrival descent upon passing the holding fix at the holding altitude constraint.