A B737NG autoflight system transitioned to level change (lvl chg) and began a descent after the crew...

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 [pounds] 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.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1096755
Time
Date	201306
Local Time Of Day	0601-1200
Place
Locale Reference	L30.TRACON
State Reference	NV
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Flying First Officer
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Procedural Clearance

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.