Narrative:

Enroute to las with the first officer as the flying pilot and myself as the pilot monitoring. We were cleared to dag for the kepec arrival into las. Before the descent; I copied the latest weather and used the performance computer to calculate data for landing and shared the info with the first officer. We reviewed the specific crossing altitudes and crossing speeds for the kepec arrival and confirmed that they were properly displayed in the CDU on the legs page. He confirmed the 'numbers' and we discussed the landing runway's specifics and target speeds that we could expect. I believe that la center cleared us to cross misen at FL240 and then later cleared us to; 'descend via the kepec two arrival.' I confirmed the clearance and watched the first officer put the appropriate altitude in the altitude window. We both discussed the new clearance and I agreed with this new altitude of 8000 ft. As we neared clarr intersection we received a call from the flight attendant's about what gate we could expect to arrive at in las and what gate their next flight would be departing from. At approximately the same time we began to descend down into some rough air and I briefed the flight attendant's on what they could expect in the descent. When I returned to the #1 frequency I was briefed by the first officer that we needed to contact las approach so I attempted to do that but there was a delay because of ATC congestion. I failed to realize that the VNAV function was inadvertently turned 'off' during this busy time. We had crossed clarr intersection at 250 and 13;000 ft and continued the descent. By the time I was able to contact las approach we were descending through 11;000 ft. Upon our check-in; he told us to climb to 12;000 ft. I assumed that he had us confused with another aircraft and confirmed that he wanted us to climb to 12;000 ft. At that point; he told us we needed to climb up to 13;000 ft and that we had descended to early on the arrival. I confirmed the clearance and we began to climb. He then vectored us to a heading of 040. As we began the climb and the turn he called out 'traffic' for us at approximately 11 o'clock position I believe. I had the traffic in sight but we never received a TA or RA on the TCAS. He then changed his mind and told us instead to descend to 10;000 ft; which we began to do. He then gave us a vector to a heading of 360 before clearing us to ipumy intersection and re-cleared us for the kepec arrival. We continued the arrival and made a normal approach and landing in las. I think that I as the PIC and we as a crew failed to properly monitor the aircraft's autopilot and VNAV systems. I think that I let other less important distractions; for example the call from the flight attendant's; the turbulence in the descent; and the frequency changes cause me to miss the fact that the aircraft's VNAV system had been inadvertently turned off or disengaged. I know it's easy for complacency to set in but that was certainly not the case here. We were working hard to confirm and fly the arrival as published. In the future I will be extra vigilant to make sure the VNAV is actually armed when it is supposed to be used and that we are executing these arrivals as they are depicted. I need to be less susceptible to these distractions and either fly or monitor the airplane first and foremost.

Google
 

Original NASA ASRS Text

Title: A B737NG deviated from the charted altitudes on the KEPEC Arrival into LAS when the aircraft came out of VNAV and the flight crew failed to catch the deviation.

Narrative: Enroute to LAS with the First Officer as the flying pilot and myself as the pilot monitoring. We were cleared to DAG for the KEPEC Arrival into LAS. Before the descent; I copied the latest weather and used the performance computer to calculate data for landing and shared the info with the First Officer. We reviewed the specific crossing altitudes and crossing speeds for the KEPEC Arrival and confirmed that they were properly displayed in the CDU on the LEGS page. He confirmed the 'numbers' and we discussed the landing runway's specifics and target speeds that we could expect. I believe that LA Center cleared us to cross MISEN at FL240 and then later cleared us to; 'Descend via the KEPEC TWO Arrival.' I confirmed the clearance and watched the First Officer put the appropriate altitude in the altitude window. We both discussed the new clearance and I agreed with this new altitude of 8000 FT. As we neared CLARR intersection we received a call from the flight attendant's about what gate we could expect to arrive at in LAS and what gate their next flight would be departing from. At approximately the same time we began to descend down into some rough air and I briefed the flight attendant's on what they could expect in the descent. When I returned to the #1 frequency I was briefed by the First Officer that we needed to contact LAS Approach so I attempted to do that but there was a delay because of ATC congestion. I failed to realize that the VNAV function was inadvertently turned 'OFF' during this busy time. We had crossed CLARR intersection at 250 and 13;000 FT and continued the descent. By the time I was able to contact LAS Approach we were descending through 11;000 FT. Upon our check-in; he told us to climb to 12;000 FT. I assumed that he had us confused with another aircraft and confirmed that he wanted us to climb to 12;000 FT. At that point; he told us we needed to climb up to 13;000 FT and that we had descended to early on the arrival. I confirmed the clearance and we began to climb. He then vectored us to a heading of 040. As we began the climb and the turn he called out 'traffic' for us at approximately 11 o'clock position I believe. I had the traffic in sight but we never received a TA or RA on the TCAS. He then changed his mind and told us instead to descend to 10;000 ft; which we began to do. He then gave us a vector to a heading of 360 before clearing us to IPUMY intersection and re-cleared us for the KEPEC Arrival. We continued the arrival and made a normal approach and landing in LAS. I think that I as the PIC and we as a Crew failed to properly monitor the aircraft's autopilot and VNAV systems. I think that I let other less important distractions; for example the call from the flight attendant's; the turbulence in the descent; and the frequency changes cause me to miss the fact that the aircraft's VNAV system had been inadvertently turned off or disengaged. I know it's easy for complacency to set in but that was certainly not the case here. We were working hard to confirm and fly the arrival as published. In the future I will be EXTRA vigilant to make sure the VNAV is actually armed when it is supposed to be used and that we are executing these arrivals as they are depicted. I need to be less susceptible to these distractions and either fly or monitor the airplane first and foremost.

Data retrieved from NASA's ASRS site as of April 2012 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.