Narrative:

During arrival into lax; our flight deviated 600 ft below assigned altitude and overflew sadde intersection. The ultimate reason for the altitude and course deviation was a disconnected autopilot that I thought had been disconnected intentionally; but did not verbally verify with the pilot flying. We were on the sadde 6 arrival for lax. The arrival and approach had all been properly briefed and the flight was going as planned until ATC gave us a clearance to fly a 070 heading after santa monica VOR for runway 24R. The clearance was given prior to crossing symon intersection. We had also been given a clearance to descend to 10;000 ft prior to symon intersection. The heading assignment at smo is a standard ATC practice that socal approach gives to arriving aircraft; but the altitude is different from the expected altitude at symon. After we crossed symon intersection; I noticed that the first officer had started a turn in heading select to the 070 heading; which was the assigned turn after smo. I verbalized that we were turning early and that our clearance was to fly the arrival until smo and not symon. The first officer acknowledged my input to the situation; corrected the error; and placed the autopilot MCP back in LNAV and the aircraft began correcting back to the STAR. After rejoining the STAR our workload began to increase rapidly because we still were not relieved of the speed restriction at symon; I was attempting [to] verify that the aircraft was on the correct flight path and the first officer was making the correct inputs into the MCP. Somewhere between symon and sadde intersection I heard the autopilot disconnect and I thought that the first officer had intentionally disconnected to make the 10;000 ft altitude assignment and speed restriction. I did not verbalize the fact that I had heard the autopilot disconnect due being distracted by ATC radio calls and I was still making sure that we were flying the arrival properly. I did not know that the first officer did hear the autopilot disconnect alert and was not aware that the autopilot was disconnected. As we rapidly approached 10;000 ft; I noticed that we were not leveling off and I stated 'altitude!' the first officer started to recover our lost altitude but we had leveled at 9400 ft before we made a quick recovery back to assigned altitude. We had also slightly overflown the sadde intersection. Socal approach told us to turn left to a new heading of 078 and squawk ident. I read back the clearance; started to push the ident button; and noticed that the transponder had been turned to standby. I asked the first officer what happened to the transponder and she replied that she had turned it to standby. This was done without my knowledge. This temporarily put me in the red. After taking a deep breath; I checked the flight instruments; observed that we were turning back to bayst and we were back/close to the assigned altitude. I turned the transponder back to altitude at the same time; trying to respond to calls from socal approach. Somewhere during all of the confusion; the autopilot was re-engaged and things started to settle down. After rejoining the arrival we continued the approach and landed in lax.I was wrong to assume that autopilot had been disconnected by the pilot flying. I always verbalize whenever I disconnect the autopilot; and autothrottles; when I am the pilot flying. I will now verbalize this whenever I hear the disconnect warning when I am pilot monitoring no matter how busy I may be. I also need to stop helping the pilot flying with inputs to the FMC when I am the pilot monitoring. I have a desire to want to be helpful but this sometimes comes across as micromanaging. I need to do a better job of monitoring; verbalizing and intervening when the situation becomes critical.

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Original NASA ASRS Text

Title: A B737 First Officer flying the LAX SADDE 6 Arrival failed to see or hear the autopilot disconnect and as the aircraft turned at SADDE it continued descending to 9;400 ft before the Captain called 'altitude.' As the flight climbed back to 10;000 ft; ATC asked them to ident at which time the Captain found the First Officer had turn the transponder OFF.

Narrative: During arrival into LAX; our flight deviated 600 ft below assigned altitude and overflew SADDE intersection. The ultimate reason for the altitude and course deviation was a disconnected autopilot that I thought had been disconnected intentionally; but did not verbally verify with the Pilot Flying. We were on the SADDE 6 Arrival for LAX. The arrival and approach had all been properly briefed and the flight was going as planned until ATC gave us a clearance to fly a 070 heading after SANTA MONICA VOR for Runway 24R. The clearance was given prior to crossing SYMON intersection. We had also been given a clearance to descend to 10;000 ft prior to SYMON intersection. The heading assignment at SMO is a standard ATC practice that SOCAL Approach gives to arriving aircraft; but the altitude is different from the expected altitude at SYMON. After we crossed SYMON intersection; I noticed that the First Officer had started a turn in Heading Select to the 070 heading; which was the assigned turn after SMO. I verbalized that we were turning early and that our clearance was to fly the arrival until SMO and not SYMON. The First Officer acknowledged my input to the situation; corrected the error; and placed the autopilot MCP back in LNAV and the aircraft began correcting back to the STAR. After rejoining the STAR our workload began to increase rapidly because we still were not relieved of the speed restriction at SYMON; I was attempting [to] verify that the aircraft was on the correct flight path and the First Officer was making the correct inputs into the MCP. Somewhere between SYMON and SADDE intersection I heard the autopilot disconnect and I thought that the First Officer had intentionally disconnected to make the 10;000 ft altitude assignment and speed restriction. I did not verbalize the fact that I had heard the autopilot disconnect due being distracted by ATC radio calls and I was still making sure that we were flying the arrival properly. I did not know that the First Officer did hear the autopilot disconnect alert and was not aware that the autopilot was disconnected. As we rapidly approached 10;000 ft; I noticed that we were not leveling off and I stated 'altitude!' The First Officer started to recover our lost altitude but we had leveled at 9400 ft before we made a quick recovery back to assigned altitude. We had also slightly overflown the SADDE intersection. SOCAL Approach told us to turn left to a new heading of 078 and squawk ident. I read back the clearance; started to push the ident button; and noticed that the transponder had been turned to Standby. I asked the First Officer what happened to the transponder and she replied that she had turned it to Standby. This was done without my knowledge. This temporarily put me in the red. After taking a deep breath; I checked the flight instruments; observed that we were turning back to BAYST and we were back/close to the assigned altitude. I turned the transponder back to ALT at the same time; trying to respond to calls from SOCAL Approach. Somewhere during all of the confusion; the autopilot was re-engaged and things started to settle down. After rejoining the arrival we continued the approach and landed in LAX.I was wrong to assume that autopilot had been disconnected by the Pilot Flying. I always verbalize whenever I disconnect the autopilot; and autothrottles; when I am the Pilot Flying. I will now verbalize this whenever I hear the disconnect warning when I am Pilot Monitoring no matter how busy I may be. I also need to stop helping the Pilot Flying with inputs to the FMC when I am the Pilot Monitoring. I have a desire to want to be helpful but this sometimes comes across as micromanaging. I need to do a better job of monitoring; verbalizing and intervening when the situation becomes critical.

Data retrieved from NASA's ASRS site 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.