Date: 201606
Local time of day: 0601-1200
Location: LAX.Airport, CA
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B LAX
ATC / advisory: TRACON SCT
Aircraft component: Autopilot
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Confusion; Communication Breakdown; Workload
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

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.


Second narrative: As the PF; we were on the SADDE 6 into LAX with the autopilot coupled to the B side. The altitude and speeds given did not all match the STAR. Prior to SYMON; we were given a speed of 280 knots and a descent to 10;000 ft; with a heading at SMO of 270. I confused the heading at SMO for a heading at SYMON; immediately the PM (Captain) corrected me and after engaging heading; I immediately engaged LNAV and avoided any deviations. That seemed to distract us both and evolved into a discussion about assigned airspeed versus published. As we approached the 70 degree turn at SADDE; I asked for clarification on airspeed; published versus assigned. The Captain and I went heads down (at the same time) to verify the programmed speed. About the time the LEGS page cycled and we both noticed we weren't turning and in a steep descent; the autopilot was no longer engaged. I immediately started recovering from the steep descent; it was recognized at 9700 ft MSL; and we recovered around 9300 ft MSL and returned to 10;000 ft. ATC immediately assigned us a heading and asked for altitude verification.As the PF; I at no time disconnected or recognized the autopilot had been disengaged. The Captain said he did; but failed bring it to my attention. This was the third time in two legs the autopilot had been disconnected without Pilot input. I do not recall hearing the disconnect and asked the Captain why he didn't point it out if he did. I'm still uncertain if we ever got the audio warning; and when it happened; there was a lot of ATC chatter and flight deck chatter and confusion.


Synopsis: 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.