Date: 202009
Local time of day: 1201-1800
Location: LGB.Airport, CA
Weather elements / visibility: 3
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent; Initial Approach
Airspace: Class E SCT
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Human-Machine Interface; Communication Breakdown
Second reporter function: Pilot Flying; Captain
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: Descending into LGB on the DSNEE arrival; we were cleared to descend via the arrival. Captain was Pilot Flying. First Officer was Pilot Monitoring. ATC cleared us to descend via the arrival for Runway 30 but didn't specify which approach to expect. ATIS stated runway in use but not approach type. Captain loaded and briefed the RNAV (Z). While descending via; and inside the branch point for the arrival; Approach Control cleared us for the ILS via the arrival. At that time we were in VNAV/LNAV with the lowest altitude on the arrival set in the MCP. Our position was a few miles East of MCKYE and descending.Upon receiving the clearance the Captain quickly loaded the ILS and executed it without verifying there were no resultant discontinuities. Because we were inside the branch point of the arrival a discontinuity did appear; even though we were going to the same runway. Once the execute (EXEC) button was pushed the discontinuity appeared and VNAV disconnected. Because we were descending at the time; we continued to descend. The crossing restriction at MCKYE was at or above 7200'. The aircraft descended to roughly 6800' before the Captain corrected and returned us to the correct altitude. We were back up to 7200' prior to MCKYE. After correcting the flight path; the correct approach was loaded in the FMC and the remainder of the flight was uneventful.Proper verification prior to execution would help prevent this type of event from occurring again. Make sure both Crew Members have seen the changes and agree on them before pushing EXEC. Also; once the arrival has commenced do not execute any FMC changes unless you are on the LEGS page and have cleared any discontinuities resulting from the approach or runway change.


Second narrative: We were on the DSNEE 4 Arrival for LGB and had previously entered the RNAV Y MIDDS transition (I did not close the MIDDS discontinuity per SOP). Weather was IMC due to fires in Southern California. We had been cleared to descend via and 4000' MSL was entered in the MCP window with VNAV PATH. Approximately 1 NM prior to DSNEE waypoint; we were given clearance for the ILS to Runway 30. I re-entered the arrival with the MIDDS transition for the ILS and immediately got a VNAV discontinuity from the FMC due to the re-entry. We were working on closing the discontinuity when I noticed the aircraft had descended 400' below the MCKYE restriction of 7200' MSL; approximately 3 NM prior to MCKYE. I immediately initiated a climb back to 7200' MSL to MCKYE and continued the descend via arrival. We did not see any traffic conflicts from TCAS and ATC did not mention anything.Expect the unexpected. I should have entered the next restriction altitude in the MCP before making changes to the arrival. Even though I did not expect a discontinuity; it would have been prudent because we were on the arrival and close to a fix with a restriction. Surprise at the discontinuity delayed action. I should have concentrated more on the restrictions and less on the FMC.


Synopsis: Flight crew reported momentarily descend below required altitude on the DSNEE FOUR arrival while programing FMC for a change from RNAV to ILS approach into LGB.