Medium Large Transport
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 145; Flight Crew Type 9800
Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Procedure
We were descending on the DSNEE 1 arrival but had been assigned several speeds; the last was 200 KIAS. I had been using speed intervention (SPD INTV) to manage the speed but switched to Level Change after passing the last intermediary altitude restriction and being asked to expedite our descent through 5000' for traffic. I announced the new mode and verified 4000' in the altitude window (the final altitude on the STAR).After we were clear off traffic; SoCal cleared us for the ILS Runway 30. I closed the discontinuity in the FMS after MIDDS; checked with the Pilot Monitoring (PM) and executed it. My PM then prompted that we could set 1600' in the altitude window for the FAF at GUNEY. I did so but failed to verify that we were in VNAV. At 3800' the PM noticed we were below the 4000' restriction at GOOFY. I reversed the descent and started climbing back to 4000'. We ended up 300' low at GOOFY. At that point; SoCal gave us a heading to join final and a descent to 2000'. We complied with the new clearance; were re-cleared for the ILS; and continued without further event.The continued use of SPD INTV would have done the same job but protected us from descending below a coded altitude on the arrival. Next time I need to descend quickly; I will try to stay in VNAV if I am continuing on an arrival that connects with an approach with step-down altitudes. Also using VVMI to verify my level of automation after an altitude selection will trap this kind of error in the future.
Captain reported descending below the altitude for crossing GOOFY intersection on the DSNEE four arrival to LGB citing management of the flight guidance system to comply with ATC speed requests put the system in a mode that did not protect for crossing altitude restriction compliance.