Date: 202101
Local time of day: 1801-2400
Location: SNA.Airport, CA
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZLA
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Training / Qualification; Workload
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: During the arrival; we were given a vector and then a direct to DSNEE. As we were still in the high teens; I suggested opening the speed window at 280 KIAS (assigned speed) and then spinning it down below 250 passing around 12-13000' (due to the Captain's relative inexperience). Sometime below 15000'; SoCal Arrival asked if we wanted the SNA 20R RNAV Z. We had already setup and briefed for the ILS but we discussed any benefits to switching and decided against it. I informed SoCal of our desire for the ILS and crosschecked my instruments and noticed we were descending through approximately 8700' still doing 280 KIAS. I gave a corrective command and immediately pressed ALT HOLD (Altitude Hold); spun back the speed to below 250 KIAS; and deployed the speed brakes. I did not say; 'My Aircraft' but time was of the essence. We slowed and continued the arrival and landing uneventfully: there was no communication from SoCal referencing the deviation.Using SPD INTV (Speed Intervene); without its protection; coupled with the distraction of SoCal's request set us up for the deviation. Additionally; the Captain is new to both the seat and the airplane.[I] suggest using all available automation to the maximum extent. We could have instead entered speeds into the CDU at the abeam waypoints and allowed the aircraft to gradually decelerate on its own (or the DESCENT page speed).


Second narrative: I was flying. Received a vector off the arrival. Chose to use speed intervene; 280kts. Just before I went to reduce speed to 250kts for 10;000; ATC asked if we wanted the Required Navigation Performance (RNP) Z. We had set up and briefed the ILS as I had been refused the RNP Z on a prior flight. First Officer (FO) and I got distracted discussing the pros and cons of accepting the RNP Z; descended below 10;000 ft. doing 280kt. The FO caught it. We leveled off; used speed brakes; got below 250kts; and continued decent. I am a new Captain. About 150 hours on the 737; which I haven't flown. I haven't flown in the last 2 months. These factors mainly contributed to a narrowing of the big picture. Additionally; I was tired. It was night. Last leg of 3 day pairing. More than anything; don't use speed intervene without a lot of extra caution. Putting the speed in the FMC is the better way to go; it only takes a couple of seconds longer.


Synopsis: B737 flight crew reported exceeding 250 knots below 10;000 feet after planning to slow from an assigned 280 knots several thousand feet above 10;000 feet to accommodate the Captain's relative inexperience.