Date: 201808
Local time of day: 0601-1200
Location: SNA.Airport, CA
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C SNA
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Situational Awareness
Second reporter function: Captain; Pilot Not Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
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: Human Factors
Primary problem: Human Factors

Narrative: SNA RNP-Z to Runway 20R. The FMC was properly programmed and MCP panel [was] in LNAV/VNAV PATH. Upon crossing KLEVR at 5000 FT IAF; we set zeros in the altitude alerter. The aircraft did not descend to cross MNNIE at 4400 FT; next mandatory altitude. I noticed level flight and immediately selected Vertical Speed to try and meet MNNIE at 4400 feet. About the same time; SoCal Approach asked us if we were descending and flying the RNP-Z. The Pilot Monitoring (PM) said 'yes' but asked if we could be cleared the Visual Approach to try and mitigate any problems. SoCal said 'no problem' and gave us a 'heading of 030 and descend to 3000 FT; call the field in sight.' As Pilot Flying (PF); the field was on my side and we had it in sight and called - in sight. At that point we were cleared for the visual and told to contact SNA Tower. The rest of the approach and landing were uneventful and we landed safely on 20R. With short distances between waypoints and mandatory descending altitude restrictions; pilots should be quicker to notice any anomaly and react quickly to stay in/on VNAV PATH even if aircraft does not do it automatically.


Second narrative: We were cleared for the RNP RNAV Z 20R Approach and level at 5000 feet. We were in visual conditions; and just using the approach for ease of use. We had zeros set after the initial approach fix and in LNAV/VNAV PATH. After crossing KLEVR; the aircraft did not descend. We noticed it quickly; and the Pilot Flying intervened by using Vertical Speed to catch the profile. Very shortly after we began to intervene; ATC asked us if we were doing the RNP Z Approach. I told him we were; but the aircraft wasn't doing what we were wanting or expecting it to do; and I asked if he could vector us for the visual approach. He gave us a heading and altitude to fly. He asked if we had the airport; which we did; and he cleared us for the visual. We landed without incident. In the future; I need to monitor more closely; and not let the good weather lull me into complacency. If we're cleared for an approach; I will monitor it better.


Synopsis: B737-700 flight crew reported failing to make a crossing restriction on the RNP-Z Runway 20R approach to SNA.