Date: 202107
Location: ZZZ.TRACON, US
Flight conditions: VMC
Aircraft: B737 MAX Series Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Autoflight System
Component problem: Improperly Operated; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Situational Awareness; Communication Breakdown; Confusion; Troubleshooting
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control
Contributing factors: Procedure; Human Factors; Aircraft; Software and Automation
Primary problem: Software and Automation

Narrative: We were descending on the arrival into ZZZ. We were descending at a high speed programmed into the FMC. I have over 10;000 hours flying the Next Gens and am comfortable with that descent profile. This was my first MAX flight. Somewhere in the descent the FMA changed without warning to MCP SPD (Mode Control Panel Speed). It went unnoticed. I have witnessed many Next Gens revert to VNV SPD (Vnav Speed); but never MCP SPD. There is a significant difference; in that MCP SPD gives no altitude protection. I have never seen anything like this. Somewhere around ZZZZZ/11000; we received a radio call and became distracted as the aircraft descended through 11000. I did not catch the deviation until 10;000 and momentum and recovery took us to about 9;500 feet. I disconnect all automation and hand flew the recovery as I tried to figure out what had just happened. We reported the deviation to ATC and continued to ZZZ without incident.I do not know why this happened. I can only guess that somehow LVL CHG (Level Change) was inadvertently selected. Another theory is that VNV could not keep up and reverted to MCP SPD. This happens sometimes in the Next Gens; but it always reverts to VNV SPD; which is much safer.


Second narrative: On arrival to ZZZ in VFR conditions. Arrival information was checked by both pilots for altitude and speed restrictions prior to descent. Aircraft was cleared by approach to descend via the arrival with a bottom altitude of 8;000 feet. The crossing restriction at ZZZZZ is 11;000 feet. Prior to reaching ZZZZZ the crew saw the aircraft was in Lvl Chg (Level Change) mode instead of VNAV and was descending through 10;000 feet. The Captain (Pilot Flying) took the controls; disconnected the autopilot and returned to altitude. The FO (Pilot Monitoring) notified ATC of the deviation but no conflicts existed and was handed to the next controller.  Upon checking the FMS the First Officer determined the altitude restriction was still active at ZZZZZ for 11;000 feet. The crew briefly discussed the incident and possible errors but did not determine how or when the autopilot entered Lvl Chg mode during the descent.


Synopsis: B737 MAX flight crew reported an altitude deviation caused by the aircraft reverting to MCP SPD that has no altitude protection.