Date: 202109
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737 MAX 9
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Human-Machine Interface
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Undershoot; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Result: Flight Crew Overcame Equipment Problem; Flight Crew Overrode Automation
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Approximately 35 minutes from landing at ZZZ at FL 350 we were given a descent clearance to cross ZZZZZ intersection at FL310 while talking to ZZZ center.  The Captain was the PM and set the alt in the MCP; confirmed by the PF FO.  The MAX 9 was programmed for the descent with winds/speeds and confirmed in VPTH.  This was approximately 30nm from ZZZZZ and a timely clearance.  This was almost 5 hours into a 5:XX all night flight.  We were working on canceling a previous write up that was noticed as being incorrect.  As we felt the aircraft descend we noticed that the A/C had put itself into VSPD and that the VDI showed us being approximately 1100 feet high.  The FO placed the autoflight system in LCHG with increased speed to increase the descent.  The plane did not respond to the inputs so the FO disconnected the autopilot and hand flew the aircraft to FL310 at ZZZZZ but crossed approximately 800 feet high at ZZZZZ.ATC very soon after changed us to ZZZ center and nothing was indicated by ATC; and we felt there were no traffic conflicts since the radio had been very quiet recently.I am writing this scenario in concern that the MAX 9 has been having issues with randomly disconnecting VPTH and putting the Autoflight system in VSPD for no apparent reason.  As we look back to the events prior; the Autoflight system was setup for success but somehow it changed modes on us without us immediately noticing.  Moving forward a takeaway from this event would be to obviously pay more attention to modes especially if distracted with other items such as maintenance writeups.  I would have also been quicker to request relief from the altitude restriction although this would never be assured to be given.  I have seen this mode change happen before but have always caught the change in mode before it becomes a concern.


Second narrative: Approximately 35 minutes from landing at ZZZ at FL 350 we were given a descent clearance to cross ZZZZZ intersection at FL310 while talking to ZZZ center.  The Captain was the PM and set the alt in the MCP; confirmed by the PF FO.  The MAX 9 was programmed for the descent with winds/speeds and confirmed in VPTH.  This was approximately 30nm from ZZZZZ and a timely clearance.  This was almost 5 hours into a 5:XX all night flight.  We were working on canceling a previous write up that was noticed as being incorrect.  As we felt the aircraft descend we noticed that the aircraft had put itself into VSPD and that the VDI showed us being approximately 1100 feet high.  The FO placed the autoflight system in LCHG with increased speed to increase the descent.  The plane did not respond to the inputs so the FO disconnected the autopilot and hand flew the aircraft to FL310 at ZZZZZ but crossed approximately 700 feet high at ZZZZZ.ATC very soon after changed us to ZZZ center and nothing was indicated by ATC; and we felt there were no traffic conflicts since the radio had been very quiet recently.I am writing this scenario in concern that the MAX 9 has been having issues with randomly disconnecting VPTH and putting the Autoflight system in VSPD for no apparent reason.  As we look back to the events prior; the Autoflight system was setup for success but somehow it changed modes on us without us immediately noticing.  Moving forward a takeaway from this event would be to obviously pay more attention to modes especially if distracted with other items such as maintenance writeups.  I would have also been quicker to request relief from the altitude restriction although this would never be assured to be given.  I have seen this happen before but have always caught the change in mode before it becomes a concern.


Synopsis: B737 MAX 9 flight crew reported that during descent the autoflight system malfunctioned and did not respond to inputs. The crew disconnected the autopilot and continued the flight by hand flying the aircraft.