Narrative:

It was day three of six for me and day three with very good first officer (first officer). Well rested; great rapport and above average crew coordination. Knew we had a maximum. It was my leg; normal ops brief; plus I briefed our concerns with the maximum issues; bulletin; MCAS; stab trim cutout response etc. I mentioned I would engage autopilot sooner than usual (I generally hand fly to at least above 10;000 ft.) to remove the possible MCAS threat. Weather was about 1000 ovc drizzle; temperature dropping and an occasional snow flake. I double checked with an additional personal walkaround just prior to push; a few drops of water on the aircraft but clean aircraft; no deice required. Strong crosswind and I asked tug driver to push a little more tail east so as not to have slow/hung start gusts 30+. Wind and mechanical turbulence was noted. Careful engine warm times; normal flaps 5 takeoff in strong (appeared almost direct) crosswind. Departure was normal. Takeoff and climb in light to moderate turbulence. After flaps 1 to 'up' and above clean 'masi up speed' with LNAV engaged I looked at and engaged a autopilot. As I was returning to my pfd (primary flight display) pm (pilot monitoring) called 'descending' followed by almost an immediate: 'dont sink dont sink!' I immediately disconnected ap (autopilot) (it was engaged as we got full horn etc.) and resumed climb. Now; I would generally assume it was my automation error; i.e.; aircraft was trying to acquire a miss-commanded speed/no autothrottles; crossing restriction etc.; but frankly neither of us could find an inappropriate setup error (not to say there wasn't one). With the concerns with the maximum 8 nose down stuff; we both thought it appropriate to bring it to your attention. We discussed issue at length over the course of the return to ZZZ. Best guess from me is airspeed fluctuation due to mechanical shear/frontal passage that overwhelmed automation temporarily or something incorrectly setup in MCP (mode control panel). Pm's callout on 'descending' was particularly quick and welcome as I was just coming back to my display after looking away. System and procedures coupled with CRM (resource management) trapped and mitigated issue.

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Original NASA ASRS Text

Title: B737MAX Captain reported an autopilot anomaly in which led to an undesired brief nose down situation.

Narrative: It was day three of six for me and day three with very good FO (First Officer). Well rested; great rapport and above average Crew coordination. Knew we had a MAX. It was my leg; normal Ops Brief; plus I briefed our concerns with the MAX issues; bulletin; MCAS; stab trim cutout response etc. I mentioned I would engage autopilot sooner than usual (I generally hand fly to at least above 10;000 ft.) to remove the possible MCAS threat. Weather was about 1000 OVC drizzle; temperature dropping and an occasional snow flake. I double checked with an additional personal walkaround just prior to push; a few drops of water on the aircraft but clean aircraft; no deice required. Strong crosswind and I asked Tug Driver to push a little more tail east so as not to have slow/hung start gusts 30+. Wind and mechanical turbulence was noted. Careful engine warm times; normal flaps 5 takeoff in strong (appeared almost direct) crosswind. Departure was normal. Takeoff and climb in light to moderate turbulence. After flaps 1 to 'up' and above clean 'MASI up speed' with LNAV engaged I looked at and engaged A Autopilot. As I was returning to my PFD (Primary Flight Display) PM (Pilot Monitoring) called 'DESCENDING' followed by almost an immediate: 'DONT SINK DONT SINK!' I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb. Now; I would generally assume it was my automation error; i.e.; aircraft was trying to acquire a miss-commanded speed/no autothrottles; crossing restriction etc.; but frankly neither of us could find an inappropriate setup error (not to say there wasn't one). With the concerns with the MAX 8 nose down stuff; we both thought it appropriate to bring it to your attention. We discussed issue at length over the course of the return to ZZZ. Best guess from me is airspeed fluctuation due to mechanical shear/frontal passage that overwhelmed automation temporarily or something incorrectly setup in MCP (Mode Control Panel). PM's callout on 'descending' was particularly quick and welcome as I was just coming back to my display after looking away. System and procedures coupled with CRM (Resource Management) trapped and mitigated issue.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.