Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Situational Awareness; Confusion; Distraction; Human-Machine Interface
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR
Flight Crew Overcame Equipment Problem
Environment - Non Weather Related; Human Factors
We were cleared for takeoff; radar vectors to 5;000 feet.  Shortly after takeoff and at acceleration altitude; as pilot flying; I called for VNAV.  Departing ZZZ there were many thunderstorm cells and we were told to expect multiple vectors. Those started early and we complied as the airplane climbed. I was also instructing my pilot monitoring to clean up flaps on schedule as he spoke to ATC.  It was just above 2;000 feet shortly after my call for autopilot on; I notice the airplane attempting to level and the throttles retarding. My first inclination was to immediately verify that we had the proper altitude of 5;000 feet set and it was. In a moment of confusion I disengaged the autopilot to continue what was then a turning climb. With the throttles now not going back into the TOGA detents; I also disengaged the autothrottles and manually pushed them forward. I verbalized 'What is going on' to my FO. It was shortly after this I realized our vertical mode: green ALT. I instructed the FO for FLCH speed 250 (we were nearing around 2;500 by this time). In the short period of this trouble shooting; I dipped the nose enough to break the 250 kts limit. When I noticed it; the speed was a little shy of 270 kts. I immediately brought the speed back to 250 kts and continued the remainder of the flight without incident.This was my first leg back after 3 months. Immediate vigilance is required and complacency is never an option. I still am unclear as to what sequence drove that airplane into green ALT (it is possible the task saturated FO hit ALT multiple times looking for AP; but he was unable to offer any clarifying information).  The point is I should have included a verification of magenta FLCH on the FMA when I verified the altitude set.  I cannot assume either the airplane or my First Officer will always deliver what I ask for.
Air carrier Captain reported the aircraft started to level off at the incorrect altitude resulting in a speed deviation while trouble shooting.