B737 Undifferentiated or Other Model
Flight Crew Type 8802
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Departure Airport; General Maintenance Action
Human Factors; Aircraft
We departed with VNAV selected on the ground; 190 heading in the heading window; and 2;500 feet in the altitude window. The takeoff was normal [until] 1;500 feet; then we were giving instructions to climb to 6;000 feet and direct [to an] intersection. At this time we were in the clouds/IMC: I noticed 160 knots in the speed window. The Captain set 6;000 feet in the altitude window and line selected direct [the intersection] in the FMS. VNAV and LNAV would not engage. Heading would not engage for a back up roll mode. I was tracking the magenta line to [the intersection] while climbing to 6;000 feet. As we were approaching [the intersection]; I proceeded to make a left turn to the next fix; when the Captain said 'check altitude'. I was following the pitch command on the flight director and noticed we were climbing through 6;400 feet. I quickly descended back down to 6;000 feet and noticed the flight director was commanding a climb and left turn. The auto throttle switch was on; but the auto throttles did not retard to maintain 250 kts. At this time the gear warning horn was continuous and the Captain [elected] to return to [departure airport]. I turned off the flight director and flew the aircraft raw data at this time to the assigned heading. The Captain managed the situation by informing the passengers; flight attendants; and Operations of our situation and intentions. As we descended; the gear horn stopped and we starting getting back auto throttle mode; pitch; and roll mode. We commenced an ILS approach and landed normally at 10;000 lbs overweight. We taxied to the gate without any further issues. The occurrence was written up in the log book along with the overweight landing.
A B737 First Officer reported an altitude excursion after system anomalies affected their autoflight capabilities.