Narrative:

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.

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Original NASA ASRS Text

Title: A B737 First Officer reported an altitude excursion after system anomalies affected their autoflight capabilities.

Narrative: 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.

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.