Narrative:

While flying the RNAV approach to runway 4; the wrong altitude was set in the altitude preselector and a descent was initiated. The error was trapped and was being corrected as ATC got a low altitude alert; and queried us. We went 300 feet below the proper altitude. This was day one of a three day trip; the second and last leg of the day. I had never flown with this captain before. We had dealt with significant delays; weather; and turbulence all day. During this approach; we were experiencing constant light and frequent moderate turbulence. The weather was near minimums for the night approaching in blowing snow. The winds were about 30 degrees off the nose; at 27 gusting to 35. This RNAV approach called for us to fly to zzzzz at 2600 feet; then descend to 2300 feet to cross ZZZZZ1; the FAF; then follow the gp (glide path) to the runway. We were at 2600 feet outside of zzzzz and having trouble getting VNAV path to engage. The turbulence was definitely a factor. It seemed every time an input was made; we hit a significant bump; making the input hard to be accurately made. Since VNAV path would not engage; the captain put 2600 in the preselector while I went heads down to re-cruise the VNAV page to get us back into VNAV path. It was definitely hectic considering everything going on. After re-cruising the VNAV page; I looked up to confirm we were back into path; and realized we were descending. We had not yet crossed zzzzz which has a minimum altitude of 2600 feet. 2300 was in the preselector. I verbalized [that] we need to immediately climb back up to 2600 feet until zzzzz. 2600 feet was selected and an immediate climb initiated. ATC gave us the low altitude alert as we were leveling back at 2600 [feet]. VNAV path was able to be re-engaged; and the remainder of the approach was flown. The significant factors contributing to this error were: 1. Complexity in getting and keeping VNAV path to engage and stay engaged for the approach.2. Turbulence causing difficult inputs. 3. Fatigue. It was late in the day. This event could be prevented in the future by more closely following the procedure of setting an altitude; confirming it with the other pilot; then selecting a vertical mode. The way the VNAV path works should also be looked into. There are a lot of steps and inputs required to get VNAV path to engage; and stay engaged. It causes a high workload in an already high workload regime of flight. With the addition of significant turbulence and fatigue thrown in; the awkwardness of the VNAV induces a very unneeded hazard. Our threat and error management training and recent focus on the pilot actively monitoring certainly helped in our situation.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737 flight crew reported that due to turbulence VNAV PATH would not engage and causing the autopilot to disengage.

Narrative: While flying the RNAV approach to Runway 4; the wrong altitude was set in the altitude preselector and a descent was initiated. The error was trapped and was being corrected as ATC got a low altitude alert; and queried us. We went 300 feet below the proper altitude. This was day one of a three day trip; the second and last leg of the day. I had never flown with this Captain before. We had dealt with significant delays; weather; and turbulence all day. During this approach; we were experiencing constant light and frequent moderate turbulence. The weather was near minimums for the night approaching in blowing snow. The winds were about 30 degrees off the nose; at 27 gusting to 35. This RNAV approach called for us to fly to ZZZZZ at 2600 feet; then descend to 2300 feet to cross ZZZZZ1; the FAF; then follow the GP (Glide Path) to the runway. We were at 2600 feet outside of ZZZZZ and having trouble getting VNAV PATH to engage. The turbulence was definitely a factor. It seemed every time an input was made; we hit a significant bump; making the input hard to be accurately made. Since VNAV PATH would not engage; the captain put 2600 in the preselector while I went heads down to re-cruise the VNAV page to get us back into VNAV PATH. It was definitely hectic considering everything going on. After re-cruising the VNAV page; I looked up to confirm we were back into PATH; and realized we were descending. We had not yet crossed ZZZZZ which has a minimum altitude of 2600 feet. 2300 was in the preselector. I verbalized [that] we need to immediately climb back up to 2600 feet until ZZZZZ. 2600 feet was selected and an immediate climb initiated. ATC gave us the low altitude alert as we were leveling back at 2600 [feet]. VNAV PATH was able to be re-engaged; and the remainder of the approach was flown. The significant factors contributing to this error were: 1. Complexity in getting and keeping VNAV PATH to engage and stay engaged for the approach.2. Turbulence causing difficult inputs. 3. Fatigue. It was late in the day. This event could be prevented in the future by more closely following the procedure of setting an altitude; confirming it with the other pilot; then selecting a vertical mode. The way the VNAV PATH works should also be looked into. There are a lot of steps and inputs required to get VNAV PATH to engage; and stay engaged. It causes a high workload in an already high workload regime of flight. With the addition of significant turbulence and fatigue thrown in; the awkwardness of the VNAV induces a very unneeded hazard. Our threat and error management training and recent focus on the Pilot Actively Monitoring certainly helped in our situation.

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.