Narrative:

Arriving into seattle we were given the GLASR9 arrival landing north. ATC gave an out of the normal but not abnormal instruction to a heading and instruction to descend to 16000 ft. As time progressed we were given the instruction to go direct to jackson and descend via the glacier 9 arrival landing north. Which would require a crossing of 16000 at jackson and 15000 thousand at woodi. The instruction was given almost at the time we arrived at 16000 ft. The decision to vto was made by me and then since the altitudes were previously briefed we could set the alerter to 12000 for hethr. The vto did not take on the first attempt but since it usually is such a quick button push to make vto work we set the alerter to 12000. At that time I noticed that it didn't take and started to work on fixing the problem but I didn't realize that the altitude didn't not capture at 16000 and the system stayed descending. At about the same time we both noticed that we were low at about 14800 I believe prior to jackson by approximately by 1 to 3 miles. We went up to 15000 and talked to the controller to get an amended clearance of 15000 and to continue on the arrival which he gave because I imagine there was no loss of separation. From that point on we continued the arrival as normal without issue.there were quite a few causes to this event. The main one is the expectation for the automation to work as it usually does. I believe we were so comfortable with the button pushes we jumped the gun on setting the alerter to 12000. We should have made sure together that the VNAV was engaged prior to resetting the alerter. Also the moment the VNAV didn't engage my first reaction should have been to fly the plane and make sure we were at the appropriate altitude before working the FMS issue. Though in the moment I believe I was very biased that since it is 2 or 3 button pushes that I could fix the issue in a short time period. So my head was down into the box instead of seeing what the plane was doing. Also at that very moment I believe there were some radio calls that took the captains attention away which reduced a second set of eyes on what was going on so we both became task saturated. Since I didn't see we passed through 16000 at this point the VNAV gave me a message of can't do a climb which for a few seconds confused me because I didn't realize we passed through 16000 which is when I noticed what was going on at about the same time as the captain. Eventually I had to stop the descent manually and return to a climb. In the future when there is such a short time period I will just let the alerter capture the altitude before making FMS changes to avoid this issue and just as importantly not set the alerter to a lower altitude till I confirm I set the VNAV properly.

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Original NASA ASRS Text

Title: Q400 First Officer reports descending below the JAKSN crossing altitude on the GLASR9 Arrival to SEA; after being vectored off the arrival and told to descend to 16000 feet then told to go direct JAKSN and descend via the arrival. The MCP altitude is reset to 12000 feet prior to capturing 16000 and the aircraft continues descending to 14800 before it is noticed.

Narrative: Arriving into Seattle we were given the GLASR9 Arrival landing north. ATC gave an out of the normal but not abnormal instruction to a heading and instruction to descend to 16000 ft. As time progressed we were given the instruction to go direct to Jackson and descend via the glacier 9 Arrival landing north. Which would require a crossing of 16000 at Jackson and 15000 thousand at WOODI. The instruction was given almost at the time we arrived at 16000 ft. The decision to VTO was made by me and then since the altitudes were previously briefed we could set the alerter to 12000 for HETHR. The VTO did not take on the first attempt but since it usually is such a quick button push to make VTO work we set the alerter to 12000. At that time I noticed that it didn't take and started to work on fixing the problem but I didn't realize that the altitude didn't not capture at 16000 and the system stayed descending. At about the same time we both noticed that we were low at about 14800 I believe prior to Jackson by approximately by 1 to 3 miles. We went up to 15000 and talked to the controller to get an amended clearance of 15000 and to continue on the arrival which he gave because I imagine there was no loss of separation. From that point on we continued the arrival as normal without issue.There were quite a few causes to this event. The main one is the expectation for the automation to work as it usually does. I believe we were so comfortable with the button pushes we jumped the gun on setting the alerter to 12000. We should have made sure together that the VNAV was engaged prior to resetting the alerter. Also the moment the VNAV didn't engage my first reaction should have been to fly the plane and make sure we were at the appropriate altitude before working the FMS issue. Though in the moment I believe I was very biased that since it is 2 or 3 button pushes that I could fix the issue in a short time period. So my head was down into the box instead of seeing what the plane was doing. Also at that very moment I believe there were some radio calls that took the captains attention away which reduced a second set of eyes on what was going on so we both became task saturated. Since I didn't see we passed through 16000 at this point the VNAV gave me a message of can't do a climb which for a few seconds confused me because I didn't realize we passed through 16000 which is when I noticed what was going on at about the same time as the captain. Eventually I had to stop the descent manually and return to a climb. In the future when there is such a short time period I will just let the alerter capture the altitude before making FMS changes to avoid this issue and just as importantly not set the alerter to a lower altitude till I confirm I set the VNAV properly.

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.