Narrative:

We were on the ILS into bzn; mt. After crossing manni (FAF) we switched to bzn tower and were cleared to land. The aircraft was flying on autopilot and the GS and course were centered on the CDI. At about 6500 feet MSL on GS (1500 feet AGL) we received an erroneous GS indication and the GS went full deflection up instantly. We were in the process of making final configuration checks for landing and didn't notice it right away. Since the autopilot was still on; it reacted by chasing the GS and pitching up rapidly. The aircraft approached critically low airspeeds at this high angle of attack; engaging the 'stick shaker' (pre-stall warning). This all happened so fast we were both still in awe wondering what was going on. The captain immediately attempted to force nose-down attitude to come out of our pre-stall condition. The autopilot had commanded in so much nose-up trim the force was hard to overcome. The captain initiated missed approach procedures and I followed his lead by commencing procedures to get the gear and flaps up. Our stall recovery procedures tell us to climb and continue climbing until out of a stall condition. The aircraft was not stalled; but on the side of safety; we treated the situation as such. In this high pitch attitude; with a light load and full power; this aircraft will climb out at 5000 to 6000 fpm. With everything that was going on; we didn't realize the missed approach altitude on the published chart of 8000 feet MSL was not set in the aircraft's altitude alerter. This all occurred at about 6500 feet MSL; so understandably we blew right through 8000 feet MSL. Once in contact with ZLC; we advised of our missed approach and were assigned 9000 feet MSL. By this time we arrested our climb at 10800 feet MSL and began descending to 9000 feet MSL. I believe the altitude was deviated from on our missed approach for 2 reasons: 1) we were in the process of cleaning up the aircraft and getting it in a safe configuration for a missed approach. 2) our procedures don't allow us to set in the missed approach altitude into the altitude alerter until within 1000 feet of field elevation. At the time of this anomaly we were at 1500 feet AGL. Hence; the proper missed approach altitude was not set in the alerter to remind us to level at 8000 feet MSL. The proper corrective actions were taken in order to keep the flight safe. We did not intentionally deviate from the missed approach altitude on the published chart; and as soon as we realized the deviation; we took evasive action to correct to the assigned altitude.

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Original NASA ASRS Text

Title: Shortly after passing MANNI on the ILS Rwy 12 at BZN; air carrier flight crew fails to level at MAP altitude when a go-around is initiated due to an autopilot pitch-up in response to an anomalous full up glide slope indication.

Narrative: We were on the ILS into BZN; MT. After crossing MANNI (FAF) we switched to BZN Tower and were cleared to land. The aircraft was flying on autopilot and the GS and course were centered on the CDI. At about 6500 feet MSL on GS (1500 feet AGL) we received an erroneous GS indication and the GS went full deflection up instantly. We were in the process of making final configuration checks for landing and didn't notice it right away. Since the autopilot was still on; it reacted by chasing the GS and pitching up rapidly. The aircraft approached critically low airspeeds at this high angle of attack; engaging the 'stick shaker' (pre-stall warning). This all happened so fast we were both still in awe wondering what was going on. The Captain immediately attempted to force nose-down attitude to come out of our pre-stall condition. The autopilot had commanded in so much nose-up trim the force was hard to overcome. The Captain initiated missed approach procedures and I followed his lead by commencing procedures to get the gear and flaps up. Our stall recovery procedures tell us to climb and continue climbing until out of a stall condition. The aircraft was NOT stalled; but on the side of safety; we treated the situation as such. In this high pitch attitude; with a light load and full power; this aircraft will climb out at 5000 to 6000 fpm. With everything that was going on; we didn't realize the missed approach altitude on the published chart of 8000 feet MSL was not set in the aircraft's altitude alerter. This all occurred at about 6500 feet MSL; so understandably we blew right through 8000 feet MSL. Once in contact with ZLC; we advised of our missed approach and were assigned 9000 feet MSL. By this time we arrested our climb at 10800 feet MSL and began descending to 9000 feet MSL. I believe the altitude was deviated from on our missed approach for 2 reasons: 1) We were in the process of cleaning up the aircraft and getting it in a safe configuration for a missed approach. 2) Our procedures don't allow us to set in the missed approach altitude into the altitude alerter until within 1000 feet of field elevation. At the time of this anomaly we were at 1500 feet AGL. Hence; the proper missed approach altitude was not set in the alerter to remind us to level at 8000 feet MSL. The proper corrective actions were taken in order to keep the flight safe. We did not intentionally deviate from the missed approach altitude on the published chart; and as soon as we realized the deviation; we took evasive action to correct to the assigned altitude.

Data retrieved from NASA's ASRS site as of April 2012 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.