Narrative:

Lga was using the localizer 31 approach the winds were strong and gusty. We were vectored onto the approach. I elected to fly the localizer with a profile descent. We were cleared down to 2;000 ft MSL. Once we were level; I selected 'profile.' we were given a heading to intercept the localizer; cleared for the approach; and instructed to maintain 1;600 ft until established. We captured the localizer and once we were established on magenta line and inside IAF as per the aom; I selected minimums of 600 ft. The aircraft immediately did an 'idle clamp' descent even though we were approximately 4 miles from FAF. The aircraft went through the FAF altitude and continued to 600 ft before FAF. The pilot not flying asked what it was doing and by the time I registered what was happening we had gone to approximately 1;100-1;200 ft. Pilot not flying said to disconnect autoflight and climb and I did so immediately. ATC called a 'low altitude alert' and gave us the local altimeter. We climbed to altitude and re-selected autoflight. The aircraft repeated its last action; so I disconnected again and stayed manual for remainder of flight. We had clear skies and unlimited visibility and the runway in sight long before cleared for approach. We elected to continue at level altitude and manually descend on approach on the correct profile. We landed on speed and in TDZ with minimal braking.

Related reports:

Google
 

Original NASA ASRS Text

Title: Following two flawed attempts to engage autoflight descent modes resulting in departures from their cleared altitude; the B717 flight crew disconnected autoflight and continued the approach successfully manually.

Narrative: LGA was using the LOC 31 approach The winds were strong and gusty. We were vectored onto the approach. I elected to fly the LOC with a profile descent. We were cleared down to 2;000 FT MSL. Once we were level; I selected 'Profile.' We were given a heading to intercept the LOC; cleared for the approach; and instructed to maintain 1;600 FT until established. We captured the LOC and once we were established on magenta line and inside IAF as per the AOM; I selected minimums of 600 FT. The aircraft immediately did an 'idle clamp' descent even though we were approximately 4 miles from FAF. The aircraft went through the FAF altitude and continued to 600 FT before FAF. The pilot not flying asked what it was doing and by the time I registered what was happening we had gone to approximately 1;100-1;200 FT. Pilot not flying said to disconnect Autoflight and climb and I did so immediately. ATC called a 'Low Altitude Alert' and gave us the local altimeter. We climbed to altitude and re-selected Autoflight. The aircraft repeated its last action; so I disconnected again and stayed manual for remainder of flight. We had clear skies and unlimited visibility and the runway in sight long before cleared for approach. We elected to continue at level altitude and manually descend on approach on the correct profile. We landed on speed and in TDZ with minimal braking.

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.