AN MD80 DROPS BELOW THE TCA CLASS B ALT OF 2000 FT WHEN 9 MI FROM IAH DURING AN ILS APCH TO RWY 8. THE ACFT HAD BEEN CLRED TO 3000 FT AND CLRED FOR THE ILS. THE LOM XING ALT WAS PUBLISHED AS 2000 FT. CREW WAS DISTR BY 'A SLOW MAIN GEAR' EXTENSION. CAPT CITES AN ALT GS CAPTURE FAILURE ON HIS AUTO FLT SYS AS CAUSAL.

Narrative

FLT WAS VECTORED TO FINAL APCH COURSE AND CLRED TO DSND TO 3000 FT AND CLRED FOR ILS RWY 8 APCH. LOC CAPTURED AND FLT GUIDANCE/AUTOPLT WAS PROPERLY ARMED FOR ALT CAPTURE AND LOC AND GS CAPTURE. THE FLT GUIDANCE SYS EITHER FAILED TO CAPTURE THE 3000 FT ALT OR LOST THE CAPTURE. LIKEWISE WITH THE GS CAPTURE. DURING THE TIME OF GS CAPTURE AT 3000 FT THE PLTS WERE DISTR BY A SLOWER GEAR EXTENSION ON 1 LNDG GEAR. DSCNT CONTINUED BELOW THE 2000 FT MINIMUM ALT. 1 ALT ALERT WARNING WAS RECEIVED. HOWEVER; THIS IS NORMAL WHEN THE CORRECT MISSED APCH ALT IS SET AND AFTER GS IS CAPTURED. WE NOTED WE WERE BELOW OUR 2000 FT ALT AT ABOUT 1400 FT MSL AND TWR ALSO CALLED TO GIVE US OUR LOW ALT WARNING. WE CORRECTED; BOTTOMING OUT AT ABOUT 1200-1300 FT MSL. WE CLBED TO REINTERCEPT THE GS FOR REMAINDER OF APCH. LESSONS LEARNED: CLOSER MONITORING OF FLT GUIDANCE STATUS; ESPECIALLY WHEN SOMETHING DIVERTS ATTN AWAY FROM MONITORING. CHANGE MD80 FLT ALT WARNING SYS SO THAT IT IS NOT NORMAL TO RECEIVE AND DISREGARD AN ALT ALERT WARNING DURING FINAL APCH. ALSO; LNDG CHKLIST CALLS FOR CAPT (PF) TO VERIFY GEAR POS. IN THIS CASE; WE WERE BOTH WAITING FOR AND MONITORING THE RED GEAR WARNING LIGHT.