Narrative:

We were on the ILS runway 12R approach. The path on the ILS runway 12R was in a clear area; with heavy rain showers on both sides of the course (about 3 mi away; left and right sides); but not on the course. We saw the hobby airport around 9-10 mi out. Captain insisted he wanted the visual approach; since we could see the hou runway 12R with ease. I questioned him; 'do you 'really' want a 'visual'?!' (I was not too cool with the idea; with rain showers nearby.) captain insisted; 'yes!' so I called the airport and houston approach cleared us for the visual approximately 2 mi before royce. I then caught that our altitude was 1800 ft. I called; 'altitude!' captain said he 'was on a visual approach.' I said; 'you got to maintain altitude!' while this was being said; he eased down to around 1600 ft!' I said; 'keep your altitude!; climb!' then; ATC called; 'low altitude alert and check your altimeter.' I acknowledged ATC. Then I told captain (we were past royce now) 'maintain your altitude so you can intercept the GS.' we had a stabilized approach around 1400 ft. I was really upset. I talked to the captain afterwards. I told him; you could not descend below 2000 ft on the approach; even on the visual approach; because of the class B airspace. He didn't realize it and immediately called on the phone to hou tower/approach. Tower told him; 'don't worry about it; since you were on the visual; but they are required to warn us.' key thing: captain will never do it again.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B737 ON APPROACH TO IAH DEVIATED FROM CHARTED ALTITUDES.

Narrative: WE WERE ON THE ILS RWY 12R APCH. THE PATH ON THE ILS RWY 12R WAS IN A CLR AREA; WITH HVY RAIN SHOWERS ON BOTH SIDES OF THE COURSE (ABOUT 3 MI AWAY; L AND R SIDES); BUT NOT ON THE COURSE. WE SAW THE HOBBY ARPT AROUND 9-10 MI OUT. CAPT INSISTED HE WANTED THE VISUAL APCH; SINCE WE COULD SEE THE HOU RWY 12R WITH EASE. I QUESTIONED HIM; 'DO YOU 'REALLY' WANT A 'VISUAL'?!' (I WAS NOT TOO COOL WITH THE IDEA; WITH RAIN SHOWERS NEARBY.) CAPT INSISTED; 'YES!' SO I CALLED THE ARPT AND HOUSTON APCH CLRED US FOR THE VISUAL APPROX 2 MI BEFORE ROYCE. I THEN CAUGHT THAT OUR ALT WAS 1800 FT. I CALLED; 'ALT!' CAPT SAID HE 'WAS ON A VISUAL APCH.' I SAID; 'YOU GOT TO MAINTAIN ALT!' WHILE THIS WAS BEING SAID; HE EASED DOWN TO AROUND 1600 FT!' I SAID; 'KEEP YOUR ALT!; CLB!' THEN; ATC CALLED; 'LOW ALT ALERT AND CHK YOUR ALTIMETER.' I ACKNOWLEDGED ATC. THEN I TOLD CAPT (WE WERE PAST ROYCE NOW) 'MAINTAIN YOUR ALT SO YOU CAN INTERCEPT THE GS.' WE HAD A STABILIZED APCH AROUND 1400 FT. I WAS REALLY UPSET. I TALKED TO THE CAPT AFTERWARDS. I TOLD HIM; YOU COULD NOT DSND BELOW 2000 FT ON THE APCH; EVEN ON THE VISUAL APCH; BECAUSE OF THE CLASS B AIRSPACE. HE DIDN'T REALIZE IT AND IMMEDIATELY CALLED ON THE PHONE TO HOU TWR/APCH. TWR TOLD HIM; 'DON'T WORRY ABOUT IT; SINCE YOU WERE ON THE VISUAL; BUT THEY ARE REQUIRED TO WARN US.' KEY THING: CAPT WILL NEVER DO IT AGAIN.

Data retrieved from NASA's ASRS site as of January 2009 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.