A flight crew descended 300 ft below assigned altitude.

Attributes
ACN	522810
Time
Date	200108
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : hub.vor
State Reference	TX
Altitude	msl bound lower : 5700 msl bound upper : 6000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-200
Flight Phase	descent : approach
Route In Use	arrival star : trinity 1
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Experience	flight time last 90 days : 75 flight time total : 6500 flight time type : 1600
ASRS Report	522810
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : far
Resolutory Action	controller : issued advisory flight crew : returned to assigned altitude
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flying the trinity 1 approach into hou. We had just reached ellin intersection and were turning to an assigned heading of 305 degrees when we were cleared from 7000 ft to 6000 ft. The assigned altitude was acknowledged and the captain initiated the descent. At the same time, the captain went off frequency to have the flight attendants 'secure the cabin.' there was significant WX in the area and I continued to adjust the radar presentation to enhance avoidance capability. Just prior to the captain returning to frequency, I made an adjustment to the radar which is located adjacent to the altitude set knob. Just as the captain came back on frequency, another aircraft was cleared to 3000 ft. I do not recall if it was acknowledged or not by the other aircraft. The captain stated, 'I'm back' and deployed the speed brake. I was curious as to why it was deployed with only 3000 ft to go until leveloff at 6000 ft. As we reached 6000 ft the captain still had a descent going and I reminded him our clearance was to 6000 ft only. He immediately recovered to 6000 ft. I did not observe the captain's altimeter, but mine reflected a leveloff and recovery from 5760 ft. As we were passed to the next frequency the controller stated he had noted a descent below or to (unsure of his exact words) 5700 ft. Nothing else was said by the controller. With the numerous things going on (radio frequency change, captain off frequency, my hands working near the altitude set knob, WX, a clearance to 3000 ft for another aircraft) I feel the captain felt sure he was cleared to 3000 ft. I felt our backing each other up and the captain's quick response to remedy the situation prevented a serious incident.

Original NASA ASRS Text

Title: A FLT CREW DSNDED 300 FT BELOW ASSIGNED ALT.

Narrative: FLYING THE TRINITY 1 APCH INTO HOU. WE HAD JUST REACHED ELLIN INTXN AND WERE TURNING TO AN ASSIGNED HDG OF 305 DEGS WHEN WE WERE CLRED FROM 7000 FT TO 6000 FT. THE ASSIGNED ALT WAS ACKNOWLEDGED AND THE CAPT INITIATED THE DSCNT. AT THE SAME TIME, THE CAPT WENT OFF FREQ TO HAVE THE FLT ATTENDANTS 'SECURE THE CABIN.' THERE WAS SIGNIFICANT WX IN THE AREA AND I CONTINUED TO ADJUST THE RADAR PRESENTATION TO ENHANCE AVOIDANCE CAPABILITY. JUST PRIOR TO THE CAPT RETURNING TO FREQ, I MADE AN ADJUSTMENT TO THE RADAR WHICH IS LOCATED ADJACENT TO THE ALT SET KNOB. JUST AS THE CAPT CAME BACK ON FREQ, ANOTHER ACFT WAS CLRED TO 3000 FT. I DO NOT RECALL IF IT WAS ACKNOWLEDGED OR NOT BY THE OTHER ACFT. THE CAPT STATED, 'I'M BACK' AND DEPLOYED THE SPD BRAKE. I WAS CURIOUS AS TO WHY IT WAS DEPLOYED WITH ONLY 3000 FT TO GO UNTIL LEVELOFF AT 6000 FT. AS WE REACHED 6000 FT THE CAPT STILL HAD A DSCNT GOING AND I REMINDED HIM OUR CLRNC WAS TO 6000 FT ONLY. HE IMMEDIATELY RECOVERED TO 6000 FT. I DID NOT OBSERVE THE CAPT'S ALTIMETER, BUT MINE REFLECTED A LEVELOFF AND RECOVERY FROM 5760 FT. AS WE WERE PASSED TO THE NEXT FREQ THE CTLR STATED HE HAD NOTED A DSCNT BELOW OR TO (UNSURE OF HIS EXACT WORDS) 5700 FT. NOTHING ELSE WAS SAID BY THE CTLR. WITH THE NUMEROUS THINGS GOING ON (RADIO FREQ CHANGE, CAPT OFF FREQ, MY HANDS WORKING NEAR THE ALT SET KNOB, WX, A CLRNC TO 3000 FT FOR ANOTHER ACFT) I FEEL THE CAPT FELT SURE HE WAS CLRED TO 3000 FT. I FELT OUR BACKING EACH OTHER UP AND THE CAPT'S QUICK RESPONSE TO REMEDY THE SIT PREVENTED A SERIOUS INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 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.