Air carrier vectored for approach descends below assigned, has TCASII alert.

Attributes
ACN	212250
Time
Date	199206
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dca
State Reference	DC
Altitude	msl bound lower : 4500 msl bound upper : 5000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dca
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp pilot : cfi
Experience	flight time last 90 days : 200 flight time total : 9000 flight time type : 1500
ASRS Report	212250
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 20000 flight time type : 3000
ASRS Report	212253
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 6000 vertical : 800
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On approach control at dca, proceeding direct dca descending to 5000 ft. Approach gave us a turn to 180 degree about 7 NM from dca for mt vernon visual. Visibility was hazy (2 mi) in broken, multiple layers. While in the turn we received a TCASII TA. As I quickly scanned to visually acquire the aircraft, ATC issued us a TA which concurred with TCASII (1 O'clock and 4000 ft MSL). I had just acquired the aircraft through the cloud layers and looked back to the TCASII to verify that it was TA aircraft on TCASII and noticed the captain was descending through 4800 ft (5000 ft was last assigned). I reacted immediately and called this to the captain's attention and rearmed 5000 ft. We were in throttle-clmp and IAS hold and I think the captain was confused on how to reverse the descent quickly. Captain then selected vertical speed and positive climb rate and I shoved the throttles up manually to expedite the climb. Descent stopped at 4500 ft and quickly returned to 5000 ft. ATC advised us of the traffic and confirmed 5000 ft assigned.

Original NASA ASRS Text

Title: ACR VECTORED FOR APCH DSNDS BELOW ASSIGNED, HAS TCASII ALERT.

Narrative: ON APCH CTL AT DCA, PROCEEDING DIRECT DCA DSNDING TO 5000 FT. APCH GAVE US A TURN TO 180 DEG ABOUT 7 NM FROM DCA FOR MT VERNON VISUAL. VISIBILITY WAS HAZY (2 MI) IN BROKEN, MULTIPLE LAYERS. WHILE IN THE TURN WE RECEIVED A TCASII TA. AS I QUICKLY SCANNED TO VISUALLY ACQUIRE THE ACFT, ATC ISSUED US A TA WHICH CONCURRED WITH TCASII (1 O'CLOCK AND 4000 FT MSL). I HAD JUST ACQUIRED THE ACFT THROUGH THE CLOUD LAYERS AND LOOKED BACK TO THE TCASII TO VERIFY THAT IT WAS TA ACFT ON TCASII AND NOTICED THE CAPT WAS DSNDING THROUGH 4800 FT (5000 FT WAS LAST ASSIGNED). I REACTED IMMEDIATELY AND CALLED THIS TO THE CAPT'S ATTN AND REARMED 5000 FT. WE WERE IN THROTTLE-CLMP AND IAS HOLD AND I THINK THE CAPT WAS CONFUSED ON HOW TO REVERSE THE DSCNT QUICKLY. CAPT THEN SELECTED VERT SPD AND POSITIVE CLB RATE AND I SHOVED THE THROTTLES UP MANUALLY TO EXPEDITE THE CLB. DSCNT STOPPED AT 4500 FT AND QUICKLY RETURNED TO 5000 FT. ATC ADVISED US OF THE TFC AND CONFIRMED 5000 FT ASSIGNED.

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.