A DC9 flight crew descended below the assigned altitude near aml.

Attributes
ACN	455220
Time
Date	199911
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : aml.vor
State Reference	VA
Altitude	msl single value : 12000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 90 flight time total : 10000 flight time type : 2000
ASRS Report	455220
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Event: when descending to 12000 ft, the master caution light illuminated as I was leveling off. Attention was diverted to the overhead panel to identify the malfunction. (A previous malfunction had occurred and corrective action performed.) because of weak lighting, proper identify was difficult. As I idented the malfunction, I also observed that I had deviated 400 ft below the assigned altitude. I immediately corrected to the proper altitude. The controller made no call and TCASII showed no aircraft within 20 mi. The malfunction was written up and the system was disabled and an MEL was documented. Chain of events: the malfunction occurred during a critical phase of flight which diverted both pilot's attention. Timely identify prevented PF from returning to flying duties. Once idented, flying duties corrected deviation. Human performance: pilot tendencies are to identify the malfunction and direct corrective action. When caution system cannot readily identify problem, then both pilot's diversion is almost imminent. Lack of self discipline caused me to fail to level the aircraft at the proper altitude. Had the caution system adequately illuminated the malfunction, this would not have occurred.

Original NASA ASRS Text

Title: A DC9 FLC DSNDED BELOW THE ASSIGNED ALT NEAR AML.

Narrative: EVENT: WHEN DSNDING TO 12000 FT, THE MASTER CAUTION LIGHT ILLUMINATED AS I WAS LEVELING OFF. ATTN WAS DIVERTED TO THE OVERHEAD PANEL TO IDENT THE MALFUNCTION. (A PREVIOUS MALFUNCTION HAD OCCURRED AND CORRECTIVE ACTION PERFORMED.) BECAUSE OF WEAK LIGHTING, PROPER IDENT WAS DIFFICULT. AS I IDENTED THE MALFUNCTION, I ALSO OBSERVED THAT I HAD DEVIATED 400 FT BELOW THE ASSIGNED ALT. I IMMEDIATELY CORRECTED TO THE PROPER ALT. THE CTLR MADE NO CALL AND TCASII SHOWED NO ACFT WITHIN 20 MI. THE MALFUNCTION WAS WRITTEN UP AND THE SYS WAS DISABLED AND AN MEL WAS DOCUMENTED. CHAIN OF EVENTS: THE MALFUNCTION OCCURRED DURING A CRITICAL PHASE OF FLT WHICH DIVERTED BOTH PLT'S ATTN. TIMELY IDENT PREVENTED PF FROM RETURNING TO FLYING DUTIES. ONCE IDENTED, FLYING DUTIES CORRECTED DEV. HUMAN PERFORMANCE: PLT TENDENCIES ARE TO IDENT THE MALFUNCTION AND DIRECT CORRECTIVE ACTION. WHEN CAUTION SYS CANNOT READILY IDENT PROB, THEN BOTH PLT'S DIVERSION IS ALMOST IMMINENT. LACK OF SELF DISCIPLINE CAUSED ME TO FAIL TO LEVEL THE ACFT AT THE PROPER ALT. HAD THE CAUTION SYS ADEQUATELY ILLUMINATED THE MALFUNCTION, THIS WOULD NOT HAVE OCCURRED.

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.