Atx aircraft descends below assigned altitude by 500'. ATC shows altitude deviation at 1200'.

Attributes
ACN	159536
Time
Date	199010
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : den
State Reference	CO
Altitude	msl bound lower : 23500 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv
Operator	common carrier : air taxi
Make Model Name	Small Transport
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 3300 flight time type : 700
ASRS Report	159536
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 50 flight time total : 13000 flight time type : 1000
ASRS Report	159531
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 24000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The following event happened 10/90, early morning during the following event happened 10/90, erly morning during a a descent into den. I was PIC of an atx X, a jet. At the descent into den. I was PIC of an atx X, a jet. At the time time we were cleared to descend to FL240 from FL310. Upon we were cleared to descend to FL240 from FL310. Upon reaching FL240, the aircraft descended to FL235 on the left altimeter. Reaching FL240, the aircraft descended to FL235 on the left altimeter. Immediate action took the aircraft back to FL240. Immediate action took the aircraft back to FL240. Center control center control requested that we call them on the ground. We later requested that we cal them on the ground. We later learned from the FAA that the computer said te aircraft went 1200' learned from the FAA that the computer said the aircraft went 1200' below assigned altitude. This cannot be correct and feel below assigned altitude. This cannot be correct and feel that that the systems reliability should be questioned. The systems reliability should be questioned.

Original NASA ASRS Text

Title: ATX ACFT DESCENDS BELOW ASSIGNED ALT BY 500'. ATC SHOWS ALT DEVIATION AT 1200'.

Narrative: THE FOLLOWING EVENT HAPPENED 10/90, EARLY MORNING DURING THE FOLLOWING EVENT HAPPENED 10/90, ERLY MORNING DURING A A DSNT INTO DEN. I WAS PIC OF AN ATX X, A JET. AT THE DSNT INTO DEN. I WAS PIC OF AN ATX X, A JET. AT THE TIME TIME WE WERE CLRED TO DSND TO FL240 FROM FL310. UPON WE WERE CLRED TO DSND TO FL240 FROM FL310. UPON REACHING FL240, THE ACFT DSNDED TO FL235 ON THE L ALTIMETER. REACHING FL240, THE ACFT DSNDED TO FL235 ON THE L ALTIMETER. IMMEDIATE ACTION TOOK THE ACFT BACK TO FL240. IMMEDIATE ACTION TOOK THE ACFT BACK TO FL240. CTR CTL CTR CTL REQUESTED THAT WE CALL THEM ON THE GND. WE LATER REQUESTED THAT WE CAL THEM ON THE GND. WE LATER LEARNED FROM THE FAA THAT THE COMPUTER SAID TE ACFT WENT 1200' LEARNED FROM THE FAA THAT THE COMPUTER SAID THE ACFT WENT 1200' BELOW ASSIGNED ALT. THIS CANNOT BE CORRECT AND FEEL BELOW ASSIGNED ALT. THIS CANNOT BE CORRECT AND FEEL THAT THAT THE SYSTEMS RELIABILITY SHOULD BE QUESTIONED. THE SYSTEMS RELIABILITY SHOULD BE QUESTIONED.

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.