Mode C problem causes descent clearance toward rather than away from traffic.

Attributes
ACN	148016
Time
Date	199006
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : msp
State Reference	MN
Altitude	msl bound lower : 21500 msl bound upper : 24500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	climbout : 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 : 200 flight time total : 25000
ASRS Report	148016
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	atc equipment other atc equipment : unspecified other flight crewa
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Climbing outbnd from msp to den cleared to FL350. Climbing through FL245, received call from msp center to descend immediately to FL200. Initiated descent toward FL200, inquired why the immediate descent of more than 4000'. Was informed that mode C readout showed us at (I believe) 21200' and descending through small transport jet altitude at FL210. Informed controller that we had started descent at FL245 and were then at FL21500 descending. Received clearance then to resume climb to FL350. Evidently faulty mode C had given a readout of descending toward climbing small transport jet. Believe a far safer action would have been either an immediate turn or an inquiry of our altitude. The descent clearance actually would have caused our descent through the small transport jet airspace. This is a case of the 'cure' being worse than the 'sickness'. Intermittent malfunctioning mode C was confirmed by later ARTCC sectors.

Original NASA ASRS Text

Title: MODE C PROBLEM CAUSES DESCENT CLRNC TOWARD RATHER THAN AWAY FROM TRAFFIC.

Narrative: CLBING OUTBND FROM MSP TO DEN CLRED TO FL350. CLBING THROUGH FL245, RECEIVED CALL FROM MSP CTR TO DSND IMMEDIATELY TO FL200. INITIATED DSNT TOWARD FL200, INQUIRED WHY THE IMMEDIATE DSNT OF MORE THAN 4000'. WAS INFORMED THAT MODE C READOUT SHOWED US AT (I BELIEVE) 21200' AND DSNDING THROUGH SMT JET ALT AT FL210. INFORMED CTLR THAT WE HAD STARTED DSNT AT FL245 AND WERE THEN AT FL21500 DSNDING. RECEIVED CLRNC THEN TO RESUME CLB TO FL350. EVIDENTLY FAULTY MODE C HAD GIVEN A READOUT OF DSNDING TOWARD CLBING SMT JET. BELIEVE A FAR SAFER ACTION WOULD HAVE BEEN EITHER AN IMMEDIATE TURN OR AN INQUIRY OF OUR ALT. THE DSNT CLRNC ACTUALLY WOULD HAVE CAUSED OUR DSNT THROUGH THE SMT JET AIRSPACE. THIS IS A CASE OF THE 'CURE' BEING WORSE THAN THE 'SICKNESS'. INTERMITTENT MALFUNCTIONING MODE C WAS CONFIRMED BY LATER ARTCC SECTORS.

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.