Corp aircraft has erroneous encoding reading, causes TCASII RA.

Attributes
ACN	229051
Time
Date	199212
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : jxn
State Reference	MI
Altitude	msl bound lower : 19000 msl bound upper : 19700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	229051
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 4800 flight time type : 350
ASRS Report	229034
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : required legal separation other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 0 vertical : 700
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Large transport at FL200 wbound. Small transport nebound at FL190 approximately 10 mi apart, head-on. Small transport mode C jumps from FL190 to FL205. Aircraft was questioned about altitude and confirmed level at FL190. TCASII equipment on large transport issued a conflict resolution descent and the pilot advised he was descending. Pilot was advised that small transport was confirmed level at FL190. Aircraft passed directly over one another. Large transport pilot later advised that he had arrested his descent at FL197 because of the earlier issuance of traffic at FL190 by ATC. If pilot had followed TCASII resolution fully, aircraft would have passed with 500 ft or less vertical separation -- decreased from the 1000 ft already established by ATC! Everyone did what they were supposed to do, and 2 aircraft were brought into dangerous proximity by a system intended to avoid collisions! Supplemental information from acn 229034: I was level at 19000 ft, there was an large transport called as traffic at 1 O'clock and 20000 ft. Evidently, my mode C went to 20500 ft, that triggered the large transport aircraft to get a TCASII warning to descend. When the large transport descended, that broke the 1000 ft separation. Both pilots had each other in sight. The altimeter that was in my airplane at that time was a loaner model, mine was out being repaired. I made some telephone calls and got a third altimeter shipped to replace this one, even though this was the third flight with this altimeter and no problems were experienced before.

Original NASA ASRS Text

Title: CORP ACFT HAS ERRONEOUS ENCODING READING, CAUSES TCASII RA.

Narrative: LGT AT FL200 WBOUND. SMT NEBOUND AT FL190 APPROX 10 MI APART, HEAD-ON. SMT MODE C JUMPS FROM FL190 TO FL205. ACFT WAS QUESTIONED ABOUT ALT AND CONFIRMED LEVEL AT FL190. TCASII EQUIP ON LGT ISSUED A CONFLICT RESOLUTION DSCNT AND THE PLT ADVISED HE WAS DSNDING. PLT WAS ADVISED THAT SMT WAS CONFIRMED LEVEL AT FL190. ACFT PASSED DIRECTLY OVER ONE ANOTHER. LGT PLT LATER ADVISED THAT HE HAD ARRESTED HIS DSCNT AT FL197 BECAUSE OF THE EARLIER ISSUANCE OF TFC AT FL190 BY ATC. IF PLT HAD FOLLOWED TCASII RESOLUTION FULLY, ACFT WOULD HAVE PASSED WITH 500 FT OR LESS VERT SEPARATION -- DECREASED FROM THE 1000 FT ALREADY ESTABLISHED BY ATC! EVERYONE DID WHAT THEY WERE SUPPOSED TO DO, AND 2 ACFT WERE BROUGHT INTO DANGEROUS PROX BY A SYS INTENDED TO AVOID COLLISIONS! SUPPLEMENTAL INFO FROM ACN 229034: I WAS LEVEL AT 19000 FT, THERE WAS AN LGT CALLED AS TFC AT 1 O'CLOCK AND 20000 FT. EVIDENTLY, MY MODE C WENT TO 20500 FT, THAT TRIGGERED THE LGT ACFT TO GET A TCASII WARNING TO DSND. WHEN THE LGT DSNDED, THAT BROKE THE 1000 FT SEPARATION. BOTH PLTS HAD EACH OTHER IN SIGHT. THE ALTIMETER THAT WAS IN MY AIRPLANE AT THAT TIME WAS A LOANER MODEL, MINE WAS OUT BEING REPAIRED. I MADE SOME TELEPHONE CALLS AND GOT A THIRD ALTIMETER SHIPPED TO REPLACE THIS ONE, EVEN THOUGH THIS WAS THE THIRD FLT WITH THIS ALTIMETER AND NO PROBS WERE EXPERIENCED BEFORE.

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.