An air carrier B737-200 flight crew starts evasive action based on an 'RA' from their TCASII. The flight crew notes that the TCASII shows the intruder above them, but during their escape maneuver they see a learjet descending below them. Apparently the learjet had a faulty mode C feature on its transponder.

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Attributes
ACN	351328
Time
Date	199610
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : elp
State Reference	TX
Altitude	msl bound lower : 28000 msl bound upper : 29200
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air taxi
Make Model Name	Learjet 25
Operating Under FAR Part	Part 135
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 100 flight time total : 17000 flight time type : 5000
ASRS Report	351328
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot conflict : airborne less severe conflict : ground less severe
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	horizontal : 750 vertical : 1700
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Received a TA and shortly after RA 'climb, climb.' first officer flying started a climb however I noticed other aircraft climbing and 300 ft above us according to our TCASII. I rolled aircraft hard right and then noticed a lear starting to descend. It passed 1700 ft below. We were about 29200 ft. Controller shortly later descended us to FL280 and told us lear had overshot its altitude. Supplemental information from acn 351873: climbing through FL240, ZAB asked us what altitude we were cleared to climb to, they were showing us out of FL290, we stated we were cleared to FL270 and climbing through FL250. At that time we switched from the secondary to primary transponder. An air carrier B737 picked up the bad mode C altitude on their TCASII and took evasive action. Upon arrival at elp maintenance found the A-4 plug loose which was responsible for the inaccurate mode C indications. The report is enclosed. Extracted pin for A-4 and verified the pin very loose. Replaced pin and system checked good.

Original NASA ASRS Text

Title: AN ACR B737-200 FLC STARTS EVASIVE ACTION BASED ON AN 'RA' FROM THEIR TCASII. THE FLC NOTES THAT THE TCASII SHOWS THE INTRUDER ABOVE THEM, BUT DURING THEIR ESCAPE MANEUVER THEY SEE A LEARJET DSNDING BELOW THEM. APPARENTLY THE LEARJET HAD A FAULTY MODE C FEATURE ON ITS XPONDER.

Narrative: RECEIVED A TA AND SHORTLY AFTER RA 'CLB, CLB.' FO FLYING STARTED A CLB HOWEVER I NOTICED OTHER ACFT CLBING AND 300 FT ABOVE US ACCORDING TO OUR TCASII. I ROLLED ACFT HARD R AND THEN NOTICED A LEAR STARTING TO DSND. IT PASSED 1700 FT BELOW. WE WERE ABOUT 29200 FT. CTLR SHORTLY LATER DSNDED US TO FL280 AND TOLD US LEAR HAD OVERSHOT ITS ALT. SUPPLEMENTAL INFO FROM ACN 351873: CLBING THROUGH FL240, ZAB ASKED US WHAT ALT WE WERE CLRED TO CLB TO, THEY WERE SHOWING US OUT OF FL290, WE STATED WE WERE CLRED TO FL270 AND CLBING THROUGH FL250. AT THAT TIME WE SWITCHED FROM THE SECONDARY TO PRIMARY XPONDER. AN ACR B737 PICKED UP THE BAD MODE C ALT ON THEIR TCASII AND TOOK EVASIVE ACTION. UPON ARR AT ELP MAINT FOUND THE A-4 PLUG LOOSE WHICH WAS RESPONSIBLE FOR THE INACCURATE MODE C INDICATIONS. THE RPT IS ENCLOSED. EXTRACTED PIN FOR A-4 AND VERIFIED THE PIN VERY LOOSE. REPLACED PIN AND SYS CHKED GOOD.

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.