The flight crew complied with a TCASII RA to descend and the intruding aircraft was below sending a ...

Attributes
ACN	336102
Time
Date	199605
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : bae airport : mdw
State Reference	WI
Altitude	msl bound lower : 5000 msl bound upper : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Brasilia EMB-120 All Series
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 100 flight time total : 7000
ASRS Report	336102
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 500
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We had just taken off from mdw and were level at 6000 ft on a 350 degree heading. The TCASII gave a TA. Looking at the TCASII vsi display, I saw a target at 1 O'clock and +500 ft, but when I looked outside I saw traffic (an E120) at 1 O'clock and about 500 ft below our altitude. We then got an RA to descend. The captain started down, and I pointed out the traffic below us. I kept scanning for traffic above us while I informed ord departure of our descent. The captain kept the lower traffic in sight during the descent. After we passed the lower traffic the TCASII announced 'clear of conflict.' the ord departure controller said, 'something is not right here -- I am going to find out what happened.' we returned to our original heading and altitude. The controller called us back and said the other aircraft (the E120) had an erroneous altitude readout from its transponder and the controller was able to confirm the other aircraft's transponder problem. This is a situation that due to a malfunction in 1 aircraft the TCASII could have caused a midair collision. I don't know how close we would have come to the other aircraft had we been in IMC. But I do know the captain did not descend the aircraft at the rate indicated on the TCASII vsi display (he descended less), in order to maintain a safe distance from the E120. Because of potential problems like this, a system inside the aircraft may be necessary to allow the pilots to check or monitor the altitude readout of the transponder.

Original NASA ASRS Text

Title: THE FLC COMPLIED WITH A TCASII RA TO DSND AND THE INTRUDING ACFT WAS BELOW SENDING A FALSE ENCODING ALTIMETER SIGNAL THAT HE WAS ABOVE.

Narrative: WE HAD JUST TAKEN OFF FROM MDW AND WERE LEVEL AT 6000 FT ON A 350 DEG HDG. THE TCASII GAVE A TA. LOOKING AT THE TCASII VSI DISPLAY, I SAW A TARGET AT 1 O'CLOCK AND +500 FT, BUT WHEN I LOOKED OUTSIDE I SAW TFC (AN E120) AT 1 O'CLOCK AND ABOUT 500 FT BELOW OUR ALT. WE THEN GOT AN RA TO DSND. THE CAPT STARTED DOWN, AND I POINTED OUT THE TFC BELOW US. I KEPT SCANNING FOR TFC ABOVE US WHILE I INFORMED ORD DEP OF OUR DSCNT. THE CAPT KEPT THE LOWER TFC IN SIGHT DURING THE DSCNT. AFTER WE PASSED THE LOWER TFC THE TCASII ANNOUNCED 'CLR OF CONFLICT.' THE ORD DEP CTLR SAID, 'SOMETHING IS NOT RIGHT HERE -- I AM GOING TO FIND OUT WHAT HAPPENED.' WE RETURNED TO OUR ORIGINAL HDG AND ALT. THE CTLR CALLED US BACK AND SAID THE OTHER ACFT (THE E120) HAD AN ERRONEOUS ALT READOUT FROM ITS XPONDER AND THE CTLR WAS ABLE TO CONFIRM THE OTHER ACFT'S XPONDER PROB. THIS IS A SIT THAT DUE TO A MALFUNCTION IN 1 ACFT THE TCASII COULD HAVE CAUSED A MIDAIR COLLISION. I DON'T KNOW HOW CLOSE WE WOULD HAVE COME TO THE OTHER ACFT HAD WE BEEN IN IMC. BUT I DO KNOW THE CAPT DID NOT DSND THE ACFT AT THE RATE INDICATED ON THE TCASII VSI DISPLAY (HE DSNDED LESS), IN ORDER TO MAINTAIN A SAFE DISTANCE FROM THE E120. BECAUSE OF POTENTIAL PROBS LIKE THIS, A SYS INSIDE THE ACFT MAY BE NECESSARY TO ALLOW THE PLTS TO CHK OR MONITOR THE ALT READOUT OF THE XPONDER.

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.

The flight crew complied with a TCASII RA to descend and the intruding aircraft was below sending a false encoding altimeter signal that he was above.