TCASII commanded an air carrier 727 to decrease its separation from a climbing dc-9.

Attributes
ACN	267086
Time
Date	199403
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 6400 msl bound upper : 7500
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 3000
ASRS Report	267086
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer pilot : cfi
Experience	flight time last 90 days : 180 flight time total : 6000 flight time type : 3000
ASRS Report	267337
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude altitude deviation : overshoot conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : took evasive action none taken : unable
Consequence	faa : reviewed incident with flight crew Other
Miss Distance	horizontal : 1200 vertical : 500
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

While making a descent to land at ord, our aircraft was assigned a level off at 7000 ft. Approaching approximately 7500 ft, we received a TCASII TA for an aircraft ahead, below us, and climbing to 6000 ft. No traffic had been issued to us by ATC. This aircraft was visually acquired, momentarily, in between cloud layers. At approximately 7100 ft our TCASII reacted with an RA to 'monitor vertical speed,' indicating a 1500 FPM descent. The first officer was flying and hesitated before reacting. The RA continued, now commanding 'descend now.' a descent was initiated. We were reasonably sure the aircraft previously seen was not a factor, but were unable to contact ATC due to radio congestion. We followed the RA commands. WX conditions were intermittent IMC. After beginning our descent, the RA changed again to 'increase vertical speed' to 2500 FPM. The RA stopped at approximately 6500 ft and our aircraft leveled off at 6400 ft. ATC was notified immediately. After the fact, ATC supervisors indicated that separation between our aircraft had been significantly compromised. The other aircraft also received a TCASII alert to climb! I feel the TCASII programming in this situation was in error and contributed to a deviation in ATC separation. I also believe that if traffic had been issued to us, we may not have reacted in the same manner. Supplemental information from acn 267337: TCASII does not know that both aircraft will level off within separation requirements. IMC prevailed at the time and we were not able to establish other aircraft visually.

Original NASA ASRS Text

Title: TCASII COMMANDED AN ACR 727 TO DECREASE ITS SEPARATION FROM A CLBING DC-9.

Narrative: WHILE MAKING A DSCNT TO LAND AT ORD, OUR ACFT WAS ASSIGNED A LEVEL OFF AT 7000 FT. APCHING APPROX 7500 FT, WE RECEIVED A TCASII TA FOR AN ACFT AHEAD, BELOW US, AND CLBING TO 6000 FT. NO TFC HAD BEEN ISSUED TO US BY ATC. THIS ACFT WAS VISUALLY ACQUIRED, MOMENTARILY, IN BTWN CLOUD LAYERS. AT APPROX 7100 FT OUR TCASII REACTED WITH AN RA TO 'MONITOR VERT SPD,' INDICATING A 1500 FPM DSCNT. THE FO WAS FLYING AND HESITATED BEFORE REACTING. THE RA CONTINUED, NOW COMMANDING 'DSND NOW.' A DSCNT WAS INITIATED. WE WERE REASONABLY SURE THE ACFT PREVIOUSLY SEEN WAS NOT A FACTOR, BUT WERE UNABLE TO CONTACT ATC DUE TO RADIO CONGESTION. WE FOLLOWED THE RA COMMANDS. WX CONDITIONS WERE INTERMITTENT IMC. AFTER BEGINNING OUR DSCNT, THE RA CHANGED AGAIN TO 'INCREASE VERT SPD' TO 2500 FPM. THE RA STOPPED AT APPROX 6500 FT AND OUR ACFT LEVELED OFF AT 6400 FT. ATC WAS NOTIFIED IMMEDIATELY. AFTER THE FACT, ATC SUPVRS INDICATED THAT SEPARATION BTWN OUR ACFT HAD BEEN SIGNIFICANTLY COMPROMISED. THE OTHER ACFT ALSO RECEIVED A TCASII ALERT TO CLB! I FEEL THE TCASII PROGRAMMING IN THIS SIT WAS IN ERROR AND CONTRIBUTED TO A DEV IN ATC SEPARATION. I ALSO BELIEVE THAT IF TFC HAD BEEN ISSUED TO US, WE MAY NOT HAVE REACTED IN THE SAME MANNER. SUPPLEMENTAL INFO FROM ACN 267337: TCASII DOES NOT KNOW THAT BOTH ACFT WILL LEVEL OFF WITHIN SEPARATION REQUIREMENTS. IMC PREVAILED AT THE TIME AND WE WERE NOT ABLE TO ESTABLISH OTHER ACFT VISUALLY.

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.