<< >> NASA ASRS Report 454945, November 1999

ARTCC controller issued advisories to an IFR british ATP and a VFR piper PA28 which were separated on their present course and altitude. However, the ATP crew had already descended and turned in response to a TCASII RA. Controller believes that TCASII in this incident caused an unsafe condition in lieu of preventing one.

Attributes
ACN	454945
Time
Date	199911
Day	Tue
Local Time Of Day	1201 To 1800
Place
State Reference	WI
Altitude	msl bound lower : 5700 msl bound upper : 6400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp.artcc
Operator	common carrier : air carrier
Make Model Name	ATP Advanced Turboprop
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zmp.artcc
Operator	general aviation : personal
Make Model Name	PA-28 Cherokee/Archer II/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	VFR
Person 1
Affiliation	government : faa
Function	controller : radar
Experience	controller military : 4 controller non radar : 19 controller radar : 19 controller time certified in position1 : 16
ASRS Report	454945
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance other spatial deviation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	controller : issued advisory
Consequence	Other
Miss Distance	horizontal : 3000 vertical : 400
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Aircraft X was level at 6000 ft sbound. Aircraft Y was level at 6400 ft nwbound receiving VFR TA's. I issued a TA to aircraft X as '12 O'clock, 1 1/2 mi, 6400 ft, a piper cherokee.' aircraft X responded that he was in a descent and a r-hand turn because of a TCASII RA. This situation is a good example of a situation where the 2 aircraft were separated and in a safe environment, and then TCASII made it a potentially unsafe environment by directing a turn and descent. There could have been traffic below that we did not know about because of marginal radar coverage.

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Original NASA ASRS Text

Title: ARTCC CTLR ISSUED ADVISORIES TO AN IFR BRITISH ATP AND A VFR PIPER PA28 WHICH WERE SEPARATED ON THEIR PRESENT COURSE AND ALT. HOWEVER, THE ATP CREW HAD ALREADY DSNDED AND TURNED IN RESPONSE TO A TCASII RA. CTLR BELIEVES THAT TCASII IN THIS INCIDENT CAUSED AN UNSAFE CONDITION IN LIEU OF PREVENTING ONE.

Narrative: ACFT X WAS LEVEL AT 6000 FT SBOUND. ACFT Y WAS LEVEL AT 6400 FT NWBOUND RECEIVING VFR TA'S. I ISSUED A TA TO ACFT X AS '12 O'CLOCK, 1 1/2 MI, 6400 FT, A PIPER CHEROKEE.' ACFT X RESPONDED THAT HE WAS IN A DSCNT AND A R-HAND TURN BECAUSE OF A TCASII RA. THIS SIT IS A GOOD EXAMPLE OF A SIT WHERE THE 2 ACFT WERE SEPARATED AND IN A SAFE ENVIRONMENT, AND THEN TCASII MADE IT A POTENTIALLY UNSAFE ENVIRONMENT BY DIRECTING A TURN AND DSCNT. THERE COULD HAVE BEEN TFC BELOW THAT WE DID NOT KNOW ABOUT BECAUSE OF MARGINAL RADAR COVERAGE.

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.