B737 at FL310 being vectored by ZID experienced TCASII RA with MD80.

Attributes
ACN	605709
Time
Date	200401
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : ape.vortac
State Reference	OH
Altitude	msl single value : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid.artcc
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zid.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : flight engineer pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 210 flight time total : 16000 flight time type : 400
ASRS Report	605709
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 15000 vertical : 500
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

We were in cruise at FL310 ZID airspace (frequency/sector 125.07). During radar vectors towards foothills VORTAC (odf) we received a TA co-altitude 4-5 NM at 11 O'clock position. Advisory increased to an RA climb 1500-1700 FPM which we followed. ATC had issued an instruction to descend which we did not follow, and I (first officer) as the PNF informed them we were following the RA climb. Had we followed ATC instructions the chances of a midair collision would have increased. It was during the climb to avoid a collision that we visually acquired this other aircraft, an MD80. Separation was reduced vertically by 300 ft and horizontally by 2.5 NM. I strongly believe that following an RA even if contrary to ATC instructions is the safest course of action.

Original NASA ASRS Text

Title: B737 AT FL310 BEING VECTORED BY ZID EXPERIENCED TCASII RA WITH MD80.

Narrative: WE WERE IN CRUISE AT FL310 ZID AIRSPACE (FREQ/SECTOR 125.07). DURING RADAR VECTORS TOWARDS FOOTHILLS VORTAC (ODF) WE RECEIVED A TA CO-ALT 4-5 NM AT 11 O'CLOCK POS. ADVISORY INCREASED TO AN RA CLB 1500-1700 FPM WHICH WE FOLLOWED. ATC HAD ISSUED AN INSTRUCTION TO DSND WHICH WE DID NOT FOLLOW, AND I (FO) AS THE PNF INFORMED THEM WE WERE FOLLOWING THE RA CLB. HAD WE FOLLOWED ATC INSTRUCTIONS THE CHANCES OF A MIDAIR COLLISION WOULD HAVE INCREASED. IT WAS DURING THE CLB TO AVOID A COLLISION THAT WE VISUALLY ACQUIRED THIS OTHER ACFT, AN MD80. SEPARATION WAS REDUCED VERTLY BY 300 FT AND HORIZLY BY 2.5 NM. I STRONGLY BELIEVE THAT FOLLOWING AN RA EVEN IF CONTRARY TO ATC INSTRUCTIONS IS THE SAFEST COURSE OF ACTION.

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.