MD80 ZAU undefined TCASII maneuver.

Attributes
ACN	505378
Time
Date	200103
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : rod.vortac
State Reference	OH
Altitude	msl bound lower : 26000 msl bound upper : 27000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.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
Route In Use	enroute other
Aircraft 2
Controlling Facilities	artcc : zau.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Route In Use	enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 10000
ASRS Report	505378
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 critical
Independent Detector	aircraft equipment : tcas other flight crewa
Resolutory Action	flight crew : returned to assigned altitude none taken : detected after the fact
Consequence	faa : investigated
Miss Distance	horizontal : 22700 vertical : 700
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance

Narrative:

Descended from FL270 to FL260 as required by TCASII alert. Climbed back to FL270 when issued 'clear of conflict.' advised ATC.

Original NASA ASRS Text

Title: MD80 ZAU UNDEFINED TCASII MANEUVER.

Narrative: DSNDED FROM FL270 TO FL260 AS REQUIRED BY TCASII ALERT. CLBED BACK TO FL270 WHEN ISSUED 'CLR OF CONFLICT.' ADVISED ATC.

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.