Air carrier X TCASII TA RA made unauthorized descent from assigned altitude. Pilot deviation.

Attributes
ACN	211549
Time
Date	199205
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : okk
State Reference	IN
Altitude	msl bound lower : 19500 msl bound upper : 21000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	other
Make Model Name	Fighter
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 7000 flight time type : 350
ASRS Report	211549
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 125 flight time total : 15000 flight time type : 400
ASRS Report	211548
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	vertical : 1500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Level at FL200, received ATC traffic call of 4 fgts at 1 O'clock FL210. We all were entering area of WX and IMC conditions. TCASII TA warning was received followed shortly by TCASII RA to descend (2500-4000 FPM). We started our descent, notified center, leveled at FL195 when clear of conflict. Started return to FL200 and notified center of same and that we deviated from altitude due to RA. They (ATC) indicated that they would file a report on RA. Chicago center (ATC) made no additional comments to us concerning the situation. We do not know if there was or would have been a loss of separation if we remained at altitude. Due to the fact that it was a formation of fgts, we decided that it would be best to follow the RA. Since we were in IMC conditions we never saw the traffic visually.

Original NASA ASRS Text

Title: ACR X TCASII TA RA MADE UNAUTHORIZED DSCNT FROM ASSIGNED ALT. PLTDEV.

Narrative: LEVEL AT FL200, RECEIVED ATC TFC CALL OF 4 FGTS AT 1 O'CLOCK FL210. WE ALL WERE ENTERING AREA OF WX AND IMC CONDITIONS. TCASII TA WARNING WAS RECEIVED FOLLOWED SHORTLY BY TCASII RA TO DSND (2500-4000 FPM). WE STARTED OUR DSCNT, NOTIFIED CTR, LEVELED AT FL195 WHEN CLR OF CONFLICT. STARTED RETURN TO FL200 AND NOTIFIED CTR OF SAME AND THAT WE DEVIATED FROM ALT DUE TO RA. THEY (ATC) INDICATED THAT THEY WOULD FILE A RPT ON RA. CHICAGO CTR (ATC) MADE NO ADDITIONAL COMMENTS TO US CONCERNING THE SITUATION. WE DO NOT KNOW IF THERE WAS OR WOULD HAVE BEEN A LOSS OF SEPARATION IF WE REMAINED AT ALT. DUE TO THE FACT THAT IT WAS A FORMATION OF FGTS, WE DECIDED THAT IT WOULD BE BEST TO FOLLOW THE RA. SINCE WE WERE IN IMC CONDITIONS WE NEVER SAW THE TFC 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.