Large transport has TCASII RA to descend. Responds.

Attributes
ACN	237076
Time
Date	199303
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : atl
State Reference	GA
Altitude	msl bound lower : 25300 msl bound upper : 26000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 13000 flight time type : 2000
ASRS Report	237076
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were level at FL260. We got a TCASII to descend. We descended to FL253 before clear of conflict. I could see an airplane on TCASII, 2000 ft above us and the spacing holding at 2000-2100 ft as we descended. When we talked to ATC they said there was an light transport jet descending to FL270 directly above us. The TCASII forecast a conflict and responded correctly but didn't know the light transport intended to level off at FL270. This is a case where the equipment responded as programmed but the response was incorrect for the situation. In looking back this gives me more faith in the system because it worked and it had no way to know the light transport was going to level off at FL270 and if it hadn't it would have gone right on top of us.

Original NASA ASRS Text

Title: LGT HAS TCASII RA TO DSND. RESPONDS.

Narrative: WE WERE LEVEL AT FL260. WE GOT A TCASII TO DSND. WE DSNDED TO FL253 BEFORE CLR OF CONFLICT. I COULD SEE AN AIRPLANE ON TCASII, 2000 FT ABOVE US AND THE SPACING HOLDING AT 2000-2100 FT AS WE DSNDED. WHEN WE TALKED TO ATC THEY SAID THERE WAS AN LTT JET DSNDING TO FL270 DIRECTLY ABOVE US. THE TCASII FORECAST A CONFLICT AND RESPONDED CORRECTLY BUT DIDN'T KNOW THE LTT INTENDED TO LEVEL OFF AT FL270. THIS IS A CASE WHERE THE EQUIP RESPONDED AS PROGRAMMED BUT THE RESPONSE WAS INCORRECT FOR THE SIT. IN LOOKING BACK THIS GIVES ME MORE FAITH IN THE SYS BECAUSE IT WORKED AND IT HAD NO WAY TO KNOW THE LTT WAS GOING TO LEVEL OFF AT FL270 AND IF IT HADN'T IT WOULD HAVE GONE RIGHT ON TOP OF US.

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.