A CL600 air carrier crew descends 900 ft below their assigned cruise altitude when their TCASII senses a rapidly descending aircraft leveling 1000 ft above 35 mi southeast of flm, ky.

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Attributes
ACN	496120
Time
Date	200012
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : flm.vor
State Reference	KY
Altitude	msl bound lower : 23100 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid.artcc
Operator	common carrier : air carrier
Make Model Name	Challenger CL600
Operating Under FAR Part	Part 135
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	arrival star : ns
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zid.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Experience	flight time last 90 days : 230 flight time total : 3000 flight time type : 1100
ASRS Report	496120
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	non adherence : published procedure non adherence : clearance other anomaly other
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 1500 vertical : 1000
Supplementary
Problem Areas	Environmental Factor Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

Cruising at FL240, we received an RA for traffic to descend. The captain complied and I reported it to ATC. After we were clear of traffic, we climbed back to FL240. Total altitude deviation was approximately 900 ft. The other aircraft leveled at FL250 as cleared. Suspect excessive descent rate of other aircraft was the problem.

Original NASA ASRS Text

Title: A CL600 ACR CREW DSNDS 900 FT BELOW THEIR ASSIGNED CRUISE ALT WHEN THEIR TCASII SENSES A RAPIDLY DSNDING ACFT LEVELING 1000 FT ABOVE 35 MI SE OF FLM, KY.

Narrative: CRUISING AT FL240, WE RECEIVED AN RA FOR TFC TO DSND. THE CAPT COMPLIED AND I RPTED IT TO ATC. AFTER WE WERE CLR OF TFC, WE CLBED BACK TO FL240. TOTAL ALTDEV WAS APPROX 900 FT. THE OTHER ACFT LEVELED AT FL250 AS CLRED. SUSPECT EXCESSIVE DSCNT RATE OF OTHER ACFT WAS THE PROB.

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.