A CL65 crew has some problems with their FMS making a descending turn over spa, sc. The PIC had to revert to heading mode to turn back to the desired radial. ATC steps in and gives an altitude restr and a turn.

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Attributes
ACN	395870
Time
Date	199803
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : spa
State Reference	SC
Altitude	msl bound lower : 28000 msl bound upper : 31000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	common carrier : air carrier
Make Model Name	Challenger CL604
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 13000 flight time type : 95
ASRS Report	395870
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 19000 flight time type : 75
ASRS Report	396169
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

We were approaching spartanburg VOR from the west at FL310. Ground speed was +500 KTS and wind from northwest at +100 KTS. Navigation was FMS in navigation mode. We were given a crossing restr also that required a rate of descent of +3000 FPM. Turn and descent were started about the same time. Outbound course was 035 degrees (left turn) with northwest winds at +100 KTS. So I expected a large heading change to stay on track. I watched the FMS go through outbound course and turn to what I thought was too much correction. The airplane was in a 30 degree bank to the left. I went to heading mode and set heading to reintercept outbound course (about 85 degrees). By this time ATC said we had a traffic conflict and maintain FL280. We were about 27700 ft. I canceled the autoplt and flew the airplane to FL280 and back on course. I expected a large change in heading to accommodate winds and course change, and had no indication of a malfunction. There also was very little time to check and verify FMS indications. Supplemental information from acn 396189: ATC called and stopped our descent at FL280 and gave us a 085 degree heading to return to course. We were then cleared to traks, on the FMS navigation mode, we went to traks but the FMS didn't make the turn at traks. We went to heading mode and made the correction. The FMS worked properly the rest of the day.

Original NASA ASRS Text

Title: A CL65 CREW HAS SOME PROBS WITH THEIR FMS MAKING A DSNDING TURN OVER SPA, SC. THE PIC HAD TO REVERT TO HEADING MODE TO TURN BACK TO THE DESIRED RADIAL. ATC STEPS IN AND GIVES AN ALT RESTR AND A TURN.

Narrative: WE WERE APCHING SPARTANBURG VOR FROM THE W AT FL310. GND SPD WAS +500 KTS AND WIND FROM NW AT +100 KTS. NAV WAS FMS IN NAV MODE. WE WERE GIVEN A XING RESTR ALSO THAT REQUIRED A RATE OF DSCNT OF +3000 FPM. TURN AND DSCNT WERE STARTED ABOUT THE SAME TIME. OUTBOUND COURSE WAS 035 DEGS (L TURN) WITH NW WINDS AT +100 KTS. SO I EXPECTED A LARGE HEADING CHANGE TO STAY ON TRACK. I WATCHED THE FMS GO THROUGH OUTBOUND COURSE AND TURN TO WHAT I THOUGHT WAS TOO MUCH CORRECTION. THE AIRPLANE WAS IN A 30 DEG BANK TO THE L. I WENT TO HEADING MODE AND SET HEADING TO REINTERCEPT OUTBOUND COURSE (ABOUT 85 DEGS). BY THIS TIME ATC SAID WE HAD A TFC CONFLICT AND MAINTAIN FL280. WE WERE ABOUT 27700 FT. I CANCELED THE AUTOPLT AND FLEW THE AIRPLANE TO FL280 AND BACK ON COURSE. I EXPECTED A LARGE CHANGE IN HEADING TO ACCOMMODATE WINDS AND COURSE CHANGE, AND HAD NO INDICATION OF A MALFUNCTION. THERE ALSO WAS VERY LITTLE TIME TO CHK AND VERIFY FMS INDICATIONS. SUPPLEMENTAL INFO FROM ACN 396189: ATC CALLED AND STOPPED OUR DSCNT AT FL280 AND GAVE US A 085 DEG HDG TO RETURN TO COURSE. WE WERE THEN CLRED TO TRAKS, ON THE FMS NAV MODE, WE WENT TO TRAKS BUT THE FMS DIDN'T MAKE THE TURN AT TRAKS. WE WENT TO HEADING MODE AND MADE THE CORRECTION. THE FMS WORKED PROPERLY THE REST OF THE DAY.

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.