Within 1 min ARTCC radar controller issued 3 altitude clrncs to the reporter in an atx small transport twin. One to descend, one to climb back to original altitude, the third clearance to descend again to 12000 ft. The reporter spotted conflicting traffic 500 ft below at 12000 ft. He immediately made a rapid climb to eliminate any further conflict.

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Attributes
ACN	418008
Time
Date	199810
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : hln airport : hln
State Reference	MT
Altitude	msl bound lower : 12500 msl bound upper : 14000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zlc
Operator	common carrier : air taxi
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 135
Flight Phase	cruise other descent other
Route In Use	enroute airway : zlc
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air taxi
Make Model Name	Caravan 1 208A
Operating Under FAR Part	Part 135
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 264 flight time total : 2100
ASRS Report	418008
Person 2
Affiliation	company : air carrier
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance flight crew : took evasive action other
Consequence	Other
Miss Distance	horizontal : 1000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error Pilot Deviation

Narrative:

I was level at 14000 ft on approach to hln. Controller issued descent clearance to 12000 ft. As I descended through 13400 ft, controller amended clearance to climb/maintain 14000 ft. As I climbed through 13700 ft, the controller reissued a descent clearance. I believed the clearance was to 12000 ft. Descending through 12600 ft, the controller called and said simply 'aircraft X, center.' I could tell from the tone of his voice all was not well and increased my visual scan while I responded to his call. The controller's response was blocked, but at 12500 ft I spotted the conflicting traffic at 12000 ft. I initiated a zoom climb to return to 13000 ft within 10 seconds, which eliminated the traffic conflict. Factors: I should have clarified the third clearance. Our aircraft are not equipped with altitude alerters. The controller issued 3 different/conflicting altitudes within 1 min. The controller might have mentioned the conflicting traffic at 12000 ft.

Original NASA ASRS Text

Title: WITHIN 1 MIN ARTCC RADAR CTLR ISSUED 3 ALT CLRNCS TO THE RPTR IN AN ATX SMT TWIN. ONE TO DSND, ONE TO CLB BACK TO ORIGINAL ALT, THE THIRD CLRNC TO DSND AGAIN TO 12000 FT. THE RPTR SPOTTED CONFLICTING TFC 500 FT BELOW AT 12000 FT. HE IMMEDIATELY MADE A RAPID CLB TO ELIMINATE ANY FURTHER CONFLICT.

Narrative: I WAS LEVEL AT 14000 FT ON APCH TO HLN. CTLR ISSUED DSCNT CLRNC TO 12000 FT. AS I DSNDED THROUGH 13400 FT, CTLR AMENDED CLRNC TO CLB/MAINTAIN 14000 FT. AS I CLBED THROUGH 13700 FT, THE CTLR REISSUED A DSCNT CLRNC. I BELIEVED THE CLRNC WAS TO 12000 FT. DSNDING THROUGH 12600 FT, THE CTLR CALLED AND SAID SIMPLY 'ACFT X, CTR.' I COULD TELL FROM THE TONE OF HIS VOICE ALL WAS NOT WELL AND INCREASED MY VISUAL SCAN WHILE I RESPONDED TO HIS CALL. THE CTLR'S RESPONSE WAS BLOCKED, BUT AT 12500 FT I SPOTTED THE CONFLICTING TFC AT 12000 FT. I INITIATED A ZOOM CLB TO RETURN TO 13000 FT WITHIN 10 SECONDS, WHICH ELIMINATED THE TFC CONFLICT. FACTORS: I SHOULD HAVE CLARIFIED THE THIRD CLRNC. OUR ACFT ARE NOT EQUIPPED WITH ALT ALERTERS. THE CTLR ISSUED 3 DIFFERENT/CONFLICTING ALTS WITHIN 1 MIN. THE CTLR MIGHT HAVE MENTIONED THE CONFLICTING TFC AT 12000 FT.

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.