<< >> NASA ASRS Report 132169, December 1989

One air carrier aircraft experiences an altitude excursion due to two acrs on frequency with same flight number. Wrong flight accepts descent clearance.

Attributes
ACN	132169
Time
Date	198912
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ffu
State Reference	UT
Altitude	msl bound lower : 30300 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zlc
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Route In Use	arrival 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 : 4400
ASRS Report	132169
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 110 flight time total : 8200 flight time type : 1150
ASRS Report	133347
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Deviation Pilot Deviation
Situations
ATC Facility	procedure or policy : unspecified

Narrative:

While at cruise FL310 ZLC advised 'AB2 descend, maintain 15,000'. Since this was about 50 mi sooner than I wanted to descend I replied 'is that pilot's discretion to 15,000 for an air carrier AB2?' controller answered 'AB2 descend now, maintain 15,000' and included an altimeter setting. I replied 'AB2 out of 310 for 15,000.' approximately 30 seconds later, B and AB2 said 'center, did you have a clearance for B air carrier AB2?' controller stated 'yes, I cleared you to 15,000' and B air carrier AB2 said 'well, somebody else accepted that clearance.' I said 'center, a air carrier AB2 accepted and read back that clearance to 15,000.' at this point we had descended to approximately 30,300'. Center controller replied 'a air carrier AB2, maintain FL310.' I climbed back to FL310. No further comments were made. I believe confusion would have been eliminated if the controller had pointed out the similar call signs and had been more careful to include the company prefix with the flight number.

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Original NASA ASRS Text

Title: ONE ACR ACFT EXPERIENCES AN ALT EXCURSION DUE TO TWO ACRS ON FREQ WITH SAME FLT NUMBER. WRONG FLT ACCEPTS DESCENT CLRNC.

Narrative: WHILE AT CRUISE FL310 ZLC ADVISED 'AB2 DSND, MAINTAIN 15,000'. SINCE THIS WAS ABOUT 50 MI SOONER THAN I WANTED TO DSND I REPLIED 'IS THAT PLT'S DISCRETION TO 15,000 FOR AN ACR AB2?' CTLR ANSWERED 'AB2 DSND NOW, MAINTAIN 15,000' AND INCLUDED AN ALTIMETER SETTING. I REPLIED 'AB2 OUT OF 310 FOR 15,000.' APPROX 30 SECONDS LATER, B AND AB2 SAID 'CENTER, DID YOU HAVE A CLRNC FOR B ACR AB2?' CTLR STATED 'YES, I CLRED YOU TO 15,000' AND B ACR AB2 SAID 'WELL, SOMEBODY ELSE ACCEPTED THAT CLRNC.' I SAID 'CENTER, A ACR AB2 ACCEPTED AND READ BACK THAT CLRNC TO 15,000.' AT THIS POINT WE HAD DESCENDED TO APPROX 30,300'. CENTER CTLR REPLIED 'A ACR AB2, MAINTAIN FL310.' I CLIMBED BACK TO FL310. NO FURTHER COMMENTS WERE MADE. I BELIEVE CONFUSION WOULD HAVE BEEN ELIMINATED IF THE CTLR HAD POINTED OUT THE SIMILAR CALL SIGNS AND HAD BEEN MORE CAREFUL TO INCLUDE THE COMPANY PREFIX WITH THE FLT NUMBER.

Data retrieved from NASA's ASRS site as of August 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.