A B737 flight crew misunderstands approach controller clearance due to nonstandard phraseology.

Attributes
ACN	519720
Time
Date	200107
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : lga.airport
State Reference	NY
Altitude	msl bound lower : 9000 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-200
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 18000 flight time type : 350
ASRS Report	519720
Events
Anomaly	altitude deviation : excursion from assigned altitude altitude deviation : crossing restriction not met non adherence : clearance
Resolutory Action	controller : issued advisory
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance

Narrative:

New york approach gave us a nonstandard descent clearance. We were at 11000 ft when the controller said 'cross 25 mi south of lga at 10000 ft, maintain 4000 ft at your discretion.' so both of us thought this meant descend from 11000 ft to 10000 ft by 25 mi south of lga and continue to 4000 ft at our discretion. At about 35 mi south of lga passing 9000 ft, descending to 4000 ft, new york approach said 'stop at 9000 ft, your clearance was not to descend until 25 DME south of lga.' (no conflict with other aircraft.) I told the new york controller 'that isn't how we understood the clearance.' he said no problem. If new york approach controller had only used proper ATC procedures such as, 'descend to cross 25 mi south of lga at or above 10000 ft, maintain 4000 ft at your discretion,' this situation could have been avoided. I guess in the future I will not accept such a vague clearance.

Original NASA ASRS Text

Title: A B737 FLT CREW MISUNDERSTANDS APCH CTLR CLRNC DUE TO NONSTANDARD PHRASEOLOGY.

Narrative: NEW YORK APCH GAVE US A NONSTANDARD DSCNT CLRNC. WE WERE AT 11000 FT WHEN THE CTLR SAID 'CROSS 25 MI S OF LGA AT 10000 FT, MAINTAIN 4000 FT AT YOUR DISCRETION.' SO BOTH OF US THOUGHT THIS MEANT DSND FROM 11000 FT TO 10000 FT BY 25 MI S OF LGA AND CONTINUE TO 4000 FT AT OUR DISCRETION. AT ABOUT 35 MI S OF LGA PASSING 9000 FT, DSNDING TO 4000 FT, NEW YORK APCH SAID 'STOP AT 9000 FT, YOUR CLRNC WAS NOT TO DSND UNTIL 25 DME S OF LGA.' (NO CONFLICT WITH OTHER ACFT.) I TOLD THE NEW YORK CTLR 'THAT ISN'T HOW WE UNDERSTOOD THE CLRNC.' HE SAID NO PROB. IF NEW YORK APCH CTLR HAD ONLY USED PROPER ATC PROCS SUCH AS, 'DSND TO CROSS 25 MI S OF LGA AT OR ABOVE 10000 FT, MAINTAIN 4000 FT AT YOUR DISCRETION,' THIS SIT COULD HAVE BEEN AVOIDED. I GUESS IN THE FUTURE I WILL NOT ACCEPT SUCH A VAGUE CLRNC.

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.