An air carrier widebody transport flight crew misinterpreted its clearance to descend.

Attributes
ACN	252171
Time
Date	199309
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : omn
State Reference	FL
Altitude	msl bound lower : 31000 msl bound upper : 38000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx tower : gso
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	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
ASRS Report	252171
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 80 flight time total : 15000 flight time type : 80
ASRS Report	252157
Events
Anomaly	altitude deviation : crossing restriction not met altitude deviation : undershoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I understood crossing restriction and read back as: 9 north of omn at 310. Controller soon questioned if we had started descent and stated restriction as 90 north at 310. Copilot had left flight deck to go to lavatory and had returned. I briefed him as 9 north at 310. ATC mentioned no conflict as if no problem had developed. Supplemental information from acn 252157: captain was programming a descent in FMC. He stated to me a crossing altitude restriction of 9N of omn at 310. Proper crew brief after pilot return to flight deck. Apparently, ATC missed 9 mi north readback and error went unnoticed until mentioned.

Original NASA ASRS Text

Title: AN ACR WDB FLC MISINTERPRETED ITS CLRNC TO DSND.

Narrative: I UNDERSTOOD XING RESTRICTION AND READ BACK AS: 9 N OF OMN AT 310. CTLR SOON QUESTIONED IF WE HAD STARTED DSCNT AND STATED RESTRICTION AS 90 N AT 310. COPLT HAD LEFT FLT DECK TO GO TO LAVATORY AND HAD RETURNED. I BRIEFED HIM AS 9 N AT 310. ATC MENTIONED NO CONFLICT AS IF NO PROB HAD DEVELOPED. SUPPLEMENTAL INFO FROM ACN 252157: CAPT WAS PROGRAMMING A DSCNT IN FMC. HE STATED TO ME A XING ALT RESTRICTION OF 9N OF OMN AT 310. PROPER CREW BRIEF AFTER PLT RETURN TO FLT DECK. APPARENTLY, ATC MISSED 9 MI N READBACK AND ERROR WENT UNNOTICED UNTIL MENTIONED.

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.