Air carrier flight crew descends too early on back course localizer approach.

Attributes
ACN	136799
Time
Date	199002
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : msy
State Reference	LA
Altitude	msl bound lower : 700 msl bound upper : 2000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : msy
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach descent other
Route In Use	arrival other enroute : on vectors
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 : 200
ASRS Report	136799
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 180 flight time total : 3500 flight time type : 300
ASRS Report	136867
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : ground less severe non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were vectored onto the localizer/back course and cleared to descend to 2000' from 3000' until ampar and cleared for the approach. Ampar dropped out of the CDU and gave the impression that we had crossed ampar. We descended to the MDA of 700! When we noticed we were down, too soon, we were at ampar intersection.

Original NASA ASRS Text

Title: ACR FLT CREW DESCENDS TOO EARLY ON BACK COURSE LOC APCH.

Narrative: WE WERE VECTORED ONTO THE LOC/BACK COURSE AND CLRED TO DSND TO 2000' FROM 3000' UNTIL AMPAR AND CLRED FOR THE APCH. AMPAR DROPPED OUT OF THE CDU AND GAVE THE IMPRESSION THAT WE HAD CROSSED AMPAR. WE DSNDED TO THE MDA OF 700! WHEN WE NOTICED WE WERE DOWN, TOO SOON, WE WERE AT AMPAR INTXN.

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.