Air carrier aircraft is descended prematurely on clearance for a visual approach and descends below glide slope. Alert approach controller intervenes and stops his descent to avoid an small aircraft in crossing situation. Loss of standard separation occurs. First officer making approach. Captain (PNF) under 100 hours in type (adjusted for takeoffs and or lndgs).

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Attributes
ACN	131585
Time
Date	198912
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other 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 : 90 flight time total : 6000 flight time type : 55
ASRS Report	131585
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : far non adherence : required legal separation non adherence other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action other
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cleared for visibility approach to 35R at dfw. Previously cleared to descend to 3000' MSL. Runway in sight. Utilizing 35R ILS as backup started final descent. Small aircraft then appeared from right side on w-erly vector at 2000'; MSL. Aircraft crossed in front and below, was in visibility contact to us. Approach control told us to level at 2500' MSL, which we did. Aircraft cleared off left side, then we continued approach. Accepting visibility approach is questionable in any situation. In any event, autoplts in 2-CREW aircraft should be left on at least until established on G/south. Very easy to get distracted outside and let altitude slip.

Original NASA ASRS Text

Title: ACR ACFT IS DESCENDED PREMATURELY ON CLRNC FOR A VISUAL APCH AND DESCENDS BELOW GLIDE SLOPE. ALERT APCH CTLR INTERVENES AND STOPS HIS DESCENT TO AVOID AN SMA IN CROSSING SITUATION. LOSS OF STANDARD SEPARATION OCCURS. FO MAKING APCH. CAPT (PNF) UNDER 100 HOURS IN TYPE (ADJUSTED FOR TKOFS AND OR LNDGS).

Narrative: CLRED FOR VIS APCH TO 35R AT DFW. PREVIOUSLY CLRED TO DSND TO 3000' MSL. RWY IN SIGHT. UTILIZING 35R ILS AS BACKUP STARTED FINAL DSCNT. SMA THEN APPEARED FROM RIGHT SIDE ON W-ERLY VECTOR AT 2000'; MSL. ACFT CROSSED IN FRONT AND BELOW, WAS IN VIS CONTACT TO US. APCH CTL TOLD US TO LEVEL AT 2500' MSL, WHICH WE DID. ACFT CLRED OFF LEFT SIDE, THEN WE CONTINUED APCH. ACCEPTING VIS APCH IS QUESTIONABLE IN ANY SITUATION. IN ANY EVENT, AUTOPLTS IN 2-CREW ACFT SHOULD BE LEFT ON AT LEAST UNTIL ESTABLISHED ON G/S. VERY EASY TO GET DISTRACTED OUTSIDE AND LET ALT SLIP.

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.