B757 crew was unable to stop descent on GS as requested by ATC.

Attributes
ACN	432450
Time
Date	199903
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lax.airport
State Reference	AB
Altitude	msl single value : 5100
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 25r other
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 14000 flight time type : 150
ASRS Report	432450
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance other anomaly
Independent Detector	other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : took precautionary avoidance action flight crew : returned to intended course
Supplementary
Problem Areas	Flight Crew Human Performance Airspace Structure Airport Aircraft ATC Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were cleared for ILS runway 25R approach at lax. On localizer and GS with autoplt coupled at approximately 200 KTS assigned. At approximately 5100 ft the controller issued 'turn left to 180 degrees and maintain 5000 ft.' by the time the autoplt was uncoupled and a turn started, the altitude reached 4650 ft. We climbed to 5000 ft and shortly afterwards received a turn back to final and a new approach clearance. The reason given by the controller was an overshoot by a plane on approach to the runway 24 complex. No phone contact was made. Uncoupling an autoplt on approach is a complicated task. In fact, once on GS and localizer it cannot be done and remains in automatic flight control. The autoplt must be disconnected, which creates a whole new set of problems. Unexpected clrncs and commands may take 10-20 seconds for a smooth transition in a big passenger aircraft.

Original NASA ASRS Text

Title: B757 CREW WAS UNABLE TO STOP DSCNT ON GS AS REQUESTED BY ATC.

Narrative: WE WERE CLRED FOR ILS RWY 25R APCH AT LAX. ON LOC AND GS WITH AUTOPLT COUPLED AT APPROX 200 KTS ASSIGNED. AT APPROX 5100 FT THE CTLR ISSUED 'TURN L TO 180 DEGS AND MAINTAIN 5000 FT.' BY THE TIME THE AUTOPLT WAS UNCOUPLED AND A TURN STARTED, THE ALT REACHED 4650 FT. WE CLBED TO 5000 FT AND SHORTLY AFTERWARDS RECEIVED A TURN BACK TO FINAL AND A NEW APCH CLRNC. THE REASON GIVEN BY THE CTLR WAS AN OVERSHOOT BY A PLANE ON APCH TO THE RWY 24 COMPLEX. NO PHONE CONTACT WAS MADE. UNCOUPLING AN AUTOPLT ON APCH IS A COMPLICATED TASK. IN FACT, ONCE ON GS AND LOC IT CANNOT BE DONE AND REMAINS IN AUTO FLT CTL. THE AUTOPLT MUST BE DISCONNECTED, WHICH CREATES A WHOLE NEW SET OF PROBS. UNEXPECTED CLRNCS AND COMMANDS MAY TAKE 10-20 SECONDS FOR A SMOOTH TRANSITION IN A BIG PAX ACFT.

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.