A B757-200 flight crew experienced an autopilot pitch over in descent that resulted in an altitude d...

Attributes
ACN	816965
Time
Date	200812
Local Time Of Day	0001 To 0600
Place
Locale Reference	navaid : zzz.vor
State Reference	US
Altitude	msl bound lower : 23500 msl bound upper : 24000
Environment
Flight Conditions	Marginal
Weather Elements	Turbulence
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Route In Use	arrival star : n/s
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 127 flight time total : 18800 flight time type : 4774
ASRS Report	816965
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time total : 5000 flight time type : 800
ASRS Report	816960
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude flight crew : overcame equipment problem
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

We were cleared to descend to FL240 maintaining 300 KTS or greater; from I believe FL290; with the center autopilot in command and autothrottles on; we were in a flight level change descent with 320 KTS selected in the airspeed window. Out of about FL250; the aircraft nosed down; exceeded 320 KTS to about 335 KTS and started a steep descent rate. It was evident it was going to blow through leveloff. By the time I got the autopilot off and nosed the plane up; we were leveling at FL235. It was a strange anomaly neither of us had experienced on the B757. My first officer speculated it almost seemed like it was trying to correct a backside of the power curve scenario. We returned to the assigned altitude and noted no conflicts existed on our TCAS.

Original NASA ASRS Text

Title: A B757-200 flight crew experienced an autopilot pitch over in descent that resulted in an altitude deviation.

Narrative: We were cleared to descend to FL240 maintaining 300 KTS or greater; from I believe FL290; with the center autopilot in command and autothrottles on; we were in a Flight Level Change descent with 320 KTS selected in the airspeed window. Out of about FL250; the aircraft nosed down; exceeded 320 KTS to about 335 KTS and started a steep descent rate. It was evident it was going to blow through leveloff. By the time I got the autopilot off and nosed the plane up; we were leveling at FL235. It was a strange anomaly neither of us had experienced on the B757. My First Officer speculated it almost seemed like it was trying to correct a backside of the power curve scenario. We returned to the assigned altitude and noted no conflicts existed on our TCAS.

Data retrieved from NASA's ASRS site as of May 2009 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.