A B757 crew slightly overshot its FL350 cruise altitude. After descending down using a slight thrust lever reduction the aircraft entered a stall buffet. Autothrottles and VNAV may have malfunctioned.

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Attributes
ACN	811714
Time
Date	200811
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 35000
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	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 150 flight time total : 17000 flight time type : 1800
ASRS Report	811714
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time total : 7200 flight time type : 650
ASRS Report	811712
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : company policies other anomaly other anomaly
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude flight crew : regained aircraft control
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance

Narrative:

Climbing to FL350; FMC set to FL330. Reset FMC to FL350 above FL330. Climbed to FL353-354. Descended to FL350 by slightly retarding throttles. Leveled at FL350 and turned on center autoplt. Speed deteriorated to slight airframe buffet. At that point autothrottle disengage EICAS and master caution lights illuminated. ADI showed no autothrottle indication; and VNAV had an amber line through it. Autoplt and autothrottle were disengaged and throttles set to maximum. Airspeed slowly accelerated; but altitude was maintained; slight airframe buffet ended almost immediately. Speed mode was selected with a higher speed. When cruise speed was reattained; VNAV; autothrottle and autoplt were reengaged and appeared to operate normally. Distrs (PA; ATC) caused inattention to flight instruments. Flight manual information is inadequate to allow us to understand these system and why they failed to respond as expected. A maintenance write-up was accomplished; outcome unknown.

Original NASA ASRS Text

Title: A B757 CREW SLIGHTLY OVERSHOT ITS FL350 CRUISE ALTITUDE. AFTER DESCENDING DOWN USING A SLIGHT THRUST LEVER REDUCTION THE AIRCRAFT ENTERED A STALL BUFFET. AUTOTHROTTLES AND VNAV MAY HAVE MALFUNCTIONED.

Narrative: CLBING TO FL350; FMC SET TO FL330. RESET FMC TO FL350 ABOVE FL330. CLBED TO FL353-354. DSNDED TO FL350 BY SLIGHTLY RETARDING THROTTLES. LEVELED AT FL350 AND TURNED ON CTR AUTOPLT. SPD DETERIORATED TO SLIGHT AIRFRAME BUFFET. AT THAT POINT AUTOTHROTTLE DISENGAGE EICAS AND MASTER CAUTION LIGHTS ILLUMINATED. ADI SHOWED NO AUTOTHROTTLE INDICATION; AND VNAV HAD AN AMBER LINE THROUGH IT. AUTOPLT AND AUTOTHROTTLE WERE DISENGAGED AND THROTTLES SET TO MAX. AIRSPD SLOWLY ACCELERATED; BUT ALT WAS MAINTAINED; SLIGHT AIRFRAME BUFFET ENDED ALMOST IMMEDIATELY. SPD MODE WAS SELECTED WITH A HIGHER SPD. WHEN CRUISE SPD WAS REATTAINED; VNAV; AUTOTHROTTLE AND AUTOPLT WERE REENGAGED AND APPEARED TO OPERATE NORMALLY. DISTRS (PA; ATC) CAUSED INATTENTION TO FLT INSTS. FLT MANUAL INFO IS INADEQUATE TO ALLOW US TO UNDERSTAND THESE SYS AND WHY THEY FAILED TO RESPOND AS EXPECTED. A MAINT WRITE-UP WAS ACCOMPLISHED; OUTCOME UNKNOWN.

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.