An air carrier medium large transport flight crew climbed above their cleared altitude. The flight crew suspects a malfunction in the autoflt guidance system, but could not verify this. The system operated perfectly for the remainder of the flight.

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Attributes
ACN	375020
Time
Date	199707
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : ewr
State Reference	NJ
Altitude	msl bound lower : 6000 msl bound upper : 7000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	departure other departure sid : sid enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 1300
ASRS Report	375020
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : far non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

ATC clearance 'climb to 6000 ft MSL, left turn 220 degrees to intercept col 350 degree radial.' as PNF, I diverted attention to FMC to set up course intercept for PF. He was flying manually. I had set 6000 ft in altitude window and then began to set up course intercept. For some unknown reason, I had to repeat the setup and while attempting second time, PF queried '6000 ft?' I looked up and confirmed '6000 ft' and noticed we were leveling at 7000 ft MSL. PF began immediate correction to 6000 ft and stated that the command bars indicated climb until he finally queried '6000 ft?' as we approached 7000 ft MSL. We have no explanation why command bars commanded climb. He also stated, autothrottles did not retard. His correction back to 6000 ft was immediate. Upon reaching 6000 ft, departure cleared us to higher (I think 12000 ft) and we began climb. Passing 6400 ft ATC requested our altitude and we responded 6400 ft climbing to 12000 ft. ATC said nothing further, nor did we. Mistakes: 1) PNF diverted attention to FMC too long at one time. 2) PF didn't monitor altimeter as well as command bars. If altitude warning and autothrottles malfunctioned, it never happened again for entire flight (no explanation). We wonder if same would have occurred with autoplt flying.

Original NASA ASRS Text

Title: AN ACR MLG FLC CLBED ABOVE THEIR CLRED ALT. THE FLC SUSPECTS A MALFUNCTION IN THE AUTOFLT GUIDANCE SYS, BUT COULD NOT VERIFY THIS. THE SYS OPERATED PERFECTLY FOR THE REMAINDER OF THE FLT.

Narrative: ATC CLRNC 'CLB TO 6000 FT MSL, L TURN 220 DEGS TO INTERCEPT COL 350 DEG RADIAL.' AS PNF, I DIVERTED ATTN TO FMC TO SET UP COURSE INTERCEPT FOR PF. HE WAS FLYING MANUALLY. I HAD SET 6000 FT IN ALT WINDOW AND THEN BEGAN TO SET UP COURSE INTERCEPT. FOR SOME UNKNOWN REASON, I HAD TO REPEAT THE SETUP AND WHILE ATTEMPTING SECOND TIME, PF QUERIED '6000 FT?' I LOOKED UP AND CONFIRMED '6000 FT' AND NOTICED WE WERE LEVELING AT 7000 FT MSL. PF BEGAN IMMEDIATE CORRECTION TO 6000 FT AND STATED THAT THE COMMAND BARS INDICATED CLB UNTIL HE FINALLY QUERIED '6000 FT?' AS WE APCHED 7000 FT MSL. WE HAVE NO EXPLANATION WHY COMMAND BARS COMMANDED CLB. HE ALSO STATED, AUTOTHROTTLES DID NOT RETARD. HIS CORRECTION BACK TO 6000 FT WAS IMMEDIATE. UPON REACHING 6000 FT, DEP CLRED US TO HIGHER (I THINK 12000 FT) AND WE BEGAN CLB. PASSING 6400 FT ATC REQUESTED OUR ALT AND WE RESPONDED 6400 FT CLBING TO 12000 FT. ATC SAID NOTHING FURTHER, NOR DID WE. MISTAKES: 1) PNF DIVERTED ATTN TO FMC TOO LONG AT ONE TIME. 2) PF DIDN'T MONITOR ALTIMETER AS WELL AS COMMAND BARS. IF ALT WARNING AND AUTOTHROTTLES MALFUNCTIONED, IT NEVER HAPPENED AGAIN FOR ENTIRE FLT (NO EXPLANATION). WE WONDER IF SAME WOULD HAVE OCCURRED WITH AUTOPLT FLYING.

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.