Altitude deviation altitude excursion when autoplt malfunctions.

Attributes
ACN	205923
Time
Date	199203
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zbw
State Reference	MA
Altitude	msl bound lower : 23000 msl bound upper : 23700
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zbw artcc : zbw
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : instrument pilot : commercial
Experience	flight time last 90 days : 200 flight time total : 10600 flight time type : 1200
ASRS Report	205923
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

We were on flight mco-bos. Originally at FL410, we had been directed to descend to FL230. We descended and were level at FL230. Captain was flying with FMC set to FL230, left autoplt engaged. After about 5 mins of level flight, without warning and uncommanded, the engines accelerated to climb EPR and the aircraft started a climb at greater than 2000 FPM. The only warning was the altitude alerter as we climbed greater than 300 ft over FL230. The captain disconnected the autoplt and autothrottles immediately, pushed the nose over to return to altitude and eased back to FL230. We probably exceeded our assigned altitude by 600-700 ft (FL23.7). I have no idea why the autoplt malfunctioned in this manner with no EICAS warnings or messages. Once back at FL230, we re-engaged the same autoplt for the remainder of the flight. There were no further problems. We wrote up the maintenance discrepancy upon arrival at bos. In this instance, the human element intervened and corrected what could have become a very serious mechanical excursion. ATC queried our altitude as we descended through 23.2 back to FL230. I do not believe there was a conflict as we had just descended from FL240 to FL230 mins prior to this incident.

Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION WHEN AUTOPLT MALFUNCTIONS.

Narrative: WE WERE ON FLT MCO-BOS. ORIGINALLY AT FL410, WE HAD BEEN DIRECTED TO DSND TO FL230. WE DSNDED AND WERE LEVEL AT FL230. CAPT WAS FLYING WITH FMC SET TO FL230, LEFT AUTOPLT ENGAGED. AFTER ABOUT 5 MINS OF LEVEL FLT, WITHOUT WARNING AND UNCOMMANDED, THE ENGS ACCELERATED TO CLB EPR AND THE ACFT STARTED A CLB AT GREATER THAN 2000 FPM. THE ONLY WARNING WAS THE ALT ALERTER AS WE CLBED GREATER THAN 300 FT OVER FL230. THE CAPT DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES IMMEDIATELY, PUSHED THE NOSE OVER TO RETURN TO ALT AND EASED BACK TO FL230. WE PROBABLY EXCEEDED OUR ASSIGNED ALT BY 600-700 FT (FL23.7). I HAVE NO IDEA WHY THE AUTOPLT MALFUNCTIONED IN THIS MANNER WITH NO EICAS WARNINGS OR MESSAGES. ONCE BACK AT FL230, WE RE-ENGAGED THE SAME AUTOPLT FOR THE REMAINDER OF THE FLT. THERE WERE NO FURTHER PROBLEMS. WE WROTE UP THE MAINT DISCREPANCY UPON ARR AT BOS. IN THIS INSTANCE, THE HUMAN ELEMENT INTERVENED AND CORRECTED WHAT COULD HAVE BECOME A VERY SERIOUS MECHANICAL EXCURSION. ATC QUERIED OUR ALT AS WE DSNDED THROUGH 23.2 BACK TO FL230. I DO NOT BELIEVE THERE WAS A CONFLICT AS WE HAD JUST DSNDED FROM FL240 TO FL230 MINS PRIOR TO THIS INCIDENT.

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.