Air carrier advtech aircraft altitude excursion at cruise altitude.

Attributes
ACN	160512
Time
Date	199010
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : pmd
State Reference	CA
Altitude	msl bound lower : 28700 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla tower : oak
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 15000 flight time type : 1500
ASRS Report	160512
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp pilot : instrument
Experience	flight time last 90 days : 120 flight time total : 3000 flight time type : 120
ASRS Report	160515
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude non adherence : published procedure
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

First officer flying with autoplt #2 and pms engaged and coupled. FMA indicated perf crz level stabilized flight. As first officer turned to respond to the cabin attendant at the cockpit door, the aircraft began a 1000 FPM descend with no change indicated on FMA. All systems disengaged and the aircraft was leveled at FL287. Medium large transport flight director system and mode problems continue unabated. Supplemental information from acn 160515: in 'perf' mode, aircraft began descending. Captain yelled, 'aircraft going down!' I disconnected the autoplt, stopped the descent and climbed back up to FL290. No further deviations occurred.

Original NASA ASRS Text

Title: ACR ADVTECH ACFT ALT EXCURSION AT CRUISE ALT.

Narrative: F/O FLYING WITH AUTOPLT #2 AND PMS ENGAGED AND COUPLED. FMA INDICATED PERF CRZ LEVEL STABILIZED FLT. AS F/O TURNED TO RESPOND TO THE CABIN ATTENDANT AT THE COCKPIT DOOR, THE ACFT BEGAN A 1000 FPM DSND WITH NO CHANGE INDICATED ON FMA. ALL SYSTEMS DISENGAGED AND THE ACFT WAS LEVELED AT FL287. MLG FLT DIRECTOR SYS AND MODE PROBS CONTINUE UNABATED. SUPPLEMENTAL INFO FROM ACN 160515: IN 'PERF' MODE, ACFT BEGAN DSNDING. CAPT YELLED, 'ACFT GOING DOWN!' I DISCONNECTED THE AUTOPLT, STOPPED THE DSNT AND CLBED BACK UP TO FL290. NO FURTHER DEVIATIONS OCCURRED.

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.