<< >> NASA ASRS Report 169273, January 1991

Altitude deviation altitude excursion from assigned as aircraft attitude direction indicator becomes erratic and autoplt follow ADI.

Attributes
ACN	169273
Time
Date	199101
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : den
State Reference	CO
Altitude	msl bound lower : 34520 msl bound upper : 35250
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zdv
Operator	common carrier : air carrier
Make Model Name	Medium 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 : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 200
ASRS Report	169273
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

In cruise at 35000', autoplt-a engaged, aircraft pitched up (not violently), much as in a mountain wave encounter. I disengaged autoplt, pushed nose over to what I thought was level flight on the ADI and altitude returned toward FL350 from about 35250', and in what showed on the ADI as level flight we entered a descent. I stopped the descent by reference to altimeter at about 34520' and returned to FL350. At this time I noticed a discrepancy between the bank on the standby horizon and the ADI and realized the standby and the first officer aid agreed, so I gave the aircraft control to the first officer, complied with procedures in quick reference handbook and returned to des for maintenance. They replaced #1 platform and digital to analog adapter and problem was fixed. This problem could have been recognized much easier for what it was if any of the INS flags would have come into view. According to maintenance, with a failure of this type no flags will come into view, and I feel this is a design deficiency that could cause a serious problem. Good cooperation from ATC, company and maintenance department.

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Original NASA ASRS Text

Title: ALT DEVIATION ALT EXCURSION FROM ASSIGNED AS ACFT ATTITUDE DIRECTION INDICATOR BECOMES ERRATIC AND AUTOPLT FOLLOW ADI.

Narrative: IN CRUISE AT 35000', AUTOPLT-A ENGAGED, ACFT PITCHED UP (NOT VIOLENTLY), MUCH AS IN A MOUNTAIN WAVE ENCOUNTER. I DISENGAGED AUTOPLT, PUSHED NOSE OVER TO WHAT I THOUGHT WAS LEVEL FLT ON THE ADI AND ALT RETURNED TOWARD FL350 FROM ABOUT 35250', AND IN WHAT SHOWED ON THE ADI AS LEVEL FLT WE ENTERED A DSNT. I STOPPED THE DSNT BY REF TO ALTIMETER AT ABOUT 34520' AND RETURNED TO FL350. AT THIS TIME I NOTICED A DISCREPANCY BTWN THE BANK ON THE STANDBY HORIZON AND THE ADI AND REALIZED THE STANDBY AND THE F/O AID AGREED, SO I GAVE THE ACFT CTL TO THE F/O, COMPLIED WITH PROCS IN QUICK REF HANDBOOK AND RETURNED TO DES FOR MAINT. THEY REPLACED #1 PLATFORM AND DIGITAL TO ANALOG ADAPTER AND PROB WAS FIXED. THIS PROB COULD HAVE BEEN RECOGNIZED MUCH EASIER FOR WHAT IT WAS IF ANY OF THE INS FLAGS WOULD HAVE COME INTO VIEW. ACCORDING TO MAINT, WITH A FAILURE OF THIS TYPE NO FLAGS WILL COME INTO VIEW, AND I FEEL THIS IS A DESIGN DEFICIENCY THAT COULD CAUSE A SERIOUS PROB. GOOD COOPERATION FROM ATC, COMPANY AND MAINT DEPT.

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.