Medium large transport on autoplt lost 450'.

Attributes
ACN	93526
Time
Date	198809
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zzz
State Reference	US
Altitude	msl bound lower : 34550 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zbw
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 : 130 flight time total : 10000 flight time type : 325
ASRS Report	93526
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
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	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

During cruise with pms in 'perf' mode and DFGS system 1 in use at FL350, the sif (stall indication failure) light on the overhead annunciator panel illuminated followed by an amber slat disagree light and CDU message of vmax 188. Autothrottles went to 'clmp' (clamp) and pitch mode of autoplt defaulted to mach hold causing aircraft to descend 450' before descent could be arrested. Airspeed never got lower than 240 (KIAS) or .73 mach. The system was recycled and reinstated in DFGS #1 and perf and it worked normally remainder of flight. The sif and slat disagree lights went out almost immediately during altitude recovery after the autoplt was disconnected. It strikes me that the -930 computer should default to mach or speed hold on the autothrottles and the altitude hold on the pitch. Altitude deviations of this sort have a great potential to be dangerous.

Original NASA ASRS Text

Title: MLG ON AUTOPLT LOST 450'.

Narrative: DURING CRUISE WITH PMS IN 'PERF' MODE AND DFGS SYS 1 IN USE AT FL350, THE SIF (STALL INDICATION FAILURE) LIGHT ON THE OVERHEAD ANNUNCIATOR PANEL ILLUMINATED FOLLOWED BY AN AMBER SLAT DISAGREE LIGHT AND CDU MESSAGE OF VMAX 188. AUTOTHROTTLES WENT TO 'CLMP' (CLAMP) AND PITCH MODE OF AUTOPLT DEFAULTED TO MACH HOLD CAUSING ACFT TO DSND 450' BEFORE DSCNT COULD BE ARRESTED. AIRSPD NEVER GOT LOWER THAN 240 (KIAS) OR .73 MACH. THE SYS WAS RECYCLED AND REINSTATED IN DFGS #1 AND PERF AND IT WORKED NORMALLY REMAINDER OF FLT. THE SIF AND SLAT DISAGREE LIGHTS WENT OUT ALMOST IMMEDIATELY DURING ALT RECOVERY AFTER THE AUTOPLT WAS DISCONNECTED. IT STRIKES ME THAT THE -930 COMPUTER SHOULD DEFAULT TO MACH OR SPD HOLD ON THE AUTOTHROTTLES AND THE ALT HOLD ON THE PITCH. ALT DEVIATIONS OF THIS SORT HAVE A GREAT POTENTIAL TO BE DANGEROUS.

Data retrieved from NASA's ASRS site as of August 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.