Altitude deviation. Excursion from assigned.

Attributes
ACN	144712
Time
Date	199004
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : hec
State Reference	CA
Altitude	msl bound lower : 28000 msl bound upper : 28700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla tower : rdu
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
Route In Use	enroute airway : zla
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument pilot : atp pilot : cfi
Experience	flight time last 90 days : 230 flight time total : 9000 flight time type : 3800
ASRS Report	144712
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
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 : regained aircraft control
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation

Narrative:

We were in the cruise phase of flight autoplt on level at 28000'. Cleared pilot discretion to FL240. Reset the altitude selector on the MCP panel for 24000' top of descent was approximately 20 mi ahead. Aircraft maintained altitude as it should have. I began to get my approach plates in order for the approach into lax. During my review of the approach plates my first officer questioned our altitude. I looked up and we had climbed 700' to 28700'. I immediately disconnected the autoplt and autothrottles and descended to 24000'. Prior to and after this occurrence the autoplt worked fine on this flight. Thinking this was just a glitch we departed lax and immediately had another discrepancy with the autoplt system. After the we wrote up the irregularity and maintenance placarded the altitude hold function inoperative. There been a lot of controversy over altitude excursion with this medium large transport autoplt system, and I can tell emphatically it is not operator error! It's about time someone stepped in and corrected this anomaly once and for all, or come up with a new system. The altitude selector was obviously sending erroneous information to both autoplt systems on our aircraft.

Original NASA ASRS Text

Title: ALT DEVIATION. EXCURSION FROM ASSIGNED.

Narrative: WE WERE IN THE CRUISE PHASE OF FLT AUTOPLT ON LEVEL AT 28000'. CLRED PLT DISCRETION TO FL240. RESET THE ALT SELECTOR ON THE MCP PANEL FOR 24000' TOP OF DSNT WAS APPROX 20 MI AHEAD. ACFT MAINTAINED ALT AS IT SHOULD HAVE. I BEGAN TO GET MY APCH PLATES IN ORDER FOR THE APCH INTO LAX. DURING MY REVIEW OF THE APCH PLATES MY F/O QUESTIONED OUR ALT. I LOOKED UP AND WE HAD CLBED 700' TO 28700'. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES AND DSNDED TO 24000'. PRIOR TO AND AFTER THIS OCCURRENCE THE AUTOPLT WORKED FINE ON THIS FLT. THINKING THIS WAS JUST A GLITCH WE DEPARTED LAX AND IMMEDIATELY HAD ANOTHER DISCREPANCY WITH THE AUTOPLT SYS. AFTER THE WE WROTE UP THE IRREGULARITY AND MAINT PLACARDED THE ALT HOLD FUNCTION INOP. THERE BEEN A LOT OF CONTROVERSY OVER ALT EXCURSION WITH THIS MLG AUTOPLT SYS, AND I CAN TELL EMPHATICALLY IT IS NOT OPERATOR ERROR! IT'S ABOUT TIME SOMEONE STEPPED IN AND CORRECTED THIS ANOMALY ONCE AND FOR ALL, OR COME UP WITH A NEW SYS. THE ALT SELECTOR WAS OBVIOUSLY SENDING ERRONEOUS INFO TO BOTH AUTOPLT SYSTEMS ON OUR ACFT.

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.