Air carrier medium large transport altitude excursion.

Attributes
ACN	103941
Time
Date	198902
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : buj
State Reference	TX
Altitude	msl bound lower : 35000 msl bound upper : 35480
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zfw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 12000 flight time type : 980
ASRS Report	103941
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : commercial pilot : instrument
Experience	flight time last 90 days : 35 flight time total : 1200 flight time type : 15
ASRS Report	104169
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

In cruise at FL350 first officer's leg using autoplt/flight guidance #2 system. We had been cleared pilot discretion descent to FL240 and 240 had been set and armed in altitude window. Aircraft remained in altitude hold mode as it should have at 35000'. I was getting the airport ATIS and the first officer was looking over the STAR when I noticed the aircraft had climbed to 35480' while the flight director still indicated altitude hold. I immediately started a descent when the center controller called us to ask about our altitude. He told us there was no conflict, no problem, and no alarms had been set off. On arriving at dfw I wrote the altitude hold problem up in the aircraft log book for maintenance action. Also we had not looked away from the panel but for only a very short time. We never felt or noticed the aircraft climb. We were flying into an area of rapidly increasing barometric pressure. Dfw was 30.66 and increasing rapidly. I believe now that it could have been a glitch in the CADC (central air data computer) correcting for the rapid rise in barometric pressure that created the indication problem. The problem could not be duplicated later and the fact remains that the autoplt remained in altitude hold and had leveled at 35480' which just about corresponds to the change in altimeter settings in the area we had just flown through.

Original NASA ASRS Text

Title: ACR MLG ALT EXCURSION.

Narrative: IN CRUISE AT FL350 F/O'S LEG USING AUTOPLT/FLT GUIDANCE #2 SYS. WE HAD BEEN CLRED PLT DISCRETION DSCNT TO FL240 AND 240 HAD BEEN SET AND ARMED IN ALT WINDOW. ACFT REMAINED IN ALT HOLD MODE AS IT SHOULD HAVE AT 35000'. I WAS GETTING THE ARPT ATIS AND THE F/O WAS LOOKING OVER THE STAR WHEN I NOTICED THE ACFT HAD CLBED TO 35480' WHILE THE FLT DIRECTOR STILL INDICATED ALT HOLD. I IMMEDIATELY STARTED A DSCNT WHEN THE CENTER CTLR CALLED US TO ASK ABOUT OUR ALT. HE TOLD US THERE WAS NO CONFLICT, NO PROB, AND NO ALARMS HAD BEEN SET OFF. ON ARRIVING AT DFW I WROTE THE ALT HOLD PROB UP IN THE ACFT LOG BOOK FOR MAINT ACTION. ALSO WE HAD NOT LOOKED AWAY FROM THE PANEL BUT FOR ONLY A VERY SHORT TIME. WE NEVER FELT OR NOTICED THE ACFT CLB. WE WERE FLYING INTO AN AREA OF RAPIDLY INCREASING BAROMETRIC PRESSURE. DFW WAS 30.66 AND INCREASING RAPIDLY. I BELIEVE NOW THAT IT COULD HAVE BEEN A GLITCH IN THE CADC (CENTRAL AIR DATA COMPUTER) CORRECTING FOR THE RAPID RISE IN BAROMETRIC PRESSURE THAT CREATED THE INDICATION PROB. THE PROB COULD NOT BE DUPLICATED LATER AND THE FACT REMAINS THAT THE AUTOPLT REMAINED IN ALT HOLD AND HAD LEVELED AT 35480' WHICH JUST ABOUT CORRESPONDS TO THE CHANGE IN ALTIMETER SETTINGS IN THE AREA WE HAD JUST FLOWN THROUGH.

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.