Altitude deviation. Excursion from assigned altitude.

Attributes
ACN	130585
Time
Date	198912
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : abq
State Reference	NM
Altitude	agl bound lower : 35000 agl bound upper : 35300
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zab
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 : 100 flight time total : 3500 flight time type : 100
ASRS Report	130585
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
ASRS Report	130188
Events
Anomaly	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
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While cruising at FL350 with the autoplt engaged, I was operating the flight guidance system in the performance cruise mode. In that mode, the aircraft tends to climb or descend slightly to maintain a particular speed. With a setting of 29.92 in all 3 altimeters, the captain's and first officer's altimeter, while reading 350, showed the center altimeter to read 35160. In this instance, the performance cruise mode climbed the aircraft to 35200 on the captain's and first officer's altimeter, and 35360 on the center altimeter. The altitude alert signaled, so I selected altitude hold and descended the aircraft back down the 200' to FL350 with the vertical speed wheel. The aircraft never went above 35200 on the captain's and first officer's altimeter and there was no conflict, but I was concerned that the altitude alert went off. I went through an altitude alert test after landing which showed the system to work fine, so I did not write it up. I am not sure why the altitude alert system went off, but I do not like a flight guidance system that fluctuates altitude to maintain speed.

Original NASA ASRS Text

Title: ALT DEVIATION. EXCURSION FROM ASSIGNED ALT.

Narrative: WHILE CRUISING AT FL350 WITH THE AUTOPLT ENGAGED, I WAS OPERATING THE FLT GUIDANCE SYS IN THE PERFORMANCE CRUISE MODE. IN THAT MODE, THE ACFT TENDS TO CLB OR DSND SLIGHTLY TO MAINTAIN A PARTICULAR SPD. WITH A SETTING OF 29.92 IN ALL 3 ALTIMETERS, THE CAPT'S AND F/O'S ALTIMETER, WHILE READING 350, SHOWED THE CENTER ALTIMETER TO READ 35160. IN THIS INSTANCE, THE PERFORMANCE CRUISE MODE CLBED THE ACFT TO 35200 ON THE CAPT'S AND F/O'S ALTIMETER, AND 35360 ON THE CENTER ALTIMETER. THE ALT ALERT SIGNALED, SO I SELECTED ALT HOLD AND DSNDED THE ACFT BACK DOWN THE 200' TO FL350 WITH THE VERT SPD WHEEL. THE ACFT NEVER WENT ABOVE 35200 ON THE CAPT'S AND F/O'S ALTIMETER AND THERE WAS NO CONFLICT, BUT I WAS CONCERNED THAT THE ALT ALERT WENT OFF. I WENT THROUGH AN ALT ALERT TEST AFTER LNDG WHICH SHOWED THE SYS TO WORK FINE, SO I DID NOT WRITE IT UP. I AM NOT SURE WHY THE ALT ALERT SYS WENT OFF, BUT I DO NOT LIKE A FLT GUIDANCE SYS THAT FLUCTUATES ALT TO MAINTAIN SPD.

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.