Altitude deviation arriving seatac in A320.

Attributes
ACN	438103
Time
Date	199905
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : jaksn
State Reference	WA
Altitude	msl single value : 12000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : s46.tracon
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude descent : vacating altitude
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 : 12000 flight time type : 2000
ASRS Report	438103
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	altitude deviation : crossing restriction not met non adherence : clearance non adherence : far
Independent Detector	other flight crewa
Resolutory Action	flight crew : became reoriented flight crew : exited adverse environment
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were given a descent clearance from FL200 to 12000 ft. Just after the flight guidance system went into the altitude capture mode at 12400 ft, we were given another clearance to cross jaksn at 12000 ft, descend and maintain 7000 ft. Problem: when you set a new altitude (7000 ft) in the altitude select window on the flight control unit -- while the flight guidance is in the altitude capture mode (it was capturing 12000 ft) -- the vertical flight mode reverts to vertical speed. The aircraft will no longer capture the altitude -- in our case 12000 ft -- even though it was already in the process of capturing and initiating a leveloff at 12000 ft. It reverts to vertical speed as soon as you select 7000 ft and will fly through 12000 ft. We crossed jaksn slightly below 12000 ft. No conflicts with traffic and ATC did not question our altitude. This is a real 'gotcha' in the glass/automated cockpit. In your mind you know the aircraft is leveling off at an altitude (12000 ft) but as soon as you change the altitude select window, you discontinue your leveloff. I have also seen this occur on non precision approachs. I would suggest that the automated flight guidance system be configured so that the altitude capture mode will not discontinue a leveloff once the altitude capture mode is engaged without direct pilot involvement. It is very easy to not realize that you are not going to level off anymore at an altitude that the aircraft was in the process of capturing and leveling off.

Original NASA ASRS Text

Title: ALTDEV ARRIVING SEATAC IN A320.

Narrative: WE WERE GIVEN A DSCNT CLRNC FROM FL200 TO 12000 FT. JUST AFTER THE FLT GUIDANCE SYS WENT INTO THE ALT CAPTURE MODE AT 12400 FT, WE WERE GIVEN ANOTHER CLRNC TO CROSS JAKSN AT 12000 FT, DSND AND MAINTAIN 7000 FT. PROB: WHEN YOU SET A NEW ALT (7000 FT) IN THE ALT SELECT WINDOW ON THE FLT CTL UNIT -- WHILE THE FLT GUIDANCE IS IN THE ALT CAPTURE MODE (IT WAS CAPTURING 12000 FT) -- THE VERT FLT MODE REVERTS TO VERT SPD. THE ACFT WILL NO LONGER CAPTURE THE ALT -- IN OUR CASE 12000 FT -- EVEN THOUGH IT WAS ALREADY IN THE PROCESS OF CAPTURING AND INITIATING A LEVELOFF AT 12000 FT. IT REVERTS TO VERT SPD AS SOON AS YOU SELECT 7000 FT AND WILL FLY THROUGH 12000 FT. WE CROSSED JAKSN SLIGHTLY BELOW 12000 FT. NO CONFLICTS WITH TFC AND ATC DID NOT QUESTION OUR ALT. THIS IS A REAL 'GOTCHA' IN THE GLASS/AUTOMATED COCKPIT. IN YOUR MIND YOU KNOW THE ACFT IS LEVELING OFF AT AN ALT (12000 FT) BUT AS SOON AS YOU CHANGE THE ALT SELECT WINDOW, YOU DISCONTINUE YOUR LEVELOFF. I HAVE ALSO SEEN THIS OCCUR ON NON PRECISION APCHS. I WOULD SUGGEST THAT THE AUTOMATED FLT GUIDANCE SYS BE CONFIGURED SO THAT THE ALT CAPTURE MODE WILL NOT DISCONTINUE A LEVELOFF ONCE THE ALT CAPTURE MODE IS ENGAGED WITHOUT DIRECT PLT INVOLVEMENT. IT IS VERY EASY TO NOT REALIZE THAT YOU ARE NOT GOING TO LEVEL OFF ANYMORE AT AN ALT THAT THE ACFT WAS IN THE PROCESS OF CAPTURING AND LEVELING OFF.

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.