A320 captain overshot intermediate altitude during STAR arrival due the fix dropping out of the FMC when going to the vertical descent mode for a higher descent rate.

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Attributes
ACN	527877
Time
Date	200110
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : snrkl
State Reference	CA
Altitude	msl bound lower : 10500 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : sct.tracon artcc : zjx.artcc
Operator	common carrier : air carrier
Make Model Name	A320
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	arrival star : mitts
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 11000 flight time type : 1700
ASRS Report	527877
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : published procedure
Independent Detector	atc equipment other atc equipment : radu other controllera
Resolutory Action	controller : issued alert flight crew : returned to assigned altitude
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Mcdu programmed for mitts 1 arrival into lax. All constraints entered correctly. ATC had held us high FL180 at 280 KTS then cleared us for descent via mitts 1 arrival, slow 250 KTS. Put 8000 ft in altitude window (lowest altitude on arrival). Pulled for open descent to get aircraft started down (we were high and needed to slow). Used maximum speed brakes for the confign. At 3 NM east of snrkl, aircraft was approaching glide path, retracted speed brake and pushed for VNAV. Managed descent 12000 ft constraint was in mcdu. Got distraction. Next call was from ATC 'are you leveling at 12000 ft?' checked the gauges. Aircraft was 1 NM east of snrkl at 10700 ft, descending. The 12000 constrained had disappeared. Selected vertical speed +300 ft. Aircraft descended to 10500 ft, started to climb back to 12000 ft at snrkl. Continued normal VNAV arrival. Don't know why the constraint fell out, but every time I go to lax the cockpit gets extremely busy, then I get rapid dscnts while slowing and just get set up for these things to happen.

Original NASA ASRS Text

Title: A320 CAPT OVERSHOT INTERMEDIATE ALT DURING STAR ARR DUE THE FIX DROPPING OUT OF THE FMC WHEN GOING TO THE VERT DSCNT MODE FOR A HIGHER DSCNT RATE.

Narrative: MCDU PROGRAMMED FOR MITTS 1 ARR INTO LAX. ALL CONSTRAINTS ENTERED CORRECTLY. ATC HAD HELD US HIGH FL180 AT 280 KTS THEN CLRED US FOR DSCNT VIA MITTS 1 ARR, SLOW 250 KTS. PUT 8000 FT IN ALT WINDOW (LOWEST ALT ON ARR). PULLED FOR OPEN DSCNT TO GET ACFT STARTED DOWN (WE WERE HIGH AND NEEDED TO SLOW). USED MAX SPD BRAKES FOR THE CONFIGN. AT 3 NM E OF SNRKL, ACFT WAS APCHING GLIDE PATH, RETRACTED SPD BRAKE AND PUSHED FOR VNAV. MANAGED DSCNT 12000 FT CONSTRAINT WAS IN MCDU. GOT DISTR. NEXT CALL WAS FROM ATC 'ARE YOU LEVELING AT 12000 FT?' CHKED THE GAUGES. ACFT WAS 1 NM E OF SNRKL AT 10700 FT, DSNDING. THE 12000 CONSTRAINED HAD DISAPPEARED. SELECTED VERT SPD +300 FT. ACFT DSNDED TO 10500 FT, STARTED TO CLB BACK TO 12000 FT AT SNRKL. CONTINUED NORMAL VNAV ARR. DON'T KNOW WHY THE CONSTRAINT FELL OUT, BUT EVERY TIME I GO TO LAX THE COCKPIT GETS EXTREMELY BUSY, THEN I GET RAPID DSCNTS WHILE SLOWING AND JUST GET SET UP FOR THESE THINGS TO HAPPEN.

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.