A300 crew overshoots assigned altitude during descent to FL240 in ZAU airspace.

Attributes
ACN	705651
Time
Date	200608
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : rfd.vortac
State Reference	IL
Altitude	msl bound lower : 23500 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	A300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : intermediate altitude
Route In Use	arrival : on vectors
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 56 flight time total : 13000 flight time type : 3200
ASRS Report	705651
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : company policies non adherence : far
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude flight crew : overcame equipment problem
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

On vector direct rfd VOR at FL330. Cleared by ATC to FL240. Selected FL240 in altitude window. I pulled altitude knob and then selected 'descend now' on FMS and profile mode. Aircraft began descent at 1000 FPM and then captured profile path and descent rate increased to about 2700 FPM. Coming through FL245; aircraft pitched down and accelerated airspeed and descent rate. Both myself and first officer noted this and I immediately disconnected autoplt and pulled nose up. By the time I arrested descent rate and started to climb; we reached about FL235. I climbed back to FL240 in about 10 seconds and leveled there. Re-engaged autoplt and continued normally. We were below FL240 and between FL235 and FL240 for less than 10 seconds. No comment from ATC and we did not discuss with ATC. Myself and first officer were both surprised and verified that FL240 was selected in altitude window. After some discussion I believe that we caused the problem by making an input into FMS while we were still in the 'palt' mode and prior to 'palt' captured. We had received a speed restr of 280 KIAS from ATC and I requested first officer to input into FMS tact menu. The input was made right after the first officer made the FL250 for FL240 call. I should have waited until we fully captured the palt.

Original NASA ASRS Text

Title: A300 CREW OVERSHOOTS ASSIGNED ALT DURING DSCNT TO FL240 IN ZAU AIRSPACE.

Narrative: ON VECTOR DIRECT RFD VOR AT FL330. CLRED BY ATC TO FL240. SELECTED FL240 IN ALT WINDOW. I PULLED ALT KNOB AND THEN SELECTED 'DSND NOW' ON FMS AND PROFILE MODE. ACFT BEGAN DSCNT AT 1000 FPM AND THEN CAPTURED PROFILE PATH AND DSCNT RATE INCREASED TO ABOUT 2700 FPM. COMING THROUGH FL245; ACFT PITCHED DOWN AND ACCELERATED AIRSPD AND DSCNT RATE. BOTH MYSELF AND FO NOTED THIS AND I IMMEDIATELY DISCONNECTED AUTOPLT AND PULLED NOSE UP. BY THE TIME I ARRESTED DSCNT RATE AND STARTED TO CLB; WE REACHED ABOUT FL235. I CLBED BACK TO FL240 IN ABOUT 10 SECONDS AND LEVELED THERE. RE-ENGAGED AUTOPLT AND CONTINUED NORMALLY. WE WERE BELOW FL240 AND BTWN FL235 AND FL240 FOR LESS THAN 10 SECONDS. NO COMMENT FROM ATC AND WE DID NOT DISCUSS WITH ATC. MYSELF AND FO WERE BOTH SURPRISED AND VERIFIED THAT FL240 WAS SELECTED IN ALT WINDOW. AFTER SOME DISCUSSION I BELIEVE THAT WE CAUSED THE PROB BY MAKING AN INPUT INTO FMS WHILE WE WERE STILL IN THE 'PALT' MODE AND PRIOR TO 'PALT' CAPTURED. WE HAD RECEIVED A SPD RESTR OF 280 KIAS FROM ATC AND I REQUESTED FO TO INPUT INTO FMS TACT MENU. THE INPUT WAS MADE RIGHT AFTER THE FO MADE THE FL250 FOR FL240 CALL. I SHOULD HAVE WAITED UNTIL WE FULLY CAPTURED THE PALT.

Data retrieved from NASA's ASRS site as of January 2009 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.