MD80 crew had altitude overshoot in ZJX class a airspace.

Attributes
ACN	495660
Time
Date	200012
Day	Sat
Local Time Of Day	1201 To 1800
Place
State Reference	FL
Altitude	msl bound lower : 19500 msl bound upper : 20000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx.artcc
Operator	common carrier : air carrier
Make Model Name	MD-88
Operating Under FAR Part	Part 121
Navigation In Use	other Other
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 : 180 flight time total : 8500 flight time type : 400
ASRS Report	495660
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : flight engineer pilot : multi engine pilot : instrument
Experience	flight time last 90 days : 250 flight time total : 3500 flight time type : 250
ASRS Report	495323
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : company policies non adherence : published procedure other anomaly other
Independent Detector	atc equipment other atc equipment : radar other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude none taken : detected after the fact
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft Company
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Autoplt on, in FMS VNAV descent mode. Aircraft was flying a bit faster than desired, so copilot switched from VNAV to vertical speed mode. This canceled the altitude capture armed mode and aircraft descended 500 ft below assigned altitude of FL200. There were no conflicts and ATC seemed unconcerned. Being a relatively new pilot on this equipment (4 months) led to my being distraction by the FMS VNAV mode of autoplt.

Original NASA ASRS Text

Title: MD80 CREW HAD ALT OVERSHOOT IN ZJX CLASS A AIRSPACE.

Narrative: AUTOPLT ON, IN FMS VNAV DSCNT MODE. ACFT WAS FLYING A BIT FASTER THAN DESIRED, SO COPLT SWITCHED FROM VNAV TO VERT SPD MODE. THIS CANCELED THE ALT CAPTURE ARMED MODE AND ACFT DSNDED 500 FT BELOW ASSIGNED ALT OF FL200. THERE WERE NO CONFLICTS AND ATC SEEMED UNCONCERNED. BEING A RELATIVELY NEW PLT ON THIS EQUIP (4 MONTHS) LED TO MY BEING DISTR BY THE FMS VNAV MODE OF AUTOPLT.

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.