Altitude overshot in descent when altitude set to qne vs qnh on frequency with ZAU, il.

Attributes
ACN	513330
Time
Date	200106
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zma.artcc
State Reference	FL
Altitude	msl bound lower : 16400 msl bound upper : 17000
Environment
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B727-200
Flight Phase	descent : approach
Route In Use	arrival star : omn vor
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
ASRS Report	513330
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance other anomaly other
Resolutory Action	flight crew : regained aircraft control flight crew : overcame equipment problem flight crew : overrode automation flight crew : became reoriented
Supplementary
Problem Areas	Flight Crew Human Performance Weather
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Location: 50 mi north omn VOR arrival. It was the captain's leg and he was in command of the airplane while I (the first officer was) in charge of the radios. We had been at our assigned altitude of 17000 ft and airspeed of 250 KIAS for several min prior to the event. While being vectored to the omn VOR (bith arrival) into mco, the autoplt did not maintain the assigned 17000 ft altitude. At some point during the turn the airplane started an unnoticed descent, and while being in VMC, I was looking into the turn to the right and also scanning the thunderstorms we had circumnaved to the west or our position. During this time the altitude alert sounded which prompted the aircraft to disconnect the autoplt and manually fly the aircraft back to our assigned altitude. The total loss in altitude was 500-600 ft. Prior to the disconnecting of the autoplt, I noticed the 'stabilizer out of trim' light illuminated. The captain switched autoplt elevator modes on the autoplt and our flight continued to mco without any further autoplt problems.

Original NASA ASRS Text

Title: ALT OVERSHOT IN DSCNT WHEN ALT SET TO QNE VS QNH ON FREQ WITH ZAU, IL.

Narrative: LOCATION: 50 MI N OMN VOR ARR. IT WAS THE CAPT'S LEG AND HE WAS IN COMMAND OF THE AIRPLANE WHILE I (THE FO WAS) IN CHARGE OF THE RADIOS. WE HAD BEEN AT OUR ASSIGNED ALT OF 17000 FT AND AIRSPD OF 250 KIAS FOR SEVERAL MIN PRIOR TO THE EVENT. WHILE BEING VECTORED TO THE OMN VOR (BITH ARR) INTO MCO, THE AUTOPLT DID NOT MAINTAIN THE ASSIGNED 17000 FT ALT. AT SOME POINT DURING THE TURN THE AIRPLANE STARTED AN UNNOTICED DSCNT, AND WHILE BEING IN VMC, I WAS LOOKING INTO THE TURN TO THE R AND ALSO SCANNING THE TSTMS WE HAD CIRCUMNAVED TO THE W OR OUR POS. DURING THIS TIME THE ALT ALERT SOUNDED WHICH PROMPTED THE ACFT TO DISCONNECT THE AUTOPLT AND MANUALLY FLY THE ACFT BACK TO OUR ASSIGNED ALT. THE TOTAL LOSS IN ALT WAS 500-600 FT. PRIOR TO THE DISCONNECTING OF THE AUTOPLT, I NOTICED THE 'STABILIZER OUT OF TRIM' LIGHT ILLUMINATED. THE CAPT SWITCHED AUTOPLT ELEVATOR MODES ON THE AUTOPLT AND OUR FLT CONTINUED TO MCO WITHOUT ANY FURTHER AUTOPLT PROBS.

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.