Flight crew deviates 300-400 ft below assigned altitude.

Attributes
ACN	302609
Time
Date	199504
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : shb
State Reference	IN
Altitude	msl bound lower : 22600 msl bound upper : 23000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid
Operator	common carrier : air carrier
Make Model Name	B727-200
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 22000 flight time type : 150
ASRS Report	302609
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other flight crewa other other : unspecified cockpit
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

During descent to FL230 on a modified milan 2 arrival where several vectors or changes in routings occurred, just prior to level off at FL230. A direct to shb VOR was given. Autoplt was being used, altitude hold engaged at FL230. Both pilots began to reset VOR's frequency/course and reprogram omega navigation. Autoplt disengaged and started rapid descent. I attempted autoplt re-engagement and commanded a return to FL230. As fast as I tried and, because autoplt did not respond quickly enough, our aircraft descended to 300-400 ft below FL230 before recovering back to FL230. Both pilots saw what was happening but could not respond quickly enough to avoid the deviation. The autoplt worked normally throughout the flight before and after incident. Since both pilots's most recent experience (previous 7-8 yrs) was in more modern glass cockpit aircraft with highly reliable navigation and autoplt system, probably, too much was expected of this older and less reliable system.

Original NASA ASRS Text

Title: FLC DEVIATES 300-400 FT BELOW ASSIGNED ALT.

Narrative: DURING DSCNT TO FL230 ON A MODIFIED MILAN 2 ARR WHERE SEVERAL VECTORS OR CHANGES IN ROUTINGS OCCURRED, JUST PRIOR TO LEVEL OFF AT FL230. A DIRECT TO SHB VOR WAS GIVEN. AUTOPLT WAS BEING USED, ALT HOLD ENGAGED AT FL230. BOTH PLTS BEGAN TO RESET VOR'S FREQ/COURSE AND REPROGRAM OMEGA NAV. AUTOPLT DISENGAGED AND STARTED RAPID DSCNT. I ATTEMPTED AUTOPLT RE-ENGAGEMENT AND COMMANDED A RETURN TO FL230. AS FAST AS I TRIED AND, BECAUSE AUTOPLT DID NOT RESPOND QUICKLY ENOUGH, OUR ACFT DSNDED TO 300-400 FT BELOW FL230 BEFORE RECOVERING BACK TO FL230. BOTH PLTS SAW WHAT WAS HAPPENING BUT COULD NOT RESPOND QUICKLY ENOUGH TO AVOID THE DEV. THE AUTOPLT WORKED NORMALLY THROUGHOUT THE FLT BEFORE AND AFTER INCIDENT. SINCE BOTH PLTS'S MOST RECENT EXPERIENCE (PREVIOUS 7-8 YRS) WAS IN MORE MODERN GLASS COCKPIT ACFT WITH HIGHLY RELIABLE NAV AND AUTOPLT SYS, PROBABLY, TOO MUCH WAS EXPECTED OF THIS OLDER AND LESS RELIABLE SYS.

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.