While approaching glide slope, large transport makes undesired descent of 1700'.

Attributes
ACN	163962
Time
Date	199011
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 2300 msl bound upper : 4000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : commercial pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 200 flight time total : 9000 flight time type : 1400
ASRS Report	163962
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : flight engineer pilot : atp pilot : commercial
Experience	flight time last 90 days : 165 flight time total : 16500 flight time type : 9000
ASRS Report	163961
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	atc equipment other atc equipment : unspecified other flight crewa
Resolutory Action	flight crew : became reoriented flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While flying for an approach to runway 22R in ord, we experienced an unscheduled descent. We were in the WX about 13 DME out at 4000' established inbound on the 22R localizer. We had been cleared for the approach and lower when established. The autoplt was engaged with navigation #2 on the 22R ILS and navigation #1 on the ord VOR. At about this time navigation #1, with lots of distraction, was tuned to the 22R ILS frequency. During this the aircraft descended out of 4000' to about 2300' with a resulting airspeed increase. Realizing this I disconnected the autoplt and started to return the aircraft to the G/south and requested airspeed. The final controller advised us of a low altitude alert for us as the recovery was being made. A normal approach and landing was made from there. I'm not sure why the aircraft descended out of 4000'. However, I feel the new confign or placement of navigation radios and F/D controls helped cause enough of a distraction to allow an unprogrammed and unnoticed descent to start.

Original NASA ASRS Text

Title: WHILE APCHING GLIDE SLOPE, LGT MAKES UNDESIRED DESCENT OF 1700'.

Narrative: WHILE FLYING FOR AN APCH TO RWY 22R IN ORD, WE EXPERIENCED AN UNSCHEDULED DSNT. WE WERE IN THE WX ABOUT 13 DME OUT AT 4000' ESTABLISHED INBND ON THE 22R LOC. WE HAD BEEN CLRED FOR THE APCH AND LOWER WHEN ESTABLISHED. THE AUTOPLT WAS ENGAGED WITH NAV #2 ON THE 22R ILS AND NAV #1 ON THE ORD VOR. AT ABOUT THIS TIME NAV #1, WITH LOTS OF DISTR, WAS TUNED TO THE 22R ILS FREQ. DURING THIS THE ACFT DSNDED OUT OF 4000' TO ABOUT 2300' WITH A RESULTING AIRSPD INCREASE. REALIZING THIS I DISCONNECTED THE AUTOPLT AND STARTED TO RETURN THE ACFT TO THE G/S AND REQUESTED AIRSPD. THE FINAL CTLR ADVISED US OF A LOW ALT ALERT FOR US AS THE RECOVERY WAS BEING MADE. A NORMAL APCH AND LNDG WAS MADE FROM THERE. I'M NOT SURE WHY THE ACFT DSNDED OUT OF 4000'. HOWEVER, I FEEL THE NEW CONFIGN OR PLACEMENT OF NAV RADIOS AND F/D CTLS HELPED CAUSE ENOUGH OF A DISTR TO ALLOW AN UNPROGRAMMED AND UNNOTICED DSNT TO START.

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.