Aircraft on coupled ILS approach made uncommanded 30 degree bank.

Attributes
ACN	315543
Time
Date	199509
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : bna
State Reference	TN
Altitude	msl bound lower : 1800 msl bound upper : 1800
Environment
Flight Conditions	IMC
Aircraft 1
Controlling Facilities	tracon : bna
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	315543
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During a coupled autoplt ILS to runway 02L at bna in IMC conditions we experienced a rapid 30 degree right bank by the autoplt which was stopped by the first officer, PF disconnecting the autoplt. The aircraft was tracking both localizer and GS, annunciating track/track. Confign was 170 KIAS, flaps 15 degrees, altitude 1800 ft AGL. The air was smooth. Flight guidance was selected to #2 dgfs. The ceiling was better than 1000 ft and visibility was 2 1/2 mi in fog. The aircraft was placarded CAT 2 only due to the captain's no autoland light inoperative. After the first officer disconnected the autoplt. We continued the approach manually. I initially suspected wake turbulence and on approach asked the tower the aircraft type and distance in trail. We were 3 mi behind a B737. On the ground, we reported the incident to ground control to pass on to the tower. I requested a maintenance check of the autoplt system because I was not certain whether it was a wake turbulence or autoplt problem. The aircraft did not pass the return to service check. The mach trim compensator was placarded and the #2 DFGS computer was replaced.

Original NASA ASRS Text

Title: ACFT ON COUPLED ILS APCH MADE UNCOMMANDED 30 DEG BANK.

Narrative: DURING A COUPLED AUTOPLT ILS TO RWY 02L AT BNA IN IMC CONDITIONS WE EXPERIENCED A RAPID 30 DEG R BANK BY THE AUTOPLT WHICH WAS STOPPED BY THE FO, PF DISCONNECTING THE AUTOPLT. THE ACFT WAS TRACKING BOTH LOC AND GS, ANNUNCIATING TRACK/TRACK. CONFIGN WAS 170 KIAS, FLAPS 15 DEGS, ALT 1800 FT AGL. THE AIR WAS SMOOTH. FLT GUIDANCE WAS SELECTED TO #2 DGFS. THE CEILING WAS BETTER THAN 1000 FT AND VISIBILITY WAS 2 1/2 MI IN FOG. THE ACFT WAS PLACARDED CAT 2 ONLY DUE TO THE CAPT'S NO AUTOLAND LIGHT INOP. AFTER THE FO DISCONNECTED THE AUTOPLT. WE CONTINUED THE APCH MANUALLY. I INITIALLY SUSPECTED WAKE TURB AND ON APCH ASKED THE TWR THE ACFT TYPE AND DISTANCE IN TRAIL. WE WERE 3 MI BEHIND A B737. ON THE GND, WE RPTED THE INCIDENT TO GND CTL TO PASS ON TO THE TWR. I REQUESTED A MAINT CHK OF THE AUTOPLT SYS BECAUSE I WAS NOT CERTAIN WHETHER IT WAS A WAKE TURB OR AUTOPLT PROB. THE ACFT DID NOT PASS THE RETURN TO SVC CHK. THE MACH TRIM COMPENSATOR WAS PLACARDED AND THE #2 DFGS COMPUTER WAS REPLACED.

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.