B737-500 reports possible uncommanded rudder deployment just passing outer marker at dia.

Attributes
ACN	448871
Time
Date	199909
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : den.airport
State Reference	CO
Altitude	agl bound lower : 1000 agl bound upper : 1500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : dia.tower
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Flight Phase	descent : approach descent : vacating altitude
Route In Use	approach : visual arrival : vfr
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 15000 flight time type : 4600
ASRS Report	448871
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : commercial
Events
Anomaly	inflight encounter : wake turbulence other anomaly other spatial deviation
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : regained aircraft control flight crew : executed go around other
Consequence	other
Supplementary
Problem Areas	Environmental Factor Flight Crew Human Performance
Primary Problem	Environmental Factor

Narrative:

First officer flying. Uncommanded rudder deflection. Visual approach to runway 35L at den. 3-4 mi behind a B737 we encountered what seemed to be heavy wake turbulence. Then smooth air for a few seconds the aircraft began to roll right. The first officer corrected with left aileron. Aircraft began to respond then rolled further right approximately 30 degrees and nose down. The first officer appeared to have full left aileron and aircraft was not responding for 2-3 seconds. I called for a go around. Aircraft responded. We landed uneventfully on runway 35R. First officer stated that during the second roll to the right, the rudder deflected 1/4 - 1/2 of its full travel uncommanded. Callback conversation with reporter revealed the following information: the reporter states that as PNF, he first assumed that this incident was due to wake turbulence, but the fact that the traffic being followed was another B737 along with the first officer reactions of full throw on the ailerons caused him to decide to report as a possible rudder problem. As a result, the flight recorder tapes were removed. Boeing personnel, air carrier personnel, pilot group and maintenance personnel were involved. All parties, including the reporter, now agree that the cause was one of wake turbulence from the reported traffic, a B737, 3-4 mi ahead of the reporter aircraft.

Original NASA ASRS Text

Title: B737-500 RPTS POSSIBLE UNCOMMANDED RUDDER DEPLOYMENT JUST PASSING OUTER MARKER AT DIA.

Narrative: FO FLYING. UNCOMMANDED RUDDER DEFLECTION. VISUAL APCH TO RWY 35L AT DEN. 3-4 MI BEHIND A B737 WE ENCOUNTERED WHAT SEEMED TO BE HVY WAKE TURB. THEN SMOOTH AIR FOR A FEW SECONDS THE ACFT BEGAN TO ROLL R. THE FO CORRECTED WITH L AILERON. ACFT BEGAN TO RESPOND THEN ROLLED FURTHER R APPROX 30 DEGS AND NOSE DOWN. THE FO APPEARED TO HAVE FULL L AILERON AND ACFT WAS NOT RESPONDING FOR 2-3 SECONDS. I CALLED FOR A GAR. ACFT RESPONDED. WE LANDED UNEVENTFULLY ON RWY 35R. FO STATED THAT DURING THE SECOND ROLL TO THE R, THE RUDDER DEFLECTED 1/4 - 1/2 OF ITS FULL TRAVEL UNCOMMANDED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THAT AS PNF, HE FIRST ASSUMED THAT THIS INCIDENT WAS DUE TO WAKE TURB, BUT THE FACT THAT THE TFC BEING FOLLOWED WAS ANOTHER B737 ALONG WITH THE FO REACTIONS OF FULL THROW ON THE AILERONS CAUSED HIM TO DECIDE TO RPT AS A POSSIBLE RUDDER PROB. AS A RESULT, THE FLT RECORDER TAPES WERE REMOVED. BOEING PERSONNEL, ACR PERSONNEL, PLT GROUP AND MAINT PERSONNEL WERE INVOLVED. ALL PARTIES, INCLUDING THE RPTR, NOW AGREE THAT THE CAUSE WAS ONE OF WAKE TURB FROM THE RPTED TFC, A B737, 3-4 MI AHEAD OF THE RPTR ACFT.

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.