Date: 201404
Local time of day: 1201-1800
Location: IAD.Airport, DC
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight phase: Final Approach
Airspace: Class B IAD
ATC / advisory: TRACON PCT
Aircraft component: ILS/VOR
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Human factors: Confusion; Human-Machine Interface
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I had set up both NAV radios prior to Descent Checklist being accomplished and they were rechecked on the Approach Checklist (for Runway 19L). Sometime after the Approach Checklist the First Officers LOC frequency was changed from 19L to 19C. I know I didn't do it and my First Officer does not recall having done it. The aircraft seemed reluctant to track the extended centerline that we observed on the Map display; it seemed to want to stay to the right of it for some reason. It showed that the LOC was captured; but didn't want to come back over to the displayed magenta line. The Tower asked if we were going to [Runway] 19L and I told them yes and we were correcting left; but the aircraft just didn't want to come back to the left. I instructed my First Officer to disconnect the autopilot/autothrottles and fly the raw data. About this time we descended below the broken layer of clouds and saw that we were slightly right of course; I looked at the frequency in the First Officer's NAV radio and it was the frequency for [Runway] 19C. My First Officer corrected visually and landed without incident.I called the Tower Supervisor on my cell once we were at the gate and asked if any further conversation was necessary about our approach. He said he had heard nothing from Potomac Approach or the Tower controllers and that there was apparently no problem. I thanked him for his time. I occasionally see the autopilots take a little extra time to line up on the extended centerline; but this is usually when the intercept angle is a bit excessive; which was not the case with our turn on; but this is what I was assuming was the problem. Once again; it didn't enter my mind to check the NAV frequency a third time; but that was the problem. And just to confirm; [Runway] 19L was loaded in the FMC and was displaying the correct extended centerline; but since the First Officer's Flight Director was the Master and APPR was selected (and not LNAV) the autopilot followed the incorrect NAV/LOC information.




Synopsis: B737 Captain reports setting up for for the ILS 19L with the First Officer flying and discovering during approach that the First Officers NAV radio has been switched to 19C ILS. Both ATC and the crew detect the deviation but the cause is not immediately discovered.