B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 11000; Flight Crew Type 1317
Distraction; Confusion; Workload; Troubleshooting; Training / Qualification; Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown
Aircraft Automation Overrode Flight Crew; Flight Crew Became Reoriented
Company Policy; Aircraft; Manuals
Enroute to BOS; #2 Navigation radio head failed [to] tune.  The new frequency on the right side would change but would not switch to the active side.  Since the left ILS frequency could not be set; the RNAV (GPS) 22L was selected; briefed; requested and flown.  When we were cleared for the approach; the approach mode would not select.  Despite numerous attempts to select approach; no guidance was achieved.  Fortunately; we were in VFR conditions and an uneventful visual approach to touchdown was flown.  The FOM briefing flow charts and briefing cards were utilized during the brief.  Neither the First Officer nor I could figure out why the approach mode would not engage.  A maintenance entry was made but the autopilot systems checked normally.  I still have no idea why the approach was not successful.  Systems training on the 737 is poor to nonexistent.  I feel ill equipped to understand or predict what the aircraft will do in any degraded system mode.  A modern multimedia systems training course should be designed and implemented to increase pilot systems knowledge of the aircraft.
A B737-800 Captain reported that the number 2 NAV radio head failed precluding an ILS and subsequently the APP mode would not arm for the RNAV (GPS) approach so a visual approach was completed.