Q400
Flight Crew Air Transport Pilot (ATP)
Distraction; Confusion; Human-Machine Interface; Troubleshooting; Situational Awareness; Fatigue
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Human Factors; Aircraft; Procedure
RNAV approach; at a fix inside the IF; lost VNAV and LNAV coupling. Instructed pilot not flying to engage NAV and VNAV; twice; all the while descending and not monitoring glidepath guidance. Was confused by the unexplained loss of coupling and distracted by trying to get it back. Although I knew the corrective action (missed approach or climb up closer to the Vpath so it will capture); for some reason; it did not occur to me at the time. I then realized that we were below glidepath; (possibly full scale; not sure now) just as we were breaking out of the clouds. No deviation calls or missed approach call from pilot not flying. Requested and received visual approach clearance; and landed. Might have been 2 previous nights of sub-optimal sleep; leading to some degree of degradation of alertness.
A Q400 VNAV and LNAV coupling disengaged on an RNAV approach inside the IF and while troubleshooting; the Captain lost situational awareness and descended more than one dot below the glidepath and fortunately became VMC for landing.