A Q400 VNAV and LNAV coupling disengaged on an RNAV approach inside the if and while troubleshooting...

Attributes
ACN	1196012
Time
Date	201408
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Q400
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT Inflight Event / Encounter Unstabilized Approach

Narrative:

RNAV approach; at a fix inside the if; lost VNAV and LNAV coupling. Instructed pilot not flying to engage navigation 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.

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.