B757-200
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Situational Awareness; Training / Qualification
Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented
Human Factors; Procedure
During the initial segment of the transition for the approach; after being cleared for the X transition for the ILS 28R; I became fixated on the glide slope indication and used the vertical speed mode to remain one dot below the glide path to allow a short segment to decelerate. In doing so we descended approximately 1000 feet below the published profile; upon which time the pilot monitoring called out the deviation and the descent rate was arrested and the published profile was regained. Lesson was to treat the initial part of this type of approach; although cleared for an ILS; like an RNAV (Area navigation) approach; flying in LNAV/VNAV (Lateral Navigation/Vertical Navigation) using the LAVS procedure until established on final. The above factors in addition to being unfamiliar with RNAV with vertical guidance transitions to ILS final. It seems in training we do either RNAV approaches; or we do ILS approaches (usually with vectors and a descent given to an intercept altitude). This was a blend of both; and my addled mind was used to being in a descent for an ILS approach and the need to remain below glide path to intercept the glidepath from below. Clearly this was not that type of approach clearance/transition.
B757-200 flight crew descended below glideslope during approach.