B737-700
Flight Crew Last 90 Days 203; Flight Crew Type 400
Communication Breakdown; Human-Machine Interface
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Aircraft; Procedure; Human Factors
Approaching [our destination] from the south we were given a late descent due to crossing traffic. We were then given multiple course changes and vectors through the Localizer in an attempt to lose altitude. We were cleared for the ILS 1L approximately five miles south of the FAF [while] at approximately 4;000 feet MSL. As we continued our descent to [cross the FAF at] 3;000 feet MSL we descended from above to below the glideslope in Level Change mode and the autopilot captured the Localizer. I had Approach mode selected but I did not verify that the glideslope or our 3;000 feet MSL altitude was captured [when] I selected zero in the MCP window and the aircraft continued its descent to approximately 2;800 feet MSL at which point I deselected the autopilot and began a climb to 3;000 feet MSL to re-intercept the glideslope at FAF crossing altitude.I should have verbalized setting zero in the MCP window which would have given the PM a chance to cross check that we had not captured the glideslope. We were both busy at the time from configuring the aircraft and communicating with ATC as we tried to descend and slow down.
As Pilot Monitoring when ATC gave us a frequency change to contact Tower; just as we were intercepting the [localizer]; I looked down to switch frequency and apparently the PF [had] zeroed out the MCP altitude and I didn't see it due to looking down (frequency change). But we hadn't yet captured the glide slope; so the aircraft continued a descent below the FAF altitude of 3;000 feet MSL down to about 2;800 feet MSL. That was a mistake----assuming the G/S had been captured. I made a comment quickly to 'climb; climb; climb----back up to 3;000 feet MSL' to recapture the glideslope. ...since we were so close to the FAF; by the time we got to 3;000 feet MSL we were then too high because the G/S started coming back down; so a slight pushover was needed to re-establish the descent path. Total G/S displacement was no more than about 1-1/2 dots; so a go-around was not initiated. The approach path was regained from that point and aircraft approach path was stabilized by 1;000 feet MSL at which point we had visual contact with the runway.
Following a hurried descent due to traffic and receipt of a clearance to the ILS the PF First Officer of a B737-700 intercepted the localizer but set the MCP altitude to zero prior to confirming GS capture. The FMS remained in Level Change and the flight descended below the glideslope before the flight crew noted the error and corrected.