Date: 201307
Local time of day: 0601-1200
Location: PDX.Airport, OR
Flight conditions: IMC
Weather elements / visibility: 10
Light: Dawn
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class C PDX
ATC / advisory: Tower PDX
Aircraft component: Autoflight System
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Confusion; Distraction; Troubleshooting
Second reporter function: First Officer
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Contributing factors: Human Factors; Procedure; Weather
Primary problem: Human Factors

Narrative: Prior to Top of Descent; we acquired the ATIS (310 KTS; 10 SM; 008 OVC); loaded the FMC (ILS 28L); ran the performance numbers; briefed the arrival (MOXEE 6); and completed the full approach briefing for the planned approach. After flying the MOXEE 6 Arrival and a handoff to PDX Approach Control; we received a vector to join the LOC 28L. We confirmed with ATC that the assigned approach was the LOC 28L rather than the ILS 28L that we were planning. Not having noted prior to arrival that the Runway 28L glideslope was listed in NOTAMs as out of service is an error that I take full responsibility for. I should have made myself more aware of all items pertaining to the flight. I saw it in the weather package NOTAMs listed as WEF. It is also my usual routine to read the destination ATIS from the weather packet as part of normal preflight planning; which did show the 28L GS OTS. I don't recall having read that. Additional review of the landing ATIS enroute should also have alerted me to the need to load and plan for the LOC 28L rather than the ILS 28L. Every opportunity was provided to me to be properly prepared for the appropriate approach; however; we arrived in the PDX area expecting to fly the ILS as briefed not the LOC. We then prepared for the LOC 28L by briefing what we thought were the pertinent changes to the approach procedure. Using the same chart we had already briefed; we reset the altimeter bug to the appropriate DDA (530 feet); reviewed the approximate descent rate (700 FPM); and selected an intercept course to HANAH; the fix prior to the FAF. We were then given a clearance to descend to 2;500 feet and a final vector to join the approach course. After review of the LEGS page; we determined that because we were joining outside of HANAH; which the chart depicted as a minimum crossing altitude of 2;900 feet; that we would stop the descent at 2;900 feet until crossing HANAH; which we did. We also confirmed that the autopilot captured the course in VOR/LOC mode as verified by the FMA. We then descended to 2;000 feet and continued with the landing configuration of the aircraft and slowing in preparation of reaching the final approach fix ADDUM.Crossing ADDUM; the FAF; I set zero in the MCP; however; what should have been set was 1;040 for YINUP; an intermediate step-down fix that we did not notice during our abbreviated re-review of the approach chart. I will make no excuses for this oversight. This is obviously no small matter. The better choice on my part would have been to request a vector off or through the course; or turns in holding; prior to joining the approach to allow myself and my crew the opportunity to catch up with the situation and do a full review of the approach chart as a LOC approach rather than an ILS. This would have also provided the time necessary to load the LOC 28L approach into the FMC. I have no doubt that the slight inconvenience would have substantially improved our chances for avoiding this serious oversight. Further complicating the situation; not long after we started our descent; it became apparent that the autopilot was no longer tracking the Localizer needle. This was first noticed by the deflection of the course indicator on the CDI and then confirmed by the FMA indicating CWS ROLL. I disengaged the autopilot at that time; turned to re-intercept the course; and re-selected VOR/LOC on the MCP. I have no idea what might have caused the autopilot to drop out of VOR/LOC mode. I can only speculate. My only guess at this point is that I might have bumped the switch when selecting zero in the MCP. But that theory is only based on the knowledge of how the system works not anything that I had noticed at the time. I must have also pitched down slightly; while turning right as indicated by the Flight Director and then reduced the power slightly to correct for the increase in indicated airspeed. Not wanting to get high or fast on the approach. A better choice would have been to correct back to the course with the autopilot engaged using HDG SEL mode to re-capture the course; this would have maintained our proper rate of descent. It was about the time we started getting the course to come back in that we broke out below the overcast layer; still slightly left of course. We acquired the runway visually and it was obvious we were below the normal glidepath. Still hand flying the aircraft; I leveled somewhere between 850 feet and 900 feet MSL while remaining in VMC conditions. It was about this time PDX Tower called us with a low altitude alert; we reported airport in sight; rejoined a normal descent path with the aid of the PAPI; and continued to a normal landing.In review; it is easy to identify many causal factors that helped lead up to the event described; many of them too slight or insignificant in and of themselves to warrant mentioning. Expectation bias and attention to detail were factors; as well as a false sense of actually proceeding in a safe manner when in fact we were in the yellow and didn't even recognize it. At the time; I felt that we had considered all that was necessary to safely complete the flight. This of all things concerns me the most. Looking back at our choices; probably the best opportunity I had to make a significant break in the chain of events was to delay beginning the approach and conduct a proper review before proceeding. I would like to think that the time spent performing a complete approach briefing as directed by the AOM in Chapter 10 would have improved the likelihood of being properly prepared for the approach and dramatically improved the outcome.


Second narrative: [This reports contains no additional information.]


Synopsis: A B737 crew briefed a PDX Runway 28L ILS after failing to notice the glideslope was out of service and when cleared for the 28L LOC they got behind the aircraft until ATC notified them of a low altitude alert.