Date: 201412
Local time of day: 0001-0600
Location: PDX.Airport, OR
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: TRACON P80
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Workload; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Weather; Human Factors
Primary problem: Human Factors

Narrative: On approach to the airport we were given an alert by ATC regarding heavy rain on final approach. There was only one other aircraft on approach at the time; 10-12 miles ahead of us. Turning base via assigned heading we were 1500 feet + below the ceiling in our area with good visibility to the airport landing runways. I informed approach control of this and they offered a visual approach however we were also 10-12 miles out at 3000 feet with some lower clouds ahead if we squared off the approach to a straight in final and the Captain decided to stay on the heading. He was also concerned that we get into the heavy rain condition unnecessarily and requested lower and was refused by ATC; but given a 10 degree heading change and a descent from 3000 feet to 2500 feet instead. He began having trouble with the automation and the plane came out of vertical speed and heading select to control wheel pitch and steering. We were then in a descent to 2500 feet and heading change for an intercept of the localizer. He was asking me to verify the correct frequencies for the approach as well as reestablish the automation that he was having trouble with; in addition was the gear down landing checklist. I am not sure why the automation came out of heading select and vertical speed but I initially just thought that this was a technique that the Captain preferred and I focused on the tasks I was given. When trying to reestablish HDG SEL and VERT SPD I was having trouble but I think this was primarily due to contrary control wheel inputs.When I looked out I noticed we were low on the approach because of visual profile as well as the VASI's indication being all red so I checked our altimeters and we were indicating 900 feet low from our last assignment. About the same time ATC asked for a confirmation of our altitude. The captain confirmed we were still visual with the airport and would continue with the visual approach to the runway. I believe there was some confusion with the captain about the visual approach; his desire to avoid any weather problems and problems with the automation while in CWS PITCH and CWS ROLL led to a loss of situational awareness and altitude. I think that the distraction of the automation going into CWS PITCH and CWS ROLL by design; default or system fault under the circumstances led to a loss of situational awareness; for the pilot flying and by direction the pilot not flying; at a critical time in the approach while other tasks needed to be managed as a priority.




Synopsis: B737 FO reported getting low on a visual approach to PDX when the pilot flying lost autopilot mode awareness.