Date: 201512
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: Widebody; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Human-Machine Interface
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: ATC Equipment / Nav Facility / Buildings; Human Factors
Primary problem: Human Factors

Narrative: Vectored toward ZZZZZ (IAF for VOR Runway XXR) at 4;000 feet. Cleared to descend to 3;000 feet; cross ZZZZZ at 3;000 feet; and cleared for the VOR XXR approach. We had plenty of distance to make our 3;000 foot restriction; but the First Officer (FO) selected FLCH for descent. At approximately 3;500 feet; I suggested performing LAVS and selecting 100 feet altitude because we had large-font altitudes in the FMS and we were cleared for the approach. The FO dialed in 100 feet; but stayed in FLCH; which I did not catch. At that point; we were discussing the approach lighting configuration when he noticed we were about 700 feet below our assigned altitude. I directed him to disconnect the autopilot and climb to 3;000 feet; and he quickly complied. As we approached 3;000 feet; the approach controller handed us off to Tower; and no mention was made about our deviation. The remainder of the approach was uneventful.Prior to our arrival; we briefed the ILS Runway YYL. The reason was because the ATIS was advertising VOR YYL approach and the VOR XXR approach; and we believed from reviewing our NOTAMS printout that the ILS for Runway YYL would be on a few minutes prior to our arrival. Also; it seemed to me that the Runway YYL approaches were more conveniently set up for our arrival. I also felt that if they assigned us Runway XXL we could get the RNAV GPS approach. We never looked at the VOR Runway XXR. When we checked in with Approach; he told us to fly 050 heading and expect the VOR XXR. I asked if we could have the RNAV approach instead. He offered us the RNAV RNP; which I declined; and clarified we wanted the RNAV GPS. He told us to stand by so he could see if our requested approach coincided with the VOR approach. After a delay of maybe 3 minutes; he vectored us to a heading of 330 and then informed us we would have to do the VOR XXL; and then after I queried; corrected himself to VOR XXR. We had adequate time to deal with the change from our briefed plan. While the FO flew the aircraft; I loaded the new approach; read the minimums for both of us to set; and briefed the approach. At 4;000 feet; the controller assigned us 3;000 feet to cross ZZZZZ at 3;000 feet; and cleared us for the VOR XXR. The FO selected 3;000 feet and selected FLCH to speed the descent; even though it seemed unnecessary to come out of VNAV because we had plenty of distance to ZZZZZ. Passing through about 3;500 feet; I suggested we complete LAVS and set 100 feet in the altitude window. He set 100 feet; which I confirmed; but I failed to notice we were still in FLCH; which meant the FMS would not honor the programmed 3;000 foot restriction at ZZZZZ. At this point; we discussed the approach lighting configuration; which we could see because it was a clear night. He stated his belief that the turn lights were for our runway. I showed him on the approach chart that those lights were for the turn to Runway XXL. I was heads-down just long enough to miss the altitude deviation. As we resolved the lighting question; Tim noticed we were 700 feet low; and then we corrected back quickly.I feel we have been adequately trained to avoid this type of situation by making sure we have VNAV selected before setting the runway altitude. In our case; I failed to properly monitor the FMAs and ensure we did not select a lower target altitude before selecting VNAV.My thought as to how to avoid this in the future: If the ATIS advertises an oddball approach that I don't think we'll be assigned; at a minimum perform a cursory review of the unique features of that approach. That way; we can avoid potentially distracting conversations after building and briefing the approach.


Second narrative: [Report narrative contained no additional information.]


Synopsis: An air carrier crew does not properly manage the Flight Management System on VOR Approach and descends below IAF altitude.