Date: 201708
Local time of day: 1201-1800
Location: MCO.Airport, FL
Flight conditions: VMC
Light: Daylight
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B MCO
ATC / advisory: Tower MCO
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Confusion; Fatigue; Workload; Human-Machine Interface
Second reporter function: Pilot Not Flying; Captain
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Detector: Automation Air Traffic Control; Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: We were assigned the PIGLT Four RNAV Arrival to land Runway 36L. 36L only has an RNAV approach so that's what I set up for even though the conditions were VFR. On the downwind leg of the PIGLT arrival we were given clearance to descend to 1600 ft and turn to a heading for an intercept to the final approach course 36L. When we reported the runway in sight we were cleared for the visual 36L. I set the VNAV for the RNAV approach and shortly after the captain noted that it might've just been easier to go direct to BERDY (the outer fix). I checked to make sure that the course line was going to intercept the extended centerline which it did. As we rolled onto final the captain asked 'why are we still descending?' At that point I realize that even though I had selected VNAV it had not engaged. I disconnected the autopilot; applied power; and initiated a climb. While I was in the process of this procedure the tower announced an altitude alert for us. When we leveled off; due to the fact that we were in VFR conditions; no other traffic around; and in clear view of the runway I elected to continue the approach visually. We continued to configure the aircraft for landing but before I could call for flaps 30 (final flap configuration) the aircraft announced 1000 feet. At that point; we were on the extended centerline; on the visual glidepath; configured; and on airspeed. I elected to continue the approach and made a normal landing.At the point of this event we were on day four of a four-day trip. All of our sign ins had been late with long overnights. However; the night before this event we were scheduled to have a 10:35 overnight which got reduced to 10 hours as we were running late. We were three hours out of Home Base Time; getting up early in the morning. In hindsight; and after a good night's sleep; I can't believe that I didn't execute a go around at the first sign of trouble. Obviously; I was more fatigued than I realized at the time.


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


Synopsis: Air carrier flight crew reported getting low while on a visual approach to MCO Runway 36L when VNAV was selected; but it did not engage.