Date: 201402
Local time of day: 0601-1200
Location: MMTO.Airport, FO
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Widebody Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
ATC / advisory: TRACON MMTO
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Fatigue; Communication Breakdown; Distraction
Second reporter function: Captain
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Airport; Human Factors
Primary problem: Human Factors

Narrative: We inadvertently descended along the localizer below glidepath to an altitude that was below published Glide Slope Intercept Altitude (GSIA). We were flying a visual approach when cleared for the ILS instead of executing the ILS. While flying the flight planned course on UJ45 for arrival into Toluca; we were cleared to descend from 15;000 to 12;000 MSL and simultaneously cleared for the ILS 2 Runway 15 approach into MMTO. The aircraft was in NAV mode pilot flying had selected airspeed edit to 220 KTS. At this point we were in PROF mode NAV Mode and descending normally. The FMS had been properly programmed by the pilot flying; we reviewed the route and approach programmed in the FMS per FOM/PHB. We were on a course to intercept the final approach course at the 14.0 TLC/12.7 I-TLC (IF) with no problems. When cleared for the approach; we transitioned to a visual approach and started a slight right turn to a shallower intercept angle for the LOC. The pilot flying had set 12;000 in the FCP and approach land; while in NAV mode between PST VOR and very close to the IF. I suggested setting 8;663 in the alt select window of the FCP; which is the DA for the approach; which the pilot flying did and I confirmed. Once cleared for the approach we transitioned to a visual backed up with the ILS for all intents and purposes and should have been in NAV Prof to MDA basically (cleared; established and protected) until established on the ILS then select approach land to finish the approach that we were conducting visually. We were cleared for the ILS but transitioned to a visual approach. This was where; I think; we made our mistake. We may not have been established on the published procedure before we set the DA. Nor did I check that we were still in PROF. I am sure we were when we set DA and I am not sure when we exited PROF; we were looking outside primarily. I'm not sure if we were in PROF or not; to be honest. We were both looking outside conducting the visual approach; it again was a good VFR day situation; nothing inhibiting our visual reference with the terrain or the airport environment. I thought we were in PROF; but couldn't possibly have been in retrospect because the aircraft did not descend along the defined/programmed VNAV path later in the approach. Again; I honestly don't remember. It was the pilot flying's first time flying into MMTO on the aircraft and due to the DVFR weather conditions and the fact that we had the landing runway in sight; both of us were looking outside noting the terrain proximity and landmarks associated with the approach; noting the published 9;912 FT terrain feature just south of our flight path; commenting that it was a good reference point for arrival into MMTO and that we will turn just past it to intercept the final approach course. I even discussed that if we were to have come in high and had a greater rate of descent; which ATC will do to us and if were IMC we could get a GPWS terrain warning in this proximity. Just discussing flight related issues in the area. In general [we were] taking advantage of the DVFR environment to familiarize the pilot flying with the aspects of the approach and terrain. We intercepted the final approach course without issue; with the published DA set in the FCP; the aircraft was descending at a normal rate. We were below the 12;200 MSL IF published altitude because we were cleared to 12;000 by MMTO Approach. But; we were below 12;000; between 12-14 DME basically in a controlled rate of descent below the glidepath. I'm not sure what altitude we were at; but we were not dangerously low and above the 10;600 MSL GSIA. Well aware and clear of the depicted terrain on the approach plate; we were referencing this and identifying it visually to enhance area familiarization. Again we were intending to descend and intercept the glidepath at GSIA while shooting the visual approach. With respect to being below the glidepath/published altitudes; inside of 12 DME (mybest recollection); I noticed outside visually that we appeared low; I came inside to check our altitude and saw we were approaching 10;000 MSL; verbally identified/announced that we were low and needed to stop our descent; almost simultaneously we got a GPWS 'terrain; terrain' announcement; and ATC queried us about our below glidepath situation. We had unintentionally descended to approximately 10;000 FT/1;500 AFE we were supposed to be at 10;600 MSL. The pilot flying responded; immediately acknowledging making a smooth corrective action by disconnecting the autopilot and climbing back to 10;600 MSL. I set 10;600 in the FCP alt select window. After a brief discussion the pilot flying; decided he would hand fly the rest of the approach. When ATC queried us regarding our departure from the published glideslope; I acknowledged that we were aware of it; leveling at GSIA 10;600 MSL and proceeding with a visual approach and would intercept the glideslope at GSIA per the course of action we decided upon. The Controller responded 'Roger' and switched us to Tower. During the time ATC was alerting us to our glidepath deviation; we received a GPWS 'terrain; terrain' warning. I believe it was due to terrain located laterally to the left of our position between 10-12 DME from TLC VOR because we were established on the localizer at approximately 500 FPM ROD and there was absolutely no terrain in front of us. We were VMC; very aware of the terrain present laterally; felt we were safely clear of it and it wasn't an issue and at no time did we remotely feel that we were in an unsafe situation. In summary; we were cleared for the ILS and shot a visual backed up with the ILS; our mistake; no doubt. At the time considering the environmental/visual conditions it wasn't unnatural to either of us; we both transitioned to that course of action. I truly think we were predisposed to conducting a visual backed up with the ILS and when cleared for the approach; descend to published GSIA. We do it routinely and even though ATC cleared us for the ILS; we heard that and read it back; that predisposition combined with our level of fatigue led us to execute a visual; intending to use the ILS as a backup. This drove our scans outside. I did notice we looked low and as I checked inside; that is when I announced we were low and almost at the same time we received the GPWS alert and ATC query. We were never in any doubt as to the safety of the aircraft; we continually kept each other situationally aware/informed of our airspeed; rates of descent which were never excessive; and terrain clearance both laterally and vertically. We were not rushed in executing our checklists; or configuring the aircraft. We adhered to sterile cockpit requirements; the conversation was professional and related strictly to the approach and arrival into MMTO. It was a comfortable well paced; evolution. At no time did we feel a go around was warranted after the GPWS alert; we were day VMC all terrain was acquired and we had the runway in sight. We reestablished the correct altitude and continued the approach. I do believe fatigue played a factor to a moderate extent in this event. We were part of a wave launch due to deice operations conducted [at our departure airport] causing a late departure. We took off 3:15 minutes late. Both of us had discussed the fact that neither had slept well prior to show time; but both agreed we were fit for duty. There was no doubt that we were tired arriving into MMTO. We did extensively brief and discuss the arrival and approach prior to TOD. While discussing this with the Captain extensively; we both realize we were fatigued more than we thought as we try to recall specific details regarding this event with concentrated effort. Regarding human error; we executed a visual backed up with the ILS instead of the complete ILS; the approach we were actually cleared to shoot. I firmly believe we were more fatigued than either of us realized. We heard ILS; but were VMC and performed a visual backed up with the ILS; inadvertently descending below GSIA momentarily. As pilot not flying; I should have based my scan inside more cross checking the instruments than looking outside. [Pilots must] be very aware of the insidious effects that fatigue will have on their decision making and the fact that we will commit more errors in this condition as a crew. I would say; verbally re confirm the approach clearance and emphasize what that actually means...even if you're VMC; clearance to shoot the ILS is not a clearance to conduct a visual backed up with the ILS; which happens the vast majority of the time to us. Be vigilant. As pilot not flying; remain inside more during a visual approach.


Second narrative: Safety was never an issue.  We remained VMC and in contact with the airfield environment at all times.


Synopsis: A Widebody cargo flight crew descending in VMC; but cleared to fly the ILS DME 2 to Runway 15 at MMTO descended below the glideslope intercept altitude of 10;600 MSL prior to intercepting the glideslope. ATC broadcast a low altitude alert and they climbed back and completed the approach. Fatigue was cited as a contributing factor.