Flight Crew Air Transport Pilot (ATP)
Fatigue; Human-Machine Interface
Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Human Factors
Flying visual approach; with GPS RNAV as backup as well as PAPI; cleared for the visual with traffic in front. First Officer is fairly new to airplane. I had to talk him through setting up the FMS for the approach as we were cleared for the visual; with a request to maintain 170 KTS to the FAF. He got it set up and I armed NAV; and joined the approach. We were established on course so I dialed in the MDA. I then transitioned outside looking at the traffic in front of us. I had previously flown this approach in IMC and had noticed that the track did not align with the runway and the vertical path started us down early; so I was looking to see if it did it again. Autopilot was still engaged. I noted that the NAV had us aligned between 36L and 36R; which was the same as I had previously experienced. I also noticed that we seemed to be low; on the approach; which also matched up to what I had seen before. A moment later I realized that it was quite low (below a 3 degree glide slope) and then that we had not crossed the FAF. I then realized that I had neglected to re-engage PROF [MD-11's version of VNAV is called PROF]; we had been in PROF; but then ATC gave us a descent; so I had transitioned to a V/S as we were off the path. We had descended a couple of hundred feet under the FAF altitude before I noticed it; turned off the auto pilot and returned to the path. At the same time the jumpseater said 'you are below PROF' continued to uneventful landing.While being tired was probably part of this; a few things contributed to it. A new (to the airplane) First Officer; who was a bit behind and missed it; I was on a visual; so really heads up. In IMC this would not have happened. There is nothing really wrong with our procedures; this was an error that was captured; and it just should have been captured sooner. My expectation bias that the approach had problems contributed to me wrongly assuming that it was the procedure design and not operator error that was causing the deviations (vertically; anyway). Having to spend more time watching the First Officer to ensure he was doing all that he needed to do added some distraction. The visibility was hazy; so we did not have a very clear picture of the runway; PAPI; etc.; and that also led to the illusions that it was not quite a far off as we were. This would make a good simulation event as there are some valuable lessons learned as to flying a visual more like IMC.
MD11 Captain reports getting low during a visual approach in hazy conditions with the autopilot engaged and ostensibly programed to fly the GPS RNAV approach. Vertical speed had been used for descent and PROF had not been reengaged to follow the vertical path.