MD-11
Flight Crew Last 90 Days 80; Flight Crew Total 13000; Flight Crew Type 6000
Confusion; Human-Machine Interface; Situational Awareness; Training / Qualification
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft; Chart Or Publication; Human Factors; Procedure
I was the pilot flying on our last leg to ZZZZ. We were cleared the RNAV approach. I fully briefed the approach; and emphasized the need to not edit the approach while in PROF because [the FMS] would not [then] adhere to [published] ALT limits. We were on the published approach and were then cleared direct to ZZZZZ [the FAF]. Prior to this we had put min Prof [DA(H) altitude] in the altitude alert window since we were cleared the approach. We were coming in to ZZZZZ at a 25 degree angle or so and; at about 2-3 miles from ZZZZZ; we were descending through 3;000 which was the ALT [crossing] restriction for this fix. I was a bit confused in that I was now thinking that we were descending because we were on the predicted [glide] path; much like when you capture the glideslope [but] are still waiting for LOC capture. It became obvious very soon that we were actually descending because the FMS [mode] had been changed to direct and the ALT at ZZZZZ was being disregarded. I intervened with vertical speed up to recapture the path and get back up to 3;000. At this same time tower called and said we were showing low on the approach and the Captain [responded that we were] correcting. We were approximately 2;450 or so MSL at the lowest based on my recollection. The Captain wanted me to limit my V speed up because he was concerned about noise; I was more concerned with getting back to a safe altitude at this point. We intercepted the path and flew a stabilized approach to a normal touchdown.
The reporter addressed some questions regarding MD-11 autoflight terminology and functionality that clarified the manner in which a small decision to accept a clearance--off of their current charted approach transition--direct to the FAF altered the programming of the active PROF ('profile' or 'VNAV') mode of the autoflight system and; combined with company approach SOP; resulted in the descent well below profile and a timely 'low altitude alert' from ATC.The SOP requires that RNAV/GPS approaches must be line selectable and are to be flown with PROF mode providing vertical path guidance. To facilitate continuous PROF guidance the flight crew is to select the MDA/DA(H) (minPROF in the narrative) in the altitude alert window; so as to allow autoflight to accomplish/direct uninterrupted required step downs; programmed glide paths; etc. Flight crews are cautioned; however; not to modify the line selected procedure once active. One reason for that restriction is that doing so may result in the loss of programmed; PROF directed crossing restrictions and; in addition; vertical path guidance defaulting to another mode (the reporter was unclear on exactly what mode that would be...and; in this case; was). The end result of the above was a Flight Director display; needles neatly centered (flybars in this case) before the modification--telling them everything was perfect--and needles still perfectly centered after the change telling them autoflight was doing exactly what it was supposed to do; now a nicely uninterrupted programmed descent to the DH...which; of course; wasn't what the flight crew wanted it to do and; worse yet; assumed it was doing.During an extensive discussion of CRM friendly SOPs the reporter commented that his preference would be to never select an altitude lower than the FAF/GS intercept altitude prior to capturing that altitude upon reaching the fix itself; only setting the MDA/DA(H) after altitude capture and then rearming PROF/VNAV for the descent beyond that point. As a First Officer; however; he is constrained from doing more than advocating such a procedure and recognizes it is contrary to the current SOP and not likely to be immediately embraced by his Captain.In this case SOP was followed. An error was made earlier in the procedure and the aircraft descended well below the FAF altitude and; had there not been an alert from ATC; recovery back to the FAF/glidepath could have been delayed to as low as 478 FT AGL; well prior to the runway.When asked for more detail as to available displays; alerts; raw data; other clues; etc. might have provided the nudge to re-analyze their situation the reporter drew a blank. It should be noted that the reporter's expertise with the system was nearly encyclopedic and he spoke glibly about a variety of technical scenarios with complete confidence and clear competence. Which renders the 'classic overload question above' even more illuminating.Finally; the reporter offered the information that he was aware of an earlier flight to a European destination that; while shooting a VOR approach using the same procedures had a nearly identical outcome with the exception that their lowest altitude was significantly lower and may have been at the MDA.The callback ended with the clear agreement that it would have been nice to have had the FAF altitude in the alert window.
An MD-11 flight crew cleared to fly an RNAV approach mistakenly modified the line selectable procedure when cleared direct from their transition route to the FAF. When doing so the crossing restriction at the FAF was deleted and the aircraft descended directly toward the DA(H). A low altitude alert was transmitted by the Tower and the flight returned to their clearance and completed the approach and landing.