Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 432
Situational Awareness
Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors; Procedure
We were instructed to cross KIRKM at FL360. The FMC was properly programmed and FL360 set in the altitude window. I noticed that the FMC was only giving us nine miles to lose 4000 FT. I mentioned it to the First Officer and he started a Vertical Speed descent. I was distracted by reviewing the STAR and approach procedures. ATC asked if we were going to make the crossing. When I looked back we were over 1000 FT high with less than five miles to the crossing. I informed ATC we would not make the crossing. He said there was traffic at FL370 and to expedite the descent. This is an ongoing issue with VNAV descents of short duration at high altitude. It comes up with unrealistic profiles. I should have watched better when it was apparent the First Officer was unfamiliar with this oddity of the VNAV system. When asked he said that he had started down at 2200 FPM but then noticed that we were low on the path so he went back up to the VNAV path by slowing the descent rate. In doing so we went back to the original problem.
Air carrier Captain reported they failed to make an altitude crossing on descent into PHX when they failed to monitor FMC performance closely enough.