B737-700
Flight Crew Last 90 Days 114; Flight Crew Type 7000
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
Weather; Aircraft
ATC issued a descent to cross BEARR Intersection at 17;000 FT. We were previously on profile so we set the constraint altitude in the MCP and continued with our descent. The local altimeter setting was 30.50 as indicated by ATC and 30.51 (ATIS altimeter setting) was previously entered into the descent planning page in the FMC. As we approached 18;000 FT; we reset our altimeters to the local setting and began the Descent Checklist. Sometime during the execution of the Descent Checklist and our passing of 18;000 FT; the VNAV disconnected and the aircraft was subsequently high on the descent path. As we were very close to our crossing restriction; there was not enough time for manual intervention to be effective and we crossed BEARR 300 to 500 FT high. We were both surprised that the VNAV would disconnect as we were on speed and had verified that the local altimeter setting was entered correctly in the descent forecast page. My suspicion is that the transition to local altimeter-based altitudes for VNAV PATH computation was too abrupt for common VNAV's pathetically narrow window for tracking the computed path. The issue was probably exacerbated when we reset our altimeter settings out of sync with the FMC's transition; which is not visibly evident to the crew. There are serious usability issues with common VNAV due to the lack of salience of mode transitions and the narrow altitude/speed ranges in which the VNAV will track the computed path. I also believe that the common VNAV is somewhat buggy as I have witnessed similar issues where the FMC will suddenly generate a large deviation from the previously computed path to a nearby constraint; to be followed by a VNAV disconnect with minimal time to intervene (e.g.; MAIER/EAGLE arrivals into Phoenix). I feel that the use of common VNAV should be re-evaluated. Promoting awareness of this failure mode would also be beneficial.
B737-700 Captain describes a VNAV disconnect descending through FL180 with a high barometric setting that he believes reveals a basic flaw in the B737 auto flight system.