B737-800
Flight Crew Last 90 Days 152
Situational Awareness; Human-Machine Interface; Workload
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Aircraft; Procedure; Weather
Four times in the last month or so; I have had issues with VNAV performance across all fleet models when altimeter settings are relatively high (30.30 plus); even with proper forecast programming. The crossing restriction on our arrival is 17;000 feet and with high altimeter settings; VNAV PATH will not stay engaged and reversion to MCP Level Automation is necessary to comply with the charted restriction. The decoupling of VNAV PATH often occurs right at the transition level; leaving very little time for Pilot intervention to even be successful. (This time the local altimeter was near 30.60 leaving 700! feet to make up. Luckily; since I was looking out for this issue; I was able to get involved about 10 miles from the fix.) As I said; I have seen this fleet wide and believe a limitation of our VNAV software is to blame. I submit this report for general awareness.
A B737 Captain reported that while descending with very high altimeter settings of 30.30 plus; the FMC VNAV PATH reverts to VNAV SPD at the transition altitude requiring pilot intervention to comply with altitude constraints.