A B737 captain reported that while descending with very high altimeter settings of 30.30 plus; the F...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1243323
Time
Date	201502
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Light	Night
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 152
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance

A B737 captain reported that while descending with very high altimeter settings of 30.30 plus; the FMC VNAV path reverts to VNAV speed at the transition altitude requiring pilot intervention to comply with altitude constraints.