777 VNAV descent logic
A few questions for the 777 gurus regarding the use of speed intervention. I understand that in descent, this causes PTH to revert to SPD, but my confusion is regarding how it this reversion affects the resulting profile.
Let’s assume we’re on profile in PTH at FL200 at 280kt. There is a crossing restriction of FL150 at the next fix and the MCP is set to FL100.
Scenario 1. If we use speed intervention to slow to 250kt, we end up above the profile. On reaching 250kt, does it attempt to recalculate a new profile, or just say DRAG REQUIRED and cross the fix above FL150?
Scenario 2. If we use speed intervention to accelerate to 310kt, will it add power to get to 310kt, then go back to idle once it reaches a new idle profile to the fix? Will it get to 310kt, and then descend at 1200fpm until intercepting the original profile? Will it simply do an idle descent at 310kt to FL150, then drive to the fix in VNAV ALT?
Is there a situation in which using speed intervention could cause VNAV to bust an altitude constraint?
I’ve tried looking for a bit more detail in the FCOM and FCTM, but with no luck. Are there any techniques you prefer using when a speed change is needed on a STAR? Would appreciate any document references available.
Thanks in advance!