There appears to be some confusion here.. " On approach logic" and VNAV.
When the FMC transitions to on approach logic, the RNP changes to a default of 0.5 NM and enables alerting features:
"UNABLE REQD NAV PERF-RNP" alerting levels are higher we check in the FMC to ensure the RNP is suitable for the approach.
MAA can be set when > 300 feet below Missed Approach Altitude, VNAV will continue to command a descent.
If more than 200 feet below vertical glide path, VNAV commands zero vertical speed until intercepting path
If FMC not in on approach logic, the pitch mode will revert to CWS pitch if a higher MCP altitude is set
The FMC will command level flight in VNAV PATH until reaching the descent path.
FMC will transition into on approach logic approximately 2 NM before the Final Approach Fix
Now, back to OP.
If engaging speed intervention:
- During a path descent with
flaps up on an idle leg, VNAV switches to VNAV SPD
- With
flaps down, VNAV remains in VNAV PTH
- When a geometric path leg is active, VNAV remains in VNAV PTH
- While a vertical angle leg (GP x.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH
If one looks at the magenta path indication with the speed window open and VNAV SPD in the FMA you may notice something as the selected speed is higher than the FMC calculated speed for that phase of the profile
