Speed Intervention?

[akafrank07]

I am unsure what speed intervention is:

If engaging in speed intervention:
- During a path decent with flaps up on an idle leg, VNAV switches to VNAV speed
- With flaps down, VNAV reminds in VNAV PTH
-When a geometric path leg is active, VNAV remains in VNAV PTH
- While a vertical angle leg (GP xxx on RTE LEGS page) is active, VNAV remain in VNAV PTH
- Benefits of using speed intervention:

Gives MCP speed control like on an ILS
Easier to comply with ATC speed restrictions
Easier to make corrections to final approach speed if wind additive changes

What is the difference between 'VNAV' and 'VNAV speed' is it another word for autothrottle?

Why is it when using speed intervention it is easier to comply with ATC speed restriction and easier to make correction to final approach speed?

Thanks

[Denti]

The difference between VNAV and VNAV speed is that in speed the airplane (FMC) will still respect altitude constraints but will not follow the VNAV path anymore and rather behave like in level change mode (idle descent for example).

I is easier to comply with speed restrictions since in intervention the MCP speed selection is active and one can simply twiddle in a different speed whereas in VNAV the speed is controlled via the FMC and any changes in speed would have to be done via the FMC which is not as intuitive. Especially during approach VNAV will simply slow to the relevant min flap speed which might be well below any normally flown speed or ATC advised speed. In more modern Boeings (737NG/748/787) VNAV is not the normal non precision approach mode anymore and speed selection will be done via the MCP anyway (IAN) as the approach is flown in APProach mode, to in VNAV.

[akafrank07]

Thanks for the answer Denti :ok: