VNAV logic....

[cagarini]

I picked the "climbing above 10,000'" situ, and started playing with constraints...

At situation start the aircraft is heading towards NOXAD with estimated 300/FL147, and it's only 5NM away from the fix, so, I decide to go for SINDA, the next waypoint, and manually enter 330/FL180 replacing the estimated 300/FL233, and EXEC uting...

Modes in the PFD continue as THR REF || LNAV || VNAV SPD but aircraft never accelerates towards the 330 KIAS, even after levelling off at FL180 still some 8NM away from SINDA.

Repeating the same scenario but this time setting 330/FL180 on the FMC, and making an MCP speed intervention to 330 after crossing NOXAD, the AFS modes remain at THR REF || LNAV || VNAV SPD  and the aircraft accelerates towards 330.

If I push the speed knob and blank the speed window, speed returns to 300 for the next fix, but leaving the 330/FL180 at SINDA that I had manually entered.

I thought that entering a speed / alt restriction in the FMC would result in speed being increased or reduced towards the target speed at an upcoming fix ?

[cavaricooper]

Is 300 your entered VNAV speed (VNAV pg)?

C

[Avi]

Enter 280/FL180 and see what you get (especially in the VNAV page).

[emerydc8]

I don't know if this is related, but I am also unable to set a Cruise Speed Segment using the LEGS page once level in cruise (e.g., when entering .85/ between 50W and 40W, I get INVALID ENTRY). I never use the LEGS page to set a cruise speed (I just use VNAV 2/3), but still I think you should be able to do this on the LEGS page.

Try the Climbing Above 10,000 situ and when you finally get to FL310, see if you can enter a cruise speed on one of the segments using the LEGS page. I'm not having any success.

[ADDENDUM]:

Enter 280/FL180 and see what you get (especially in the VNAV page).

If you put in 280/FL180 at SINDA, it will slow to 280kts right away (even before crossing NOXAD) and VNAV 1/3 will display AT SINDA 280/FL180.

Also, if you put in a constraint further down the road, past NOXAD and past SINDA, say you insert 280/FL200 at LUCAS, it will slow to 280 immediately (even before crossing NOXAD). At that point, LUCAS is still about 60 miles away.

[Hardy Heinlin]

Jon, the 744 FMC does not allow speed constraints in the cruise legs, only in legs before the T/C and after the T/D.

Also, the 744 FMC does not allow Mach entries in any legs.


|-|ardy

[Hardy Heinlin]

... aircraft never accelerates towards the 330 KIAS, even after levelling off at FL180 still some 8NM away from SINDA.

Speed constraints are always maximum speed limits. That is, they command a speed reduction if your selected speed is higher than the limit. They never command the speed to increase.

Your ECON speed is 300 and your constraint is 330. 300 does not exceed the 330 limit, so it stays in ECON 300. That's correct.

To see an effect, enter a cost index of 9999. This sets the ECON speed to 345, but the command speed on the PFD will be 330.


|-|ardy

[emerydc8]

Jon, the 744 FMC does not allow speed constraints in the cruise legs, only in legs before the T/C and after the T/D.

Also, the 744 FMC does not allow Mach entries in any legs.


|-|ardy

You are right -- it's only available on the NG.

The 747-8 FMC has cruise speed segments as a new feature. Cruise speed segments are constraint Mach or speed mode entries made on cruise waypoints.

Honeywell NG FMS Pilot's Guide 3-77 (Sept. 2014).

Speed constraints are always maximum speed limits. That is, they command a speed reduction if your selected speed is higher than the limit. They never command the speed to increase.

This is really good info! I didn't know about either of these. I guess that's because I've never tried either. Thanks, Hardy.

[cagarini]

Thank you all - excellent way of learning :-)

In the situ I was actually way before TOD, at NOXAD, but as you pointed out, and I wasn't aware off, what pervails is speed set at VNAV pg, namely ECON 300.

This is why when I speed intervene, speed does change towards the selected ( in the MCP ) value, but then promptly returns to 300 in this situ.