VNAV Profile and AT Mode

[FLEX1978 373]

Well what is the speed restriction on the SID? On the Lambourne arrival your initial alt restrictions are FL250 & FL150 at LOGAN & SABER.  Check the associated speed restriction.

 

Cant test the arrival, I am enroute to EGNX  

 

http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=94&Itemid=143.html

 

Strange that you are having issues, VNAV has been working fine for me with almost 200 hours 

 

EDIT - Here's a little info to read..

 

Using the Speed Intervention switch will revert to VNAV SPD, where the aircraft will fly a target speed, and in some cases disregard the path. For instance, when in VNAV PATH, if you are above target speed and there is a speed restriction approaching, it gives the path priority. 

 

 

In the B777, when descending in VNAV PTH mode after T/D, speed intervention (i.e., pressing the IAS/MACH selector) results in a mode change to VNAV SPD. Automatic mode reversion from VNAV PTH to VNAV SPD occurs in certain under- or overspeed conditions (before switching to VNAV SPD, the system first allows the aircraft to deviate up to 150 ft from the computed path in an attempt to correct the airspeed). VNAV SPD is also used for early descents ("DES NOW" prompt on the FMS or altitude intervention when within 50 NM from T/D); in that case, VNAV PTH is automatically re-engaged when the computed idle descent path is captured.

In general, descent modes can be divided in two classes: speed-on-pitch and speed-on-thrust. LVL CHG is speed-on-pitch, V/S and FPA are speed-on-thrust. For the VNAV PTH descent mode it is less obvious what's actually happening: in VNAV PTH descent the aircraft basically follows a computed idle thrust descent path. But, while the initial autothrottle action (closing the thrustlevers) appears identical to what happens in LVL CHG, VNAV PTH is not a speed-on-pitch mode! It only appears that way as long as the conditions on which the FMS based its path computations are identical to the actual flight conditions. In fact, VNAV PTH is basically a sort of speed-on-thrust mode, except that it assumes idle thrust for the entire descent and pilot inputs (additional drag or thrust) are required if airspeed corrections are necessary. 

[3-2-1-Now 126]

OK well that gets even stranger then.

 

In VNAV PTH why can I sometimes use SPD INTV and it remain in VNAV PTH, and other times it will fail to transition from VNAV SPD to VNAV PTH when *NOT* using SPD INTV?

 

Best regards,

Robin.

[voske 36]

Speed restriction on the LAM3A arrival is 250 knots at 12 miles from LAM.

[777simmer 117]

When you use speed intervention, then you are no longer in Vnav Path.

This is normal and as in real life.

 

Once you get passed a certain point though, the FMC (or maybe I should say the "logic") switches to "on approach" and from then on you can use speed intervention to control speed according to your flap setting WHILE Vnav stays in Vnav Path mode :-)

(If you dont use speed intervention while using Vnav on approach, then you will see the target speed jumping to succesively slower speeds even if you dont extend the flaps......far from ideal and thus we use speed intervention there)

 

I am not sure which waypoint has to be sequenced for the "on approach" mode to activate, but I think it is described somewhere in the FMC chapter. (I would think it is the initial approach fix though).

[3-2-1-Now 126]

Hi,

 

Yes - it seems to be when passing a waypoint and it sequencing the next that it switches modes, but in my test flight I missed when it did it. I'll fly it again.

 

Thanks for confirming VNAV SPD mode when in SPD INTV.

 

Best regards,

Robin.

[777simmer 117]

Ok, page 11.31.30 of FCOMv2.pdf hold the info we need :-)

 

A couple of points there put the logic in "on approach" mode.

Sorry that I did not know them by hart anymore (I should know them).

 

I am not going to copy and past things from the manual as this is not allowed.

But one additional thing that puts you "on approach" is extending flaps!

 

Good stuff.

 

 

Also impressive that you picked up on these little subtleties.

For many that come from older Boeing types, one of the things they have to really get used to is looking at that FMA (Airbust requires this as well).

Not the MCP tells you what the airplane will do, but the FMA!

 

Well done :-)

[FLEX1978 373]

I am personally fond of SOPS that require readout's of FMA changes.  Although not SOP on the 777 I still read out the changes in my head like I am flying Airbus.

[777simmer 117]

I am personally fond of SOPS that require readout's of FMA changes. Although not SOP on the 777 I still read out the changes in my head like I am flying Airbus.

Well, as you know it is all up to the company to create SOPs.

 

I personally like calling out FMA status as well.

Keeps both guys in the loop!

 

It so happens that our company just changed the SOPs!

We now DO call out FMA modes changes :-)

Following suit how most 777 operators do it (I was told), we now call out pitch and roll changes (so boxed FMA modes) and the corresponding MCP value (where applicable) once above 400ft. Speed changes and AT modes are not called out unless they are ordered by ATC.

 

examples (AP engaged):

When Vnav path engages (Vnav Path boxed) - call out "Vnav Path" - followed by "checked" from the other guy.

changing heading (HDG SEL boxed) - "heading 320 set, Heading select" - followed by "checked" from the other guy.

changing altitude to 14000ft (FLCH boxed) - "one four thousand set, Flight level change" - followed by "checked" from the other guy.

changing altitude to 14000ft (Vnav SPD boxed) - "one four thousand set, Vnav Speed" - followed by "checked" from the other guy.

etc.

[FLEX1978 373]

That sounds perfect to me. It keeps you both in the loop, and reinforces your actions.

 

Regards

[3-2-1-Now 126]

Ok, page 11.31.30 of FCOMv2.pdf hold the info we need :-)

 

A couple of points there put the logic in "on approach" mode.

Sorry that I did not know them by hart anymore (I should know them).

 

I am not going to copy and past things from the manual as this is not allowed.

But one additional thing that puts you "on approach" is extending flaps!

 

Good stuff.

 

 

Also impressive that you picked up on these little subtleties.

For many that come from older Boeing types, one of the things they have to really get used to is looking at that FMA (Airbust requires this as well).

Not the MCP tells you what the airplane will do, but the FMA!

 

Well done :-)

 

Thanks. B)

 

I'll have a read of the manual pages you mentioned.

 

Best regards,

Robin.