On 10/21/2018 at 7:48 PM, Jetlinker said:

So if ATC says slow from 300 to 280 on an arrival, unlike with the -400, ..............To make such an adjustment with the 400 VNAV means reverting to VNAV SPD, reducing descent rate, and within a short time becoming well above intended path. 

Not necessarily so in the case of the 400, because there is an easy way to do this.  All things being equal, all that you need do to lose the 20 kts during a descent and still remain on the VNAV PTH profile is to amend the FMC speed to 280 kts and carefully apply a little Speedbrake to bleed off the excess speed. 

In simple practical terms, I suppose the main difference in terms of handling a VNAV PTH descent between the 400 and the -8 is that the 400 will usually require more pilot intervention (i.e. small manual adjustments to the thrust or application of speedbrake as necessary to achieve and maintain the desired speed and remain on the target descent profile), whereas the -8 will more or less do it all for you - provided of course that in both cases the FMC's are programmed correctly!

It could be argued that the 400's behaviour here is preferable to the -8, simply because it requires more pilot intervention and this will keep them more 'in the loop' and less reliant on the use of the automatics.  However, no matter what aircraft type you are actually flying maintaining a high level of situational awareness, coupled with a sound knowledge of what the automatics are doing at all times, is absolutely essential during every flight and especially so during the take-off, descent and landing.    

Edited October 23, 20187 yr by berts
addition

On 10/21/2018 at 2:08 PM, emvaos said:

(NG FMC) The offidle descent path is constructed slightly shallower than a traditional idle path descent to allow a margin of speed control during the descent. The VNAV descent mode is VNAV PTH and autothrottle SPD for the entire descent profile, similar to today’s 777 or 747-400 on-approach mode logic. This enables the autothrottle to remain active during the descent and automatically make small thrust adjustments to maintain the descent speed while remaining on path.

Sorry, No, the AT don't stay on speed for the entire descent profile, yes they remain active and make small adjustments.

All this is happening behind the scenes, as a pilot on PFD I see AT modes changing from

IDLE (maybe 15%)

THR REF

THR 

HOLD

SPD

those modes are in FCOM very clear defined. 

What is happening behind the scenes we don't see it, we don't have any annunciations of that nature.

During all these years of operating -8 with NG FMC or on the 400 with the NG FMC for that matter none of these airplanes had the AT on SPD for the entire descent profile (from lets say FL390) up to the point where the APP mode logic kicks in.

Unfortunately sometimes there is wrong information in many books, it doesn't matter Boeing, Airbus etc. 

I know that you have this info in your books but autothrottle SPD for the entire descent profile on the FMA is totaly incorrect.

https://www.dropbox.com/s/xenta01f787ofhu/APP MODE.png?dl=0

https://www.dropbox.com/s/51ced3fq6rriwse/DES IN HOLD NOT SPEED WHEN ON P.png?dl=0

https://www.dropbox.com/s/l35nrzui2rgmv1u/FMA.png?dl=0

https://www.dropbox.com/s/iskwkb23xnet920/IDLE DEF.png?dl=0

https://www.dropbox.com/s/20kwy8rj6vvpe69/SLOW VP.png?dl=0

I don't think Vangelis intened to include early descents, the geometric portion of the descent below 10000 or higher constraint and during approach mode.  I thought is was pretty clear that the discussion was in regards to the descent from TOD to first constraint.

Yes they are included Dan.

I’ve never seen THR REF in a descent. That would be a max reference power setting, so unless you’ve really jacked something up I can’t imagine how you would end up in that mode 😅. THR in a descent is for FLCH which means VNAV has been disengaged. The only three VNAV descent modes are ALT, SPD AND PTH. 

I just listed  the AT modes, THR REF is one of them but not for descend as you pointed out.

A little explanation of   AT mode annunciations then:

THR - autothrottle applies thrust to maintain the climb/descent rate required by the
pitch mode.
THR REF - thrust set to the reference thrust limit displayed on EICAS.
IDLE - displays while the autothrottle moves thrust levers to idle; IDLE mode is
followed by HOLD mode.
HOLD - thrust lever autothrottle servos are inhibited. The pilot can set the thrust
levers manually.
SPD - autothrottle maintains command speed. Command speed can be set using
the MCP IAS/Mach selector or, by the FMC, as displayed on the CDU CLIMB,
CRUISE, or DESCENT page. Autothrottle will not exceed thrust limits displayed
on EICAS. Speed protection is not provided when the pitch mode is V/S.

Re FLCH SPD is a pitch mode and AT operates in THR, followed by HOLD mode in descent. If a selected altitude is captured the AT mode will change to SPD.

ALT and PTH are part of AFDS Pitch Modes while SPD is part of AT modes, you need to be able to make a difference between them. On FCOM V2 section 4 is very clear, more in-depth reading is necessary and unfortunately not enough space and time for this here.

 

On 10/24/2018 at 7:09 AM, killthespam said:

I just listed  the AT modes, THR REF is one of them but not for descend as you pointed out.

A little explanation of   AT mode annunciations then:

THR - autothrottle applies thrust to maintain the climb/descent rate required by the
pitch mode.
THR REF - thrust set to the reference thrust limit displayed on EICAS.
IDLE - displays while the autothrottle moves thrust levers to idle; IDLE mode is
followed by HOLD mode.
HOLD - thrust lever autothrottle servos are inhibited. The pilot can set the thrust
levers manually.
SPD - autothrottle maintains command speed. Command speed can be set using
the MCP IAS/Mach selector or, by the FMC, as displayed on the CDU CLIMB,
CRUISE, or DESCENT page. Autothrottle will not exceed thrust limits displayed
on EICAS. Speed protection is not provided when the pitch mode is V/S.

Re FLCH SPD is a pitch mode and AT operates in THR, followed by HOLD mode in descent. If a selected altitude is captured the AT mode will change to SPD.

ALT and PTH are part of AFDS Pitch Modes while SPD is part of AT modes, you need to be able to make a difference between them. On FCOM V2 section 4 is very clear, more in-depth reading is necessary and unfortunately not enough space and time for this here.

 

This is certainly true of the original 400's.  The FLCH SPD behaviour is quite interesting too, because it will also advance or retard the thrust levers to maintain a vertical speed which is proportional to the altitude change requested.  With a higher altitude set in the ALT window the reference thrust limit changes to CLB when CRZ is displayed and to CON in the case of an engine failure.  

1 hour ago, berts said:

This is certainly true of the original 400's.  The FLCH SPD behaviour is quite interesting too, because it will also advance or retard the thrust levers to maintain a vertical speed which is proportional to the altitude change requested.  With a higher altitude set in the ALT window the reference thrust limit changes to CLB when CRZ is displayed and to CON in the case of an engine failure.  

Yeah, if I remember correct it adjusts as to reach the selected altitude in 2 minutes (THR mode). If unable it will go to idle then hold in a descent or to THR REF in a climb.