Hehe, seriously Paul give it a rest mate... I could point you towards discussions I have been involved in dating back over 5 years regarding FLCH logic.

 

Honestly mate, quit digging and give it a rest

Hehe, seriously Paul give it a rest mate... I could point you towards discussions I have been involved in dating back over 5 years regarding FLCH logic.

 

Honestly mate, quit digging and give it a rest

How about you quit claiming to be the source of all knowledge and back up a few of your comments with actual, verifiable facts?

 

 

[Not gonna quote the whole thing to save space]

 

Well said Kevin.

 

 

 

Despite the furore, no one has actually described a scenario where the use of V/S will get the job done best.

 

I have.  Two, in fact:

1. Slow-down-and-go-down (VNAV and FL CH will level off to decel, whereas you can force a V/S with V/S, while rolling back the selected speed)
2. Non-precision approaches

 

How about you quit claiming to be the source of all knowledge and back up a few of your comments with actual, verifiable facts?

 

Can we stop...please?

 

Let's be real here.  It's very hard to show verifiable facts from tribal knowledge.  Seriously.  Go ask N90 why they use certain procedures that - on their face - make little sense.  The only thing that they can give you is "it just works for us," and the only way you could really prove it is by trying to find poor (not bad) weather days in the NY Area and charting those against overall delays per hour.  The lack of delays is the "proof."  That isn't very scientifically verifiable, but you just have to take the operational word on it.

Firstly I never claimed to be the source of all knowledge, one of the great things about aviation and most things in life is that you are always learning right up to the grave..

 

Secondly, if you actually read my posts along with all the other contributions in this thread you will see that we have clearly explained everything.

 

So, to get back to what I originally pointed out, I know exactly what you are like around here Paul, and frankly cannot be arsed to get dragged Into this any further with you....

 

The whole thing is hilarious... Now stop digging! That hole is getting deep..

 

 

I have. Two, in fact:
1. Slow-down-and-go-down (VNAV and FL CH will level off to decel, whereas you can force a V/S with V/S, while rolling back the selected speed)
2. Non-precision approaches

V/S will let the pilot to manage the throttle, in fact it mandates it. But VNAV on a T7 will try to use continuous descent so it will only level out to decelerate if the energy state is to high and in that situation, V/S won't help, you need to use spoilers.

 

For non precision approaches, of course V/s works, but I would suggest FPV/FPA would usually work better.

 

 

Now stop digging! That hole is getting deep..

Still all mouth and no trousers

 

 

V/S will let the pilot to manage the throttle, in fact it mandates it. But VNAV on a T7 will try to use continuous descent so it will only level out to decelerate if the energy state is to high and in that situation, V/S won't help, you need to use spoilers.

 

That's not entirely true, and I don't think you're properly analyzing what's going on and how all of this works.  Instead of trying to win the argument, just step back and have a look at it given everything we've pointed out here.

 

V/S will dump to V/S + SPEED if the A/T is on (for most aircraft so equipped).  If the A/T is not on, then you are correct that the pilot must then manage the throttles.  VNAV isn't the bees knees feature that you're making it out to be.  VNAV will not try to use a continuous descent.  It will try to use an IDLE descent to the first restriction and a geometric path from then on (which is actually more of an emulation of FPA or V/S than anything).

 

Either way you cut it, though, FL CH and VNAV will level off if you decrease the speed either through SPD INTV or changing the speed on the VNAV page.  Through using both V/S and SPEED, you can avoid that leveling off.

 

Imagine this scenario:

You're in VNAV in the descent.  ATC says "reduce speed 220 knots or less for spacing, if able."  The latter is added in by controllers for two reasons: to reassert that pilots should still maintain safe speeds, and to concede that the restriction in the descent is known to be difficult.  Had they wanted to reduce speed immediately, they would have said "reduce speed 220 knots or less, immediately (or similar)" or "amend your altitude to [higher intermediate altitude], at [higher amended altitude] reduce speed 220 knots or less..."

 

So, the controller just issued an instruction to reduce speed knowing that it will be difficult in the descent.  As such, the priority is to continue descending, but reduce speed if able.  In order to satisfy both, you cannot be in a pitch for speed mode.  Since FL CH and VNAV are pitch for speed modes (VNAV is until it hits GeoPath, but speed changes will still throw it off in GP), reducing the speed using SPD INTV will make you level off.  If you'd dumped from VNAV to V/S, it would set your current V/S in the window, and your current VNAV speed in the SPEED window.  As such, you could then reduce the speed selector to 220 without affecting your V/S.  Since the current V/S was pitched for that higher speed, the reduction in the speed selection would not have an effect, initially.  From there, you could reduce the V/S some to let the speed bleed back, while still descending.  Two birds with one stone.

 

Note that nowhere in there did I mention using the spoilers.  You could certainly use them, but I'd argue that their use shows a poorer quality of airman, as a result of a poorer understanding of the tools in front of him.

 

 

 

For non precision approaches, of course V/s works, but I would suggest FPV/FPA would usually work better.

 

Sure - V/S and FPA are really very similar in their design (there's a reason they share a button on the MCP).  One sets a flight path angle through pitch, while the other sets a vertical speed through pitch.  I consider them to be one in the same as far as modes go.  FPA makes matching things up to IAPs a lot easier (particularly when they note a particular glideslope on the chart).  Neither offer the speed protection that you were praising in FL CH and VNAV.

 

To be honest with you, I generally use FPA for approaches and V/S only in the slow-down-go-down scenario I mentioned earlier.  That's the only time I see use for it, but that doesn't mean I won't use it for IAPs if I felt the need for its use.  I don't often use the Mach feature on the MCP (in favor of setting a CRZ speed on the VNAV CRZ page), but that doesn't mean it doesn't have a place there.

VS can be used for various things.

 

1)ATC instruction to keep a certain climb/descend rate

2)very often ATC vectors are delay vectors. You get step by step desends while flying those headings. To prevent idle descends and subsequent level flight with high(er) thrust setings (high fuel flow/loud/etc) we use VS -500ft/min or so to stretch the descend so to say.

3)If you have the feeing that an ATC instruction to reduce airspeed while you are also descending is meant as "reduce NOW and quick" (high traffic density) then using VS -100ft/min actually does the job far better than FLCH. After you have reached that speed you can revert to FLCH again.

4)I probably could come up with more

 

The thing is, like I said before "there is a right time for VS and a wrong time for it, just like there is a right time vor Vnav and a wrong time for Vnav"!

 

There really is no point fighthing over this.

Kyle, try to resist! It will get you nowhere! Anyway, just a FYI ( despite my apparent gift of being the all knowing) I decided to fire of an email to a 777 captain that happens to share the same DNA as me.

 

Funnily enough he told me exactly what I have known from pretty much every other F/O, SFO, captain and trainer out in my region, including something I was not aware off. He will use it in descent in turbulence, when flying a CDA (off path) also at low altitude where FLCH is prohibited and when a specific descent rate is requested.

 

Amazing that this has gone on for two pages! But hey, I'm sure paul could teach them all a thing or two about how best to operate the aircraft.

 

 

Kyle, try to resist! It will get you nowhere!

 

As usual, I go for the debate, not the outcome.

VS can be used for various things.

 

1)ATC instruction to keep a certain climb/descend rate

2)very often ATC vectors are delay vectors. You get step by step desends while flying those headings. To prevent idle descends and subsequent level flight with high(er) thrust setings (high fuel flow/loud/etc) we use VS -500ft/min or so to stretch the descend so to say.

3)If you have the feeing that an ATC instruction to reduce airspeed while you are also descending is meant as "reduce NOW and quick" (high traffic density) then using VS -100ft/min actually does the job far better than FLCH. After you have reached that speed you can revert to FLCH again.

4)I probably could come up with more

 

The thing is, like I said before "there is a right time for VS and a wrong time for it, just like there is a right time vor Vnav and a wrong time for Vnav"!

 

There really is no point fighthing over this.

 

"There really is no point fighthing over this."    WELL said !  :smile:

 

Interesting discussion (and confirmation) of  this on PPRUNE:

http://www.pprune.org/tech-log/508355-vertical-speed-selected-airbus-boeing.html

 

Guy

VNAV will not try to use a continuous descent.  It will try to use an IDLE descent to the first restriction and a geometric path from then on (which is actually more of an emulation of FPA or V/S than anything).

 

Geometric path is a customer option on the 737, it doesn't come with the 777. On the 777, all VNAV is is basically FLCH that will respect altitude/speed constraints.

VS can be used for various things.

 

1)ATC instruction to keep a certain climb/descend rate

2)very often ATC vectors are delay vectors. You get step by step desends while flying those headings. To prevent idle descends and subsequent level flight with high(er) thrust setings (high fuel flow/loud/etc) we use VS -500ft/min or so to stretch the descend so to say.

3)If you have the feeing that an ATC instruction to reduce airspeed while you are also descending is meant as "reduce NOW and quick" (high traffic density) then using VS -100ft/min actually does the job far better than FLCH. After you have reached that speed you can revert to FLCH again.

4)I probably could come up with more

 

The thing is, like I said before "there is a right time for VS and a wrong time for it, just like there is a right time vor Vnav and a wrong time for Vnav"!

 

There really is no point fighthing over this.

 

 

Best post in this thread! Will help to improve my flying rather than my fighting!

 

Thanks, Rob.

Geometric path is a customer option on the 737, it doesn't come with the 777. On the 777, all VNAV is is basically FLCH that will respect altitude/speed constraints.

It may not have a name on the 777, but it's definitely a similar concept. What you have written regarding the 777's VNAV logic is not true.

 

FCOM 11.31.25:

The path after the first restriction is calculated as a point to point VNAV path. If the path is too shallow to descend at idle, it will command speed on throttles and shallow the descent with the elevators.

 

If it were truly FL CH that respects altitude, it would descend at idle thrust in steps to each altitude. This is not how the 777, behaves, however.

It may not have a name on the 777, but it's definitely a similar concept. What you have written regarding the 777's VNAV logic is not true.

 

FCOM 11.31.25:

The path after the first restriction is calculated as a point to point VNAV path. If the path is too shallow to descend at idle, it will command speed on throttles and shallow the descent with the elevators.

 

If it were truly FL CH that respects altitude, it would descend at idle thrust in steps to each altitude. This is not how the 777, behaves, however.

 

I think I might have been thinking back to the diagram of the descent path when in speed intervention, which is obviously not quite the same.

 

However I just tested it, and PMDG 777 does seem to follow what I thought was the case. I arranged an arrival on a star and set two altitude constraints, one at FL200 and another at 12,000, 80nm apart to see what would happen. It descended from cruise to be level at FL200, then stayed at at FL200 for ~60nm before setting idle thrust and diving for 12,000. This doesn't seem consistent with what the FCOM says, but it doesn't have any definitive diagrams.

 

 

However I just tested it, and PMDG 777 does seem to follow what I thought was the case. I arranged an arrival on a star and set two altitude constraints, one at FL200 and another at 12,000, 80nm apart to see what would happen. It descended from cruise to be level at FL200, then stayed at at FL200 for ~60nm before setting idle thrust and diving for 12,000. This doesn't seem consistent with what the FCOM says, but it doesn't have any definitive diagrams.

 

Take a look a few pages before.  There's a page with three diagrams illustrating initial descent and associated parameters for a level-off.  If it stayed there for 60nm, it was probably working in some of the DES NOW logic and hanging out at the new level until it recaptured an ECON path.  I'm on my other computer so I don't have the manuals in front of me.

 

If you do the tutorial flights, you'll see it work in GP for the entire descent after the first restriction.