Vertical speed selected AIRBUS BOEING
 

Hello, I have a question. Why pilots select manually the vertical speed whereas there is managed mode ? Is it to regulate and follow a descent path or just to respect an ATC constraint ? Indeed the managed mode of an Airbus or Boeing 737 can proceed automatically without human intervention. On videos from the flight deck we can see often the pilot select the Vertical Speed but why ?

I've been looking on the web for days but I don't find any interesting answers.

Have you found any uninteresting answers?

Actually RVSM requires the vertical speed to be within 1500 feet within the last 1000 feet of the cleared Level,Selecting V/S mode gives us direct control of our vertical speed..it is even advisable to revert to the V/S mode even below RVSM level to reduce the vertical speed and hence avoid any undesirable TCAS advisories..

Sometimes ATC want you to descend early and you don't want to descend yet so you use VS to keep a low rate of decent since your not on a profile. Sometimes you are below an ideal profile so you use VS to keep a slow descent until you reach the ideal profile.
Sometimes ATC want a specific ROD or ROC.
Sometimes you only want to climb a thousand feet and a full power climb isn't warranted since you don't want to cause TCAS conflicts or discomfort to passengers….

There are many many reasons :ok:

VS, as you say, has many uses. I always start my descents using VS to make a very smooth power reduction. Once power levers are back switch to FL change or some other mode if appropriate. It is a way to fly as smoothly as you can handfly.

V/S only for low level continuous descent and change of altitude 2000 ft or less.
A high rate of descent request by ATC will trigger a LVCH and selected speed to keep thrust idle and maintaining the requested rate.

Thanks to all of you for your very useful feedback !

- V/S is for reducing what a managed or open climb/ descent will give you.

- it's also for targeting a rate from ATC I.e. We're level at FL390 and ATC ask us to descend to FL300 and be level in 6 minutes. Easiest way is VS -1500. A managed descent below the profile on the A320 will only give 1000 FPM and an open descent will give a high rate.

- ATC in places like Germany often ask for a rate of descent I.e. 'minimum 1500 FPM'

- Different ICAO states have different minimum rates of descent. E.g. The ICAO standard is 1000 FPM hence the reason the A320 gives a minimum 1000 FPM in a managed descent. However the UK only has a minimum rate of 500 FPM so we can halve our rate until on profile and thus be 'commercial'.

- VS is very useful for controlling a CDA around an approach. It's far more accurate for achieving a 'glide star, loc star, super star'.

- VS is helpful to stabilise the rate through a turbulent level on climb in my opinion.

- VS as has been mentioned is a controlled way of approaching a level to avoid a TCAS TA or RA. Something some Ryanair pilots could do with learning. I've only ever had them from them. I'm sure they are the exception not the rule.

WhyByFlier
 

"- VS as has been mentioned is a controlled way of approaching a level to avoid a TCAS TA or RA. Something some Ryanair pilots could do with learning. I've only ever had them from them. I'm sure they are the exception not the rule. "

If you looking for a bite you will get a few with that daft comment! Its an SOP in FR to select V/S 2000FPM with 2000ft to go and then 1000FPM with 1000ft to go to target altitude/FL - I have been polling these skies awhile and do not hear FR causing RAs.

So consider this "a bite" and get your head down and go back to your scarebus :ugh:

Sigh sigh and sigh

De Facto why sigh, sigh, sigh? It's a legitimate way!

And if you're questioning my mental arithmetic, question your own first.

6 minutes, 1500 FPM, 9000 Feet to be lost. The penny will drop.

Utrinque, it wasn't an attempt at a bite. I have only had TA ( no RAs) from Ryanair aircraft. No other airline has caused them within my experience. I did end by saying 'I'm sure they (implying the pilots and circumstances at the time) were the exception not the rule'. I also said SOME not all. It's always been around BCN and PMI too for what it's worth! We all make mistakes and lose situational awareness. Especially us over worked LoCo operators. I'm not claiming some violation or lack of understanding. Relax!

In brief, LNAV/VNAV (managed modes for Airbuses?) are not always appropriate to use. Although they are normally very efficient, accurate and allow for a high degree of automation over extended periods of time, the downside is that they are somewhat inflexible, or require some time and effort to program correctly, in order to get the desired output.

The basic modes (such as, on Boeings, VS, LVL CHG, HDG, VORLOC, SPEED etc, I am led to believe these are referred to as open modes on airbus?) are normally appropriate to use to achieve short-term goals. Their operation is simpler and allows for a high degree of flexibility, at the cost of some increased in vigilance required.

As a pilot, training will teach you what each mode does. With experience you will decide when each mode, and thus what level of automation, is appropriate.

RAM777
 

RAM777 makes a very good point! So often overlooked!

Lots of ways to skin a cat LVCHG, VS, OPN DES/CLB.

IMHO as an old school guy I like to control my vertical rate and not let the aircraft dictate it by a speed for vertical control. I use VS quite often and make simple calculations as has been mentioned; gives you nice smooth vertical path. From my military days we would set pitch angles from AOA indicator. Basic pitch and power seems at time to be a lost art in today's computerized vertical guidance, which is a nice reference not the holy grail.

Boeing LVCHG does not chase the selected speed or path as bad as Airbus with OPN Mode,CLB or DES; which can at times turn into a real roller coaster ride trying to chase speed all over the sky.

How do you maintain "the requested rate" (being the high rate of descent requested by ATC) in Level Change, Idle and Selected speed? Or do you mean you'll just take whatever rate it gives you? ;)

To sum up, managed descent, DES or VNAV, has limitations. It has been made with given parameters. And of cours it is as food as the lateral flight plan on which it is based. There is a great number of reasons to use V/S instead.

So it is very usual to use V/S or OP DES / FLCH tactically. It is simple, cheap and very effective and it doesn't take a PhD to use, anyway...

SIGH SIGH SIGH
 

De facto

Please don't tell me that you are one more of the legion of airbus pilots who will use an ALT CSTR in the waypoint and then use managed descent!

Sigh, sigh, sigh

RVSM requires VS not to exceed 1000' FPM if traffic is 1000' higher/lower and within 5(10?) miles.

Some SOP masochists think the world comes to an end if you touch the VS selector. But how do you comply with RVSM requirements if the SOP police say "don't touch"? :ugh:

VS makes nicer level off's. Allows you to select the VS approaching level off vs. the rate selected by the programmer who never piloted an a/c with 1/3 of the passengers afraid of flying.

Well, that depends on what aircraft you're flying. Some are obviously worse than others...

VS makes nicer level offs. Some VNAV's are slightly better than others but none are smoother than VS.

Also used for enroute climbs. Climb thrust is quiet in the cockpit but significantly louder in the cabin, especially behind the wings. VNAV goes to climb power even for 1000' or 2000' changes. VS is pilot selectable. :ok:

Because I don't like 4000 FPM VNAV descents...

Vertical speed
 

The use of vertical speed selected when in à non précision approche, or when requested by atc, or to change level by 1000 ft

Well, to get best use of the managed descent with a constraint you'd need to descend at or within a few miles or so of the white arrow. ATC may want you to go down now - in managed with a constraint, making a decent early we will have what is effectively a commanded VS as required by ICAO standard minimum rates (unless within dest elevation +5000). You'll also probably end out rocketing towards your cleared level in an idle descent, possibly within RVSM.:= This could be extremely butt clenching if the winds uplinked or inputted to the FMGC are not actual winds. VS and mental arithmetic always work.:ok:

Also if you are beginning descent outside 200NM - it will not perform a PERF DESCENT - it'll perform a PERF CRUISE descent or STEP DESCENT which means it plans on a VS of 1000 FPM anyway and I don't believe it's use considers restrictions.

Then there's the way Airbus have decided to programme managed descent profiles - idle to the first constraint and geometric to the next constraints if cleared below the next constraint. Personally I like to perform a DIR TO after each constraint to reset to an idle descent to the next - it's more fuel efficient and typically faster on the basis of maintaining a higher TAS - unless wind below is less.

There are many ways of flying the Airbus - learn that and you'll be a long way towards good CRM and a pleasant day out.

I love how pprune gets all heated up on a simple VS debate:E
I aint flying airbus...
Maybe one should get familiar with his aircraft energy management rather than selected VS at -1500 ft for 9000 feet...just saying...just saying..

There seems to be a hunt and seek with LVL CH, resulting in uncomfortable pitch changes for the sponsors who make our pay checks possible. Vigilance is required for propel speed controll with high rates of descent.

And a hunt and seek with VS throughout 9000 ft.
Now we all agree that an initial VS to let the aircraft enter its pitch down smoother is a sweet touch for the pax but a bad habit when used continuously for 6 minutes,for eye opening,be level in 6 mins means just that,level in 6 mins not 1500 ft /min.

I agree, to smoothly use VS you need an extra minute to start descent and level off. 1500 fpm would not work so use 1800 to 2000 fpm. You can still give your pax a smooth ride.

The Airbus uses about 10% VS to ALT* (alt aquire) - roughly 150 feet. Within 300 feet of assigned level you are within tolerance and protected area. 6 mins of 1500FPM is smooth, reduces excessive N1 and N2 changes and is sure to work.

I trust the Airbus, I could use your way and sometimes do, it is one of many ways, but you need to understand and know the 'gotchas'.

As far as I'm concerned it's a perfectly legitimate use of VS.

We shall have to agree to disagree.

Once established at 1500 fpm yes you will lose 9,000 feet in 6 minutes if you don't have to initiate a descent or level off. The average descent rate, of course averages every second of your descent. Just add a minute and everything will be smooth.

I wrote this because he implied that the use of VNAV or DES is better, and sigh, sigh, sighed someone. Let's not make a mountain out of a molehill.

But I strongly oppose the don't touch "philosophy". It is a dangerous mentallity.

With V/S you can make an optimized descent to an altitude constraint. In DES, the systems is thought for a generic situation, not a specific one. This generic situation is a descent with some altitude restriction. But sometimes you are given a restriction well ahead of the TOD, which is more a step descent than a constraint during a descent to approach. In such cases an idle descent is not always the ideal solution, and therefore DES is not adecuate. But sometimes it is. It depends...

What I do, everytime I am given a constraint, is:

1st: I look at the estimated altitude passing over the waypoint before touching anything.
a) If it is lower than the constraint given an idle descent is in order, I will program the FMGS and use DES.
b) If it is higher than the constraint given, there is no need for idle descent. I make a simple calculation, using my GS in NM/min and the levels to go, and I choose a tactical TOD for either 1,000, 2,000 or 1,500 fpm. I allow for a 1,000 fpm at the last level. The more the levels, the more the rate. At this tactical TOD I set the target V/S. Approaching the point I can see how I go, If high, I'll delay selecting 1,000 fpm for the last level. If I am low, I can keep like that or do it earlier. If I used DES, I would go idle at some 3,000 to 4,000 fpm. Sometimes people use DES when below profile, they get 1,000 fpm al the way, and then they converge to the VNAV path and the last 1,000 ft are made at idle, high rate, then level off... very untidy!

Managed modes are great, except when selected modes can give a more optimized solution. V/S is a great tool. It also allows you to decide the rate at which you gain or loose speed, when accelerating or decelerating. I hate to see a pilot feel unconfortable because the speed is not being reduced as quick as he would like to (for instance in a base vector closing to the LOC) and then overshoot, when they can just use V/S to increase deceleration.

A pilot has to make the airplane do what he wants it to do. Generic solutions and philosophies might not be useful in specific situations.

I agree, all AP modes have their purpose depending on what you are trying to do. Know what mode you need and use it. Don,t let automation control you, you control automation or just hand fly.

Nice waste of a thread.....:ok:

It ain't Rocket science....:ugh::{

I didnt want to start a new topic and my question somehow fits here I guess, so here we go:

I am a passionate simulator flyer and do not fly the real birds. Lately, a question concerning the managed mode in the A320 (or any other bus) came up.
If ATC gives you a late descent clearance , lets say 10-20 nm after T/D for whatever reason, what will the autopilot do in DES mode ? In "my" 320 it will descend as steep as it can to get back onto the profile. This means, it will even accept and perform descend rates around 7000 fpm which leads to a soon overspeed warning and and so much speed that the speedbrakes are not able to handle. I thought that the Airbus autopilot will never bring you in such situations just for getting you back on your vertical profile. I Thought the flight envelope that is supposed to protect you even from own, dangerous inputs will also do that in managed modes.

The developer of the plane tells me, this is what the bus will do. I still cannot believe it so please help me on this one:
Does the DES mode have a vertical speed limit ?

DES mode operates within a 20 kt speed range around the managed target speed to maintain the descent path.
If the aircraft gets high on the computed descent path:
The speed will increase towards the upper limit of the speed range, to try to get back the aircraft on the path with IDLE thrust.
If the speed reaches the upper limit of that 20 kt speed range, THR IDLE is maintained, but the autopilot does not allow the speed to increase any more.

Thanks! One last question : If I'm reaching the upper Speed target, the a/p tells me to DECELERATE which I do using speedbrakes. You say it does not allow it... How? It does not have other options than to decrease descent rate by pulling the nose up, correct? Will it even do that if I do not use speedbrakes although it is telling me to DECELERATE?

Thanks again!

DECELERATE will show up only if you pass over the TOD still at level but will dissapear as soon as you start the descent.
If by maintaining the upper limit of the speed range the airplane is still not able to get back on path, it may suggest you to use some speed brakes. Using the speed brakes at that time is not to decelerate, but to increase the rate of descent.
Still, the usual method to get back on path would be to select a higher speed, making sure to maintain IDLE THR by selecting OPEN DES.

'VS makes nicer level offs. Some VNAV's are slightly better than others but none are smoother than VS.

Also used for enroute climbs. Climb thrust is quiet in the cockpit but significantly louder in the cabin, especially behind the wings. VNAV goes to climb power even for 1000' or 2000' changes. VS is pilot selectable'



Seriously, you are evaluating your use of the automation based on the 'noise in the cabin' ? :ugh: