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Vertical speed selected AIRBUS BOEING

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Vertical speed selected AIRBUS BOEING

Old 19th Feb 2013, 07:37
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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.
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Old 19th Feb 2013, 13:46
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Have you found any uninteresting answers?
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Old 19th Feb 2013, 15:43
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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..
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Old 19th Feb 2013, 15:52
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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
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Old 19th Feb 2013, 16:00
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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.
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Old 20th Feb 2013, 02:13
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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.
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Old 20th Feb 2013, 07:44
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Thanks to all of you for your very useful feedback !
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Old 20th Feb 2013, 07:54
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- 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.
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Old 20th Feb 2013, 08:13
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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
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Old 20th Feb 2013, 08:23
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- 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.
Sigh sigh and sigh

Last edited by de facto; 21st Feb 2013 at 13:55.
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Old 20th Feb 2013, 08:56
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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!

Last edited by WhyByFlier; 20th Feb 2013 at 09:03.
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Old 20th Feb 2013, 09:29
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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.
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Old 20th Feb 2013, 09:44
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RAM777

RAM777 makes a very good point! So often overlooked!
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Old 20th Feb 2013, 10:22
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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.

Last edited by jimmyg; 21st Feb 2013 at 03:03.
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Old 20th Feb 2013, 11:00
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Originally Posted by De Facto
A high rate of descent request by ATC will trigger a LVCH and selected speed to keep thrust idle and maintaining the requested rate.
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?
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Old 20th Feb 2013, 13:36
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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...
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Old 20th Feb 2013, 13:37
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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
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Old 20th Feb 2013, 14:14
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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"?

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.
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Old 20th Feb 2013, 22:46
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VS makes nicer level off's.
Well, that depends on what aircraft you're flying. Some are obviously worse than others...
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Old 21st Feb 2013, 00:58
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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.
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