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Vertical speed selected AIRBUS BOEING

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Vertical speed selected AIRBUS BOEING

Old 21st Feb 2013, 01:50
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Because I don't like 4000 FPM VNAV descents...
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Old 21st Feb 2013, 09:56
  #22 (permalink)  
 
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Vertical speed

The use of vertical speed selected when in ŕ non précision approche, or when requested by atc, or to change level by 1000 ft
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Old 21st Feb 2013, 11:13
  #23 (permalink)  
 
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What, exactly, is wrong with doing that? I'm not averse to using VS when appropriate (i.e. to comply with RVSM requirements) but otherwise why not use all of the tools that Monsieur Airboos gives us?
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.

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.

Last edited by WhyByFlier; 21st Feb 2013 at 11:14.
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Old 21st Feb 2013, 13:32
  #24 (permalink)  
 
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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
I love how pprune gets all heated up on a simple VS debate
I aint flying airbus...
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Quote:
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?
Maybe one should get familiar with his aircraft energy management rather than selected VS at -1500 ft for 9000 feet...just saying...just saying..

Last edited by de facto; 21st Feb 2013 at 13:39.
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Old 21st Feb 2013, 13:36
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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.
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Old 21st Feb 2013, 14:06
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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.

Last edited by de facto; 21st Feb 2013 at 14:08.
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Old 21st Feb 2013, 14:23
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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.
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Old 21st Feb 2013, 15:17
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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.

Last edited by WhyByFlier; 21st Feb 2013 at 15:20.
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Old 21st Feb 2013, 22:48
  #29 (permalink)  
 
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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.
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Old 22nd Feb 2013, 14:05
  #30 (permalink)  
 
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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
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.

Last edited by Microburst2002; 22nd Feb 2013 at 15:27.
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Old 22nd Feb 2013, 22:58
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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.
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Old 22nd Feb 2013, 23:24
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Nice waste of a thread.....

It ain't Rocket science....
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Old 6th Dec 2013, 05:03
  #33 (permalink)  
 
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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 ?
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Old 6th Dec 2013, 14:41
  #34 (permalink)  
 
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Originally Posted by docromano
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.
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Old 6th Dec 2013, 14:56
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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!
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Old 6th Dec 2013, 17:50
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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?
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.
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Old 9th Dec 2013, 05:38
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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'



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