go down and slow down
 

I know that the controller should not reduce the speed of the aircraft while the aircraft involved is descend, because it's almost impossible for the pilot to do these two things at the same time.
Could anybody make some detailed explaination about the reason for that?
Thanks a lot!

They can "go down and slow down" they do it all the time otherwise they couldn't land!

What pilots are not able to do is both instruction quickly and at the same time, unless they start applying speed brakes and other slowing/descent techniques which are not the most appropriate way of flying the aircraft.

I'm sure a pilot will be along soon to give a more educated explanation.

To give you an idea, with my very limited experience:

It takes roughly 3 miles to loose 1000' and 10 miles to go from cruise speed to a sensible arrival speed. They are realy slippery and without introducing extra drag (ie dunlops or speedbrake, flaps as a slow down method is a no no due to operating close to the flap limits) they will do both slowly.

Descents are ideal with the thrust levers at flight idle and the pitch controlling the speed. If you need to slow down as well as go down then the pitch attitude must be increased (more nose up) this in turn reduces the descent rate.

Nick

ps its all about energy management

A Simple Explanation
 

You program the FMS [Boeing]with scheduled descent speeds and forecast winds based on the planned profile often constrained by ATC eg. MARGO FL260- for ScTMA landers. All that produces a Top of Descent point [TOD] where ideally the thrust levers retard to flight idle & you start the descent.

Asking for an increase in speed results in an increase in the rate of descent [ROD], and a profile below the ideal "slope". Costs money by using more fuel in the lower levels when thrust applied.:uhoh:

Asking to slow down reduces the ROD, and therefore any ATC restrictions can now only be achieved by using the speed brake which increases the ROD to a certain extent only. Function of IAS. Using the gear to increase drag would be a really bad day !:=

The NG wing is designed to be efficient and "glides" well. :)

If say, a speed reduction is necessary, try to inform the crew before TOD so the desired speeds can be entered into the FMS to produce a modified TOD. Late reductions in speed are also bad news. Shows a lack of forward planning & a lack of understanding of aircraft performance.:ugh:

Answer : arrange jumpseat rides and see what happens aloft.:ok:

Yes, but how to arrange those....?

Going back to the original question, as Arch Stanton says, aircraft manage it all the time! What you won`t get is a heavy jet reducing from 250kts to 180kts while also expediting descent. There are factors at play beyond just the performance capabilities of the aircraft too. The minimum clean (as in, flaps, slats and gear remain stowed) speed of a typical commercial jet aircraft can vary from approx 195kts (737NG, A320 family, light load) to approx 240kts (heavy load in 747). Pilots can be reluctant to reduce below those speeds unless they`re within 20 miles or so of landing.

There are techniques we can use to manage descent and speed control. Using a heading, for example, which won`t affect other aircraft to allow an aircraft to expedite descent while remaining fast. Rather than asking for reduced speed and descent at the same time, we can ask for descent THEN speed reduction. Where language commonality allows, we can add explanation - for instance that we will need a major speed reduction in x number of miles, therefore expedite descent now while you`re still fast.

In the case of turboprops, they have a massive pair (usually) of spinning speedbrakes which can allow a swift descent at almost any part of their speed range.

Aircraft can be operated more flexibly than used to be the case, and the least flexible aircraft of all are the simulated variety! Many experienced controllers found that certain simulator blip drivers way out west in a country noted for snow would do the Ì can`t go down and slow down`routine when an actual aircraft would have no problem at all. One blip driver in particular, who took great pleasure at ruining sim runs for experienced controllers from other lands, really needs to get a life. A heavy jet belonging to his country`s air force went from FL280 to landing (threshold elevation close to sea level) in about 35 track miles the other day, at the pilot`s request...

Not to Scale

In the sandpit we are faced regularly with the local airline wanting to land the entire fleet in a 10 minute period, it does not work, we try and space you out by speed control and levels to give the approach unit a fighting chance to land you without too much delay. We tell you what to do to help us, and ultimately you, otherwise you could end up over your destination at 25,000ft and have to corkscrew down in turn, which is less cost efficient? :)

If the "entire fleet" [ of undefined size ] all pitch up within a 10 minute period, send them to the hold and peel them of the stack for the approach as and when.

Sounds like fuel saving is in the company's own hands by arranging their inbounds a bit better to avoid delays. A question of communication perhaps?

Back to minimum clean speed is quite a useful technique to use cost wise.

Minimum clean speed is used all the time but when you are faced with multiple inbounds then you have to speed sequence or vector them all over the sky, not easy with 30+ aircraft on the frequency, ah the joys! :ok:

'Minimum Clean Speed' is used by Area when in the poop, either by one's own doing or when Approach makes love to the pooch (and even then they whisper the words 230kts in hope that you will try and make them stand still in the sky!!!!)

IAS is not supposed to be used until below FL250 (as we officially found out recently) so one's hands are tied!

Whatmean dude if you have 30+ on frequency you will need more than 'min clean' to save your butt! :rolleyes:

So, Fox3snapshot, you admit to transferring traffic at DESDI and BUBIN above FL250??? Here I was thinking they had to be below 13,000'... :}

Not that I wish to get into a contest over who "makes love to the pooch" more between area and approach, but in approach we leave the pooch reaching for a post-coital cigarette afterwards. :E

"unless they start applying speed brakes and other slowing/descent techniques which are not the most appropriate way of flying the aircraft."

Why not? That is what they are there for!!!

Go down/slowdown is not the ideal situation but I get it all the time. It can be done. Various techniques and just pulling airbrake doesn't really do much. You have to slowdown first. That will then give you more minutes to go down.

FOX

Come on man, you are in the ME, 30+ happens, not my screw up for min clean but you do what you gotta do.

Don't mention zee bloody Germans......going into FRA the other day, we went from no speed control (330kts and about 1500 fpm ROD) to being told to slow to 210kts and keep 2500 fpm ROD minimum:hmm: feckin idjits

For descent and speed - North America's ATC will often specify...
"Expediate descent and maintain 8,000 feet, speed 230. Altitude first then speed".
That airspace is a paradise. Sorry for the guys in W. Europe...
Ask your ATC what they want first...
xxx
In the 747, level flight, reducing 340 to 250 takes... 9 NM (340-250= 90 NM).
Or 300 KIAS down to 250 KIAS = "50" or... 5 NM.
Works in other planes too. Worked in the 727, certainly will do in a 737.
And dont forget your gear - Your best brakes...!
xxx
:ok:
Happy contrails

Typically a jet descend into the TMA is 270-300kts (indicated) ish which is above best glide speed. During descent, if asked to slow down, it means being to asked to slow towards best glide speed which means angle and therefore rate of descent will decrease. Also (for those of you who remember trig) for a given angle of descent, at a lower forward speed we will have a lesser rate of descent. eg. 3 deg angle of descent: 250kts groundspeed rod=1327fpm but at 400kts groundspeed for the same angle rod=2123 fpm. Also, if required, the speed brake is most effective at increasing angle of descent at high speed but is apparently not the preferred method to get down for pax comfort reasons.

sr

Think of your car. If you take your foot off the accelerator whilst going up a hill, it slows down quickly. If you do it on the flat is slows down, but not so quickly. If you do it on a slight down hill, it slows down, but hardly at all. If you do it on a steep down hill, it does not slow down, and will even speed up. The principles which cause these things to happen in your car, are exactly the same ones which govern whether a plane can slow down rapidly or not. (ie gravity, and drag). In your car, you can increase drag by using the brakes. In a plane, we can use the speed brakes, and also the flaps. The problem is that the heavier the aircraft is, the smaller the deceleration resulting from these devices is. And so it is much easier to slow down on the flat, and then descend, rather than to try to decelerate whilst going down hill.

The physics of it all, is basically the conservation of energy. Potential energy (energy due to height), must go somewhere as you descend. This energy is converted into kinetic energy (energy related to velocity), and also wasted due to drag. If your rate of loss of potential energy (ie rate of decent), is the same as the rate at which energy is lost due to drag, then you will neither accelerate nor decelerate. If you descend quicker than this, there will be some spare energy left over, which is converted to kinetic energy and so you speed up. If you descend slower than this, the drag is taking away too much energy, so some has to come from somewhere else too. It is taken from the kinetic energy, and hence you slow down.

GS-Alpha

Great physics lesson thanks! :ok:

Interesting information you guys have posted but try telling your Ops department to adjust the arrival times to allow ATC to serve you better, also, why do airline companies insist on using similar callsigns xxx62, xxx72, xxx762, etc? Are they trying to make things difficult? :confused:

For most jets it IS difficult, but not for the Fokker jets...F100 and F70. We can go down, slow down, turn, whatever you like! I know it surprises ATC sometimes just how quickly we can lose height!

Now gaining that height quickly in the Fokker- that is another matter! ATC, please don't expect us to climb like a Boeing!