A320/321 Speedbrake question...

[Wingswinger]

Let's say you are at 250 knots and suddenly given a shortcut. Should you stay at 250 knots with the speedbrake until you level off, then reduce speed and confirgure, or reduce speed first, configure, then descend as quick as you can??

Go down then slow down. Keep your speed at 250kts, SPDBK out. Level off then reduce speed. The air is thicker low down therefore total drag is higher and the TAS is closer to the IAS thus the distance required to decelerate will be less.

[PhoenixRising]

As a new twist to the original question, what is the generally accepted best method of recovering the profile? Let's say you are at 250 knots and suddenly given a shortcut. Should you stay at 250 knots with the speedbrake until you level off, then reduce speed and confirgure, or reduce speed first, configure, then descend as quick as you can??

With the speedbrake out, get the speed back immediately to 180kts and CONF 2. Using speedbrake and/or gear in open descent you'll have no problem getting back on the profile. Your speed is also low enough to carry on configuring to get stabilised, rather than finding yourself at 250tks, clean, at 7 miles..

[Wingswinger]

The trouble is you are not going to get to below 200 kts quickly to allow you to select flap 2. You could disconnect the A/P and level-off to reduce speed rather than accept the soft elevator response of the A/P and the 5-700 fpm descent rate you would have while the speed reduces. In the meantime, more track miles are being consumed than need to be. Go down first.

[PhoenixRising]

I disagree. It's very easy to find yourself getting into an unstabilised approach doing what you you suggest. I'm talking about the intermediate approach, say 20 track miles and you find that you are high due to an unexpected ATC shortcut. Have used both methods to get down many times and the one that always works best is slow down, get plenty of drag out, then go down. You can easily achieve 2000fpm or more with 180kts and CONF 2 and use of gear or speedbrake, with the added advantage of the lower groundspeed. Also no problem with configuring further to meet the 1000' window stabilised, fully configured and on speed which most airlines require these days.

[A4]

Agreed Phonix.

A4

[F4F]

Phoenix, agree as well.

best is slow down, get plenty of drag out, then go down

Lower speed with same ROD = higher gradient

[Wingswinger]

I think we may be disagreeing about slightly different scenarios. My mental picture was of being roughly in a downwind or long base position at about 30 miles from touchdown (i.e. roughly 10,000ft) when you are given a short cut (which, by the way, you don't have to accept). In which case I would stand by what I have posted. I've been doing it (and teaching it as a "fix") for years. It works.

The TAS(approx) for 250kts IAS at various heights is as follows:
15000ft - 312kts
10000ft - 290 kts
5000ft - 272kts
3000ft - 260kts

Do you think you will cover fewer miles in a deceleration starting from 312kts or starting from 260kts? At 3000ft the air is denser hence there will be more total drag so the rate of deceleration will be greater. In addition, if the deceleration segment is level, there is no sacrifice of kinetic energy loss against potential energy loss so, again, the deceleration will be more rapid. If that doesn't completely solve the problem, you are into flap 2, gear down, 180kts and V/S -2000 to capture the glideslope from above by 1500ft or you won't stable at 500ft.

If you are thinking of a senario much closer and lower than 30 miles/10000ft/250kts, for example, 15 miles/5000ft/250kts then I would say don't accept the short cut.

[Caudillo]

An Paddy Eile
Thanks for your reply - very interesting point you make about the exceedence in VS becoming Open Climb, I hadn't been aware of that at all. I'll go away and read up on that one. There's still a lot about the airbus that's news to me in fact!

However, regarding the descent - I'm not so sure. Do excuse my unfamiliarity with the type, however I'm sure that I'm correct when saying given a height and a speed and an availability of drag, you will only ever be able to get down to another height and speed in a certain minimum period of time or a certain minimum amount of track miles covered.

I've just read back on that, and it's about as clear as mud so - if as you say, I try to vs down in a little more than a 3 degree slope, my airspeed x will increase to airspeed x + some. Fine. So if I'm unwilling to accept the airspeed increase I can go down in open climb at the original speed x, but I won't achieve the rate I wanted. All things being equal of course. I can increase my rate in this last case by spoiler, gear, holding my hand out of the window. Naturally, I'd also get an equivalent tempering of the speed increase in my vs descent if I do those.

It was the same on my last type, also a jet, and it's the same on pretty much anything I can think of. Ultimately an aircraft is subject to the same aerodynamic laws, whether its an airbus or a starling.

Of course I appreaciate what you say about the protection from overspeed offered by the open des mode, but descent in vs is hardly anything close to the bone as long as you're reasonably alert and vigilant, which I am sure we all are in any case..!
Cheers

[el #]

A very informative thread. Easy to assimilate because the plain language and practical "approach" to the matter
As an aviation enthusiast I understand much better now "how do you guys do it".

Thanks!

[VMK]

Fuel Metering Valve. What is it?

and.... why i read in FCOM3 3.02.07

ENG 1 (2) FUEL CTL FAULT

the procedures only apply for ground. Will it not happen on air???


why only GND.....?

[Dream Land]

Reversions

exceedence in VS becoming Open Climb

Knowing a few of these will be useful in the bus.

[An Paddy Eile]

I must agree that all the posts here are sensible and logical. (with the exception of course of VMK's post ) I agree that you can either descend, slow down and configure in a certain number of track miles or you can't, and you shouldn't throw yourself into a situation where you're not entirely sure whether it will work or not and ultimately throw it all away at 5 miles when you're still struggling to get it below 200 knots, but....

I've been trying to think about a better way of explaining what I'm looking for...

Let's say you're at 250 knots (ATC speed restriction) passing 10,000 feet or so, on a heading of North. ATC have you on what is effectively a long left base leg for RWY 27. Your estimated track miles, based on maintaining the current heading until you get a final vector for the approach at about 15 miles out, leaves you with about 35 miles to run. Just lovely!

But there is the possibility that you could turned in earlier for a shorter final of say 7 or 8 miles...

If they turn you now, your track miles would reduce to say 27 or 28 miles. Still no problem with speedbrake!!


But if they turned me later would I still be OK?? Do I need to use speedbrake now to cover both possibilities? If I knew how much extra RoD or increase in descent angle I should get, I could work that out. If I will only get a few hundred feet per minute extra with speedbrake, then I should really use it now, because it will have a longer period of time to adjust my profile. But, if I could effectively double my descent gradient, then I can wait and see what they give me as I'll still be able to manage the situation later.

Now, I realise of course that I can pay attention to what happens when using speedbrake over the next few weeks/months and I will have my answer. But, with PPRuNe available to tap into the knowledge of so many, I just can't resist!!!

So, does that make it a little clearer?? Does anyone know, or has anyone ever thought about the effectiveness of speedbrake this way before?? Am I alone in planet ODD??

Hope that helps!!

Thanks everyone!

[Wingswinger]

The important thing is to keep the F-plan sequenced so that it is giving you accurate distance-to-go. That is useful information.

If you think you need SPDBK, you need it. I don't bother with trying to calculate the required VS in such circumstances. Just use the SPDBK until current altitude + 1000ft (to allow for deceleration from 250kts) x 3 = track miles to go. If it's not working, get speed below 250 and drop the gear. Don't worry about finesse, fix it quickly. I use a "gate": On glide at GRN DOT with gear down by 2000ft above TDZ, decelerating and configuring. If there's a tailwind and EAI is on, make that 2500ft. If I don't make this gate I'm probably going around.

It's better not to accept the short cut and tell them how many miles you want.

HTH,

WS

[An Paddy Eile]

Thanks Wings!

That's quite helpful actually! I've always checked my profile by working the remaining track miles and figuring out what height I need to be at. never thought about doing it the other way around!!

Thanks!

APE