Cruise speed vs top of descent

A forum to discuss all aviation items (not for latest aviation news and military aviation news)

Moderator: Latest news team

Post Reply
Atco EBBR
Posts: 127
Joined: 21 May 2012, 13:11

Cruise speed vs top of descent

Post by Atco EBBR »

Question for the pilots on the forum:

Say you're cruising at FL240 at a IAS of 290-310 kts. If you're asked by ATC to reduce to 250kts (hypothetically ;) ), how does this affect your top of descent, if at all?

Sabena320
Posts: 547
Joined: 13 Jun 2004, 00:00
Location: Europe
Contact:

Re: Cruise speed vs top of descent

Post by Sabena320 »

If we have to reduce and maintain this reduced speed during the complete descent the top of descent will come forward. We'll have to start the descent earlier as we cannot descend as steeply as when we still had 300kts. Of course, this is when you follow an optimum glide with engines in idle. You can always use the speed brakes to increase the rate again, but this breaks your efficiency ;)

Atco EBBR
Posts: 127
Joined: 21 May 2012, 13:11

Re: Cruise speed vs top of descent

Post by Atco EBBR »

Thanks for the reply, I knew that a lower speed implied a lower rate of descent, but I didn't know that that was not compensated by the lower speed...

So basically, if I want to sequence using speed control instead of vectoring, I need to first descend with a good rate and then reduce the speed.

Sabena320
Posts: 547
Joined: 13 Jun 2004, 00:00
Location: Europe
Contact:

Re: Cruise speed vs top of descent

Post by Sabena320 »

Atco EBBR wrote:
18 Oct 2019, 12:52
So basically, if I want to sequence using speed control instead of vectoring, I need to first descend with a good rate and then reduce the speed.
If aircraft have to reduce their speed below a speed that is 'normal' for descent it would indeed be better to let them descend a bit earlier so that they are below the altitude profile and then they can reduce the speed and is the less steeper descent more manageable. Of course the most efficient (excluding operational or time costs, but fuel wise) is to reduce the speed already just prior descent and have a continious descend to avoid a level segment at lower altitude. But of course in busy airspace or airports this is most of the time not feasible I assume ;)

The most difficult is when they keep you high during vectoring, and then ask you to reduce to 210-220kts and descend at the same time. This is very difficult to manage as the speed brakes have very few effect with such a low speed. In that case you have to give priority to speed or to altitude.

jan_olieslagers
Posts: 3085
Joined: 24 Jun 2006, 08:34
Location: Vl.Brabant
Contact:

Re: Cruise speed vs top of descent

Post by jan_olieslagers »

This makes for most interesting reading, thanks for sharing!

Myself limited to FL120 or so for lack of pressurisation/oxygen bottles, and to 100 kts Vne for lack of budget :( cannot really contribute to the facts, so sorry...

jan_olieslagers
Posts: 3085
Joined: 24 Jun 2006, 08:34
Location: Vl.Brabant
Contact:

Re: Cruise speed vs top of descent

Post by jan_olieslagers »

:) thank you!

If I wish to descend rapidly - which is standard practice under certain conditions - a sideslip is the standard answer. Which will indeed increase drag dramatically, while at the same time reducing wing efficiency, thus reducing lift. And I absolutely love side-slipping! But I can imagine it is one of those things one doesn't do to the unsuspecting innocent passengers of an airliner...

Reading over your words, I think I understand that, the bigger the plane, the finer the inputs should be; both on the controls and on the throttle(s). Many SEP pilots seem to treat their throttle like a kind of on/off switch, except in cruise...

Atco EBBR
Posts: 127
Joined: 21 May 2012, 13:11

Re: Cruise speed vs top of descent

Post by Atco EBBR »

Of course the most efficient (excluding operational or time costs, but fuel wise) is to reduce the speed already just prior descent and have a continious descend to avoid a level segment at lower altitude. But of course in busy airspace or airports this is most of the time not feasible I assume ;)
Yes indeed, top of descent is in most cases much earlier than Brussels Acc. It will require very performant, interconnected ATC systems to do a pre-sequencing using speed control starting at cruise level. Something to aim for :)
The most difficult is when they keep you high during vectoring, and then ask you to reduce to 210-220kts and descend at the same time. This is very difficult to manage as the speed brakes have very few effect with such a low speed. In that case you have to give priority to speed or to altitude.
If you encounter such a situation, you can be sure that the controller has screwed him/herself :D. If you have a lot of traffic that needs to be vectored for a sequence, you need to get them down and speed reduced asap. If you reduce while they're still high on profile, they'll stay too high too long, and any benefit from the speed reduction will be lost (with the same airspeed (IAS), groundspeed will increase by +/- 6-10 kts per 1000 feet)
That's until cost index n°6 airline arrives in the way with their 245kts as from ToD though... :?
Most airlines (unless the pilots are paid by the hour :roll: ) use a descent speed of about 280kts.
No idea who you're talking about :lol: But rest assured, these 245kts airliners very often get slapped with a speed of 300kts on first contact :) . The pilots are usually very happy with that, they often ask spontanuously if there is any speed control... If I can, my reply is 'negative, free speed', which would make all other pilots happy...
jan_olieslagers wrote: ↑Yesterday, 17:53
:If I wish to descend rapidly - which is standard practice under certain conditions - a sideslip is the standard answer. Which will indeed increase drag dramatically, while at the same time reducing wing efficiency, thus reducing lift. And I absolutely love side-slipping! But I can imagine it is one of those things one doesn't do to the unsuspecting innocent passengers of an airliner...
Hmmm... always dreamed to do that on short final too... but nope, not allowed :lol:
Unless you're in the Gimli glider, that is :D
However, if you're still too high and/or too fast close to the airport, increasing speed doesn't make sense anymore (of course). It's time to slow down and increase drag with the landing gear & flaps
Funny thing is, I recently had a talk with a colleague from approach who had a plane (E190 iirc) at FL80 15nm final. I was surprised to hear that in order to catch the glide they ask to reduce instead of increase. That's not a thing that would happen in an ACC environment :) What would be a typical maximum speed to extent the gear?
Descent planning (or energy management) is one of the most difficult thing to learn for ab-initio F/O's. It can be tricky as a descent is never the same twice; it depends on traffic, wind, speed, distance to be flown etc... On top of that, it also needs to be performed in an efficient way; that's where you can save fuel.
Probably for the same reason, sequencing arrivals is on of the most difficult things to learn for trainee controllers. You really need to grow experience on what speeds and levels are 'right' and what is too fast/slow, too high/low...

User avatar
HQ_BRU_Lover
Posts: 327
Joined: 22 May 2013, 20:44

Re: Cruise speed vs top of descent

Post by HQ_BRU_Lover »

737MAX wrote:
19 Oct 2019, 09:23
jan_olieslagers wrote:
18 Oct 2019, 17:53
:) thank you!

If I wish to descend rapidly - which is standard practice under certain conditions - a sideslip is the standard answer. Which will indeed increase drag dramatically, while at the same time reducing wing efficiency, thus reducing lift. And I absolutely love side-slipping! But I can imagine it is one of those things one doesn't do to the unsuspecting innocent passengers of an airliner...
Hmmm... always dreamed to do that on short final too... but nope, not allowed :lol:

However, if you're still too high and/or too fast close to the airport, increasing speed doesn't make sense anymore (of course). It's time to slow down and increase drag with the landing gear & flaps.

To give an idea; the rate of descent at minimum clean speed (approx 210kts on a 737NG) is around 1000ft; increasing to easily 3000ft+/min with 300kts.

Descent planning (or energy management) is one of the most difficult thing to learn for ab-initio F/O's. It can be tricky as a descent is never the same twice; it depends on traffic, wind, speed, distance to be flown etc... On top of that, it also needs to be performed in an efficient way; that's where you can save fuel.
Very interesting read guys, let's continue this way.

@737MAX: can you give an example of descent planning, f.e. ARVOL STAR 25L in EBBR? Or giving an example is impossible as winds, traffic, weight are not known?

Poiu
Posts: 832
Joined: 14 Nov 2015, 09:38

Re: Cruise speed vs top of descent

Post by Poiu »

In fact it is the angle of descent and not the rate which is what you are after.
Aircraft have an optimum glide speed, which depending upon weight, is around 200-220kts for a 737/320.
The closer to that speed, the shallower the descent, so reducing the speed will bring the top of descent forward.
The highest AOD is achieved with gear and flaps extended, that’s why the E190, in your example, asked to reduce in order extend as much flaps as possible.
The 245ers appreciate a clear instruction:eg “maintain 300kts”, free speed means 245 according company procedures.
What would be extremely useful on the arrivals from the west is the track miles at the moment descent below FL80 is given.

User avatar
HQ_BRU_Lover
Posts: 327
Joined: 22 May 2013, 20:44

Re: Cruise speed vs top of descent

Post by HQ_BRU_Lover »

737MAX & Poiu: thanks a lot for these very useful reactions. I've used to fly on the sim with the 737NG (PMDG package) which came close to reality what concerns graphics/cockpit features/... But reading these kind of replies on this forum shows to be very aware that flying a sim is not flying in real life (yes, I know that a lot longer - but just to confirm the great value of both posts).

Thanks once again both.

Atco EBBR
Posts: 127
Joined: 21 May 2012, 13:11

Re: Cruise speed vs top of descent

Post by Atco EBBR »

Thanks to everybody for replying, very interesting reading.
Arriving via ARVOL 25L in Brussels means FL240 at the boundary (which is too low) and you will have to maintain FL80 until you are abeam with RWY25R north of Brussels as departing traffic is cleared to FL60/FL70 below you. You become then too high if you are all by yourself, and possibly too low if ATC requires extra miles due to traffic. When possible for ATC, you'll be given the track miles they expect you to fly so you can adapt accordingly (ironically, that is usually not given when there is too much traffic and that's when you would need it the most...).
It's strange that you say FL240 at the boundary is too low. Whenever possible, I use 'when ready descent...' and almost never I see an aircraft leveling of at FL240 to start the descent later. Coming from KOK yes, then quite often descent starts 10 nm or more past KOK...

The problem with the track miles is that you have to calculate this case by case, the system doesn't know this. Say it takes around 5 seconds to calculate and you'll know why you can get that information only when it's calm... And we do realize it's very useful information when it's busy. A good arrival managing system should be able to handle that, hopefully that'll come with next ATC system (2022-2025?)
Probably for the same reason, sequencing arrivals is on of the most difficult things to learn for trainee controllers. You really need to grow experience on what speeds and levels are 'right' and what is too fast/slow, too high/low...
I can imagine! Add to that annoying pilots who want to fly different speeds all the time -)
I have to say that in Belgium, ATCO's are quite well trained for efficient vectoring.
The trick is not to allow pilots to choose their speed ;)
And thanks for the compliment :)

Another question for the pilots here: when we vector, you are given a heading. Could a modern aircraft fly a track, just as easily? That would be easier and more precise...

jan_olieslagers
Posts: 3085
Joined: 24 Jun 2006, 08:34
Location: Vl.Brabant
Contact:

Re: Cruise speed vs top of descent

Post by jan_olieslagers »

The trick is not to allow pilots to choose their speed
:lol: WHAAhahahaha! :lol:
Could a modern aircraft fly a track, just as easily?
I cannot speak about modern planes, but from a programmer's point of view it would be a lot easier. I have over and again insisted that, in the 21st century, magnetic information should be used only in extreme cases - after all, who uses a compass, today? Using only "true" headings would, for one example, get us rid of the changing of runway ID's every so many years. And true headings are not that difficult to calculate with trigoniometric difference calculations between (gps-derived) coordinates. Adjusting for magvar* adds a lot of complexity, since magvar changes both with location and with time. There are solutions to that, but the problem shouldn't exist.

*magvar: Magnetic Variation, the discrepance between true heading as measured on a map or chart, and what a magnetic compass indicates. For example, runway id's such as 11-29 at EBAW Antwerp are based on the runway's magnetic heading, which changes with the years - I remember when that same runway was called 12-30. And no, they did not install a pivot under it :) It would be easier and clearer for all concerned to base them upon "true" heading, as measured on a chart or map. And the ID's would remain constant, saving paint and effort and changes to the AIP and other publications.
Here in Western Europe, magvar is negligible, making the point rather moot; but I seem to remember it can be some 30 degrees in California, and even more as one gets nearer to the poles, either North or South.

DIBO
Posts: 562
Joined: 28 Mar 2009, 14:54

Re: Cruise speed vs top of descent

Post by DIBO »

....that's not talking about wind drift, but doing a topic drift :-)

jan_olieslagers
Posts: 3085
Joined: 24 Jun 2006, 08:34
Location: Vl.Brabant
Contact:

Re: Cruise speed vs top of descent

Post by jan_olieslagers »

:) Guilty, your honour.

Atco EBBR
Posts: 127
Joined: 21 May 2012, 13:11

Re: Cruise speed vs top of descent

Post by Atco EBBR »

How many track miles from the boundary to the ILS25L for a usual approach into BRU?
I'd say 60 nm if you get the ARVOL-BUN shortcut and can intercept at 10nm. Probably around 80 to 90 nm if you have to follow the STAR and intercept at around 20 nm.
On the 737 you can only select a heading on the autopilot, you would need to adapt the heading by yourself to continue on the requested track. But for modern planes ( :lol: ) like the 787, you can switch that autopilot mode between heading and track easily.
I guess you're talking about the 737NG? Surprised to hear that an autopilot track selection does not exist on that plane, it's not as if the design predates GPS... I suppose the MAX - if we see it in the air again - has the feature as well? Any idea about other types?
Here in Western Europe, magvar is negligible, making the point rather moot
Well, if you have spend tremendous time and effort in repainting runways, and updating AIPs and FMS's, just because magvar changed by 1 degree and the magnetic runway direction is now 014 iso 015, then I think you have a very valid point! Magnetic is a relict, everything true please!

Post Reply