En Route Speed Control

[VectorLine]

Hello there, pilot type people.

I am an ATCO at Swanwick - South Coast Sectors - and am finding an increased need for speed control for aircraft in the cruise and from the top of descent.

It really is a learning curve for all us ATCOs because Mach numbers are not something we have used often in the past (except for Oceanic controllers) and setting decent speeds is very much trial and error.

I find that I also have to give a mach number and an IAS for later in the descent. I never seem to get the same Mach number when I ask a pilot his speed - even for same company, same type, same Flight Level.

The number of times I have given a slower speed to following aircraft and find that they are catching the one ahead is amazing. It does tend to be RYR, EZY and BAW that seem to get me every time. Im not saying you are lying about your speed (although in a busy moment I have been known to speculate!), but it is probably that I don't understand how you operate.

So, on to the questions...
1) Can you please tell me, do you fly the same cruise speed every time? or does it vary with temp, FL, weight etc.. if you could mention your a/c type and company that would also help..

2) Do you convert to flying IAS at the same FL each time or does that also vary? I am convinced that some airlines/pilots (particularly those with tight schedules at EGSS!!!) fly Mach numbers as long as they can in order to maintain a high ground speed - which really p155es me off when trying to stream Lorel arrivals via MAY or MID.

On a point of note - if you are flying a speed that is particularly slower than normal, it would be really good if you could tell me - I was trying to stream 5 a/c the other day (again for EGSS/EGGW). As I issued a speed to an EZY, I was told that they wanted to fly M0.69 to FL200, then 280kts. I think that is very slow and it screwed my sequencing. I could not get the pilot to tell me why the slow mach number - he just sounded very smug and seemed to enjoy the fact that he was causing me so much trouble.

3rd question - Many pilots ask me "is there any speed control at GWC" (speed limit point) - is that a rhetorical question or are you hoping that I don't know that GWC is the speed limit point and say No????

Sorry for the long post, but Ifelt that some explanation for the questions was in order.

cheers
VL

[Intruder]

1) Can you please tell me, do you fly the same cruise speed every time? or does it vary with temp, FL, weight etc...

It differs with load, FL, schedule, and fuel. In the 747-400, for example, "normal" Mach runs between .83 and .85. If in a rush, maybe as high as .86; if low on fuel, as low as .81. Lower-than-normal FLs (<280) bring it down further; headwinds bring it up.

2) Do you convert to flying IAS at the same FL each time or does that also vary?

It varies slightly, and is usually done automagically by the FMS. It also depends on the IAS target -- the higher the IAS, the lower the transition altitude.

Again in the 744, if we're flying Mach .85, we'll take 330-350 KIAS when we can get it, and will set 310 KIAS when in a crowd, because it's at the top end of the "expected" 290-310.

[Bealzebub]

Most aircraft operate at a mach number above around FL300 and at IAS below that. Cruise mach number does vary depending on weight, winds etc. If you want something ask for it. That is in fact what most ATCO's do ! We generally fly at around M0.79/M0.80 on the 757 although this may be higher on occaisions up to M0.82 or lower 0.78 in turbulence. Below around Fl 300 (this FL may vary slightly) the IAS will be around 290 - 300 knots. If the profile is getting a little high the excess height may be traded for an increase in airspeed rather than using speedbrake unless the controller has asked for a specific speed in the descent.

I can assure you that co-operating with ATC is a vital part of our function at all times. That is particularly so when we are relying on those same controllers to help expedite our arrival as safely and efficiently as possibly. I am not sure how someone sounds "smug" over the radio but it sounds like you might have a personal issue worthy of addressing ?

The reason you are asked if there is any speed control over GWC or wherever is because it takes time to slow an airliner down and for planning purposes it is often very helpful to know this information in advance. Most pilots will be able to asses how busy the airspace is from the overall level of ATC chatter and not bother to ask, but simply comply with the speed restriction. If there is any doubt you may be asked, and of course ATC will often advise there is no restriction well before the aircraft reaches the speed limit.

We all know how busy ATC is, particularly in the London area, and we admire the effort and dedication that goes in to controlling aircraft so efficiently in what is a high pressure environment. I am a little shocked and dissapointed that you have laboured under the impression we seek to make your life difficult or set out to cause you trouble. such behaviour would be self defeating and generally unthinkable. Of course we would all like high speed unimpeeded access to the final approach fix. We all know that isn't going to happen unless it is very quiet indeed.

Remember also that the problems are not all yours. You "ATCO type people" will often spout twenty consecutive instructions without a break and then wonder why the backlog of aircraft all trying to establish handover check in, all step on one another. Of course we know this is just symptomatic of the high levels of traffic.

I dont know how long you have been doing the job but if it is a long time ( which I seriously doubt) can I suggest you book a holiday. If it is a short time then you might be interested to know that many airlines still offer a familiarisation jumpseat ride for the purpose of seeing the problems and operation from the other side of the fence. You might want to avail yourself of this facility. I think you would find it useful.

[DJ Mixmaster]

Hi!
Just noticed this post and saw that nobody has replied.
I am a bit fresh and not flying commercially yet, so bear with me.
I'll try to answer you but it's all relying on my ATPL-theory knowledge and not from flying jets.
1) Cruise speed is a specific Mach number and ideally the speed for optimum fuel consumption. Both cruise speed and level will vary and is specifically calculated for each flight. A jet engine is most efficient at the top of the troposphere, where the temp is
-56 degrees celsius. In ISA this will be approximately FL 350.
2) For each flight you have a different crossover altitude where you switch from IAS to Mach and vice versa. Mach varies with the square root of the absolute temperature, which is rarely the same. Hope I have clarified a couple of things.

[alexban]

1) The FMC valculates the Econ speed depending of weight,altitude,temp...One important thing that varies from company to company is the " cost index" . This is a value that defines the overall costs of trip fuel compared to other direct hourly costs.This value is set by the company for comercial reasons,and has a big impact on speed.
As a example ,for a 737 clasic setting the CI at 30 will result in cruise speed of aprox M .74 , setting CI at 80-100 will give you aprox M.77 (econ speed)
For a 737 NG CI 100 - M.8 CI 30 -M .75 (aprox)
I wonder what are CI used by other comp for 737 clasic/NG
We use 80/100.
This is Econ Speed calculated by the Fmc.If in a hurry you can increase speed at a fuel cost,of course.
Also ,other important thing is turbulance. If this is the case ,a 737 clasic/NG will keep a speed of aprox .72-.73/ .76 (also depending of weight,alt,temp..)
2) When preparing for descent you have in the fmc two speeds selected (by fmc or yourself) for descent: mach number and IAS. I usually select ,for 737 , .76-.77/300kts .As the airplane begins descent ,it mantains the selected mach number.The IAS will increase as the plane descents,and when the speed reaches the selected IAS value it will keep it.
The change from mach to ias depends of course of the selected speeds,and usually occurs somewhere between FL 250 -FL 280.
From your ex with EZY ,mach .69 on a 737 is aprox max range speed.Maybe some fuel/time considerations on that crew. 280kts it's the turbulance descent speed above FL 100
3) most airports have speed restriction point. Depending on traffic,bussy or not ,the controller could authorize a pilot to disregard the SLP,and use his econ descent speeds.Sometimes ,for ex at IST the SLP will tell you to keep 250 KTS from 40 NM distance.Maybe,if not a busy day,you can keep a higher speed,why not.
Rest assured ,the pilot it's not testing your knowleadge of the SLP,he just want's to know if he can fly faster than the restricted speed on that specific day.
We usually ask something like this :" Any speed restrictions?" You can say,yes ,keep 250 at XXX or ,no,you may keep high speed. Or something like this.You decide,of course.
And yes,saddly there may be some pilots that do not respect exactly the ATC indicated speeds (due to considerations of their own ) .This is not such a safe thing to do.Maybe some use of radar vectors,to wake them up? :)
I hope this helps
Brgds
Alex

[VectorLine]

Intruder, DJ and Alexban - thanks for your replies. Quite comprehensive. It's good to be able to use this forum to get a feel for what goes on in the cockpit.

Now that several airlines are re-starting fam. flights, we atcos can once again have a first hand look at how atc affects you and what we can realistically expect from you.

Keep the replies coming please...


Bealzebub;

it sounds like you might have a personal issue worthy of addressing ?

No - I work hard to provide the best service I can - Best Fls I can negotiate with other sectors/centres, shortcuts at night and minimal speed control. And when I am unable to help (e.g. getting +FL270 for EGLL-LSZH from the French) I try to explain the reason to the pilot in question. I just find it annoying when a pilot who can't/won't help me out refuses to say why (unless it's something unsuitable for public broadcast?).

I am a little shocked and dissapointed that you have laboured under the impression we seek to make your life difficult or set out to cause you trouble
I didn't say that. I gave one example which I found annoying and gave 3 airlines that I personally find difficult to understand the speeds they fly. Hence my post - I am trying to improve my knowledge

I dont know how long you have been doing the job but if it is a long time ( which I seriously doubt) can I suggest you book a holiday. If it is a short time then you might be interested to know that many airlines still offer a familiarisation jumpseat ride for the purpose of seeing the problems and operation from the other side of the fence. You might want to avail yourself of this facility. I think you would find it useful.

Five years my friend. Long enough to have a fair idea of what I'm doing and to have had a good amount of time on fam flights. But short enough to be enthusiastic enough to try to do the best I can. In the near future, I will be spending an entire day with an EZY crew and attending a 'customer awareness' day with NATS and some representatives from various airlines.

As I said, I am trying to improve my understanding of the flying side.

Thanks again to the others for their informed replies.
VL

edited to remove an angry response :ouch:

[Bealzebub]

Yes perhaps the reply was a little caustic, however your comments of :
----------------------------------------------------------------------------------
"Im not saying you are lying about your speed (although in a busy moment I have been known to speculate!), "

" I am convinced that some airlines/pilots (particularly those with tight schedules at EGSS!!!) fly Mach numbers as long as they can in order to maintain a high ground speed - which really p155es me off when trying to stream Lorel arrivals via MAY or MID."

"I could not get the pilot to tell me why the slow mach number - he just sounded very smug and seemed to enjoy the fact that he was causing me so much trouble."

"Many pilots ask me "is there any speed control at GWC" (speed limit point) - is that a rhetorical question or are you hoping that I don't know that GWC is the speed limit point and say No????"
----------------------------------------------------------------------------------

Did rather set the tone didn't it ? The reply was generally a factual answer to your questions which were (save for some of the above comments) very good points ! In the 25 years I have been doing this job I have had the pleasure of meeting many air traffic controllers and most of those have been on familiarisation flights with me. My crew and I have always made a special effort to not only ensure the flights are as informative as possible, but also as relaxed, friendly and hospitable as possible. This seems to be borne out by the many thank you letters that have resulted from such trips. Of course not many have seemed quite as opinionated as your good self. Anyway you probably wont have the "misfortune" to have me on any of your famil' flights, and I am sure you are correct in your added presumption that "you wouldn't learn much". Nevertheless wherever you do go I sincerely hope that do learn something.

;)

(edited to remove the sarcastic bit :E )

[Giles Wembley-Hogg]

If the unmodified FMC speeds were used by the 757s and 767s operated by one British airline, then the 757s would come down at 0.79ish/277kts and the 767s would use 260kts almost the whole way down.

In practice most of us modify this to something of the order of 0.8/290 or 0.8/300. In turbulence we have to use 0.78/290 (Flying Manual restriction).

As I am sure you know, speed control is best used to maintain a gap rather than create one and it only works really well if the speeds are set up before the descent commences. I appreciate that as a South Banker you often get us fairly late from the French, so that will not always be possible.

Perhaps 757/767
Fast 0.8/320
Med 0.8/300
Slow 0.78/270

(As an aside, most of dislike flying much below 0.78. I think this is partly because we rarely operate there and partly because we need to check both the Mach graph and Vref+100 at those sort of speeds).

Hope this helps.

G W-H

[VectorLine]

G W-H, Thanks, good stuff, thanks - by the way do you have an ATC connection? or have you visited a few times?

Bealzebub; thanks for your reply. I do agree that when I see those comments put together, the post could be taken in a tone that was not intended. I have retracted my statement about not learning anything as I realise now that I bit at the hook.

However, am I correct in detecting a sarcastic tone in your recent reply? I'm sure that no one would describe 5 years experience as 'Veteran' - I am still learning, hence my questions

[5milesbaby]

Good post V-L, I think there are many of us that need to learn from this due to the multiple times we now find ourselves faced with bunching.

I too have found that an IAS restriction off a Mach has been ignored (ie A/C flew Mach to much lower therefore going to IAS 320+ and eventually reducing to 280kts imposed, not good for streaming).

As for sounding 'smug' on the RTF I can assure many that it is done and is easily noticable.

From a Swanwick perspective the SLP's are to be observed unless a speed restriction has been already imposed. However, the amount of times I still see an A/C given 300kts slow on its own accord due to an SLP of 250kts is incredible, and has been debated lots on here too, but it still happens. Swanwick can rarely lift the SLP restriction, the rare occasions we do its either due to a) prior co-ordination with Terminal Control to request it, or b) its 'night time' so speeds and descent profiles can be mostly ignored.

A question I have on the subject is to what Mach minimums the B737 series 300 and 700/800 and A319 can do at FL400/FL380 for the A319/NG's and FL360/FL340 for the 300's. Also would you prefer to be vectored all over the sky at 280kts to get the 10nms in trail, or have an early speed reduction and kept on route? Either way it results in 10 nms+ by MID/MAY, I would expect speeds rather than huge turns of possibly 70degs+ are better for Pax comfort. Would telling you the position of the number 1 with the IAS imposed for it help to get you to help us get in trail?

No guesses that we are talking RYR/EZY/EZS for the trailing, and its only getting worse as the fleets get larger. Unfortunately the trailing is crucial for the Vaton sector to integrate the MAY traffic with the MID and OCK traffic, and then slot through the tiny gap between the LHR stacks. All input is welcome, and any suggestions on how we can improve what we already do by using the modern A/C characteristics (ie FMS/offsets etc) is always welcome.

On a side, as V-L says our fam flight program is SLOWLY regenerating itself. From our perspective I also think seeing our side of things on the Dover and Worthing sectors, as well as others (ie Clacton for the same matter of GW/SS arrivals) could be hugely beneficial in understanding why our streaming is needed. I can guarantee a good day out - even if we cannot provide Duty Free half way through ;) ;)

[Giles Wembley-Hogg]

Not much help when you are streaming 737s for TC Capital I know, but a friend tells me that the 777s operated by a large British airline using the most often used cost index (CI 100) will descend at 0.795ish/295kts. These are the figures generated by the FMC, but seem popular with the crews (I am told).

Hope it helps add to your "rule of thumb" figures.

G W-H

[BOAC]

Good thread, VL and 5mb, and let's hope we can all learn from it! Again I hope that we all try to do what you ask. From my perspective, BA 737, cruise M/IAS varies as above with all those things, plus the wind - a strong head programmes the FMC to fly faster and conversely. Cabin service can affect cruise speed on short sectors too, but we do (I hope) TRY to let you know early and they ARE subordinate to your requests.

Personally I prefer early speed adjustment to vectors (unless they are towards LGW :D ) The point of transition from Mach to IAS is almost impossible to pin down. Information on the one ahead/behind? Yes please - we can then see the 'big picture' (even if we have no hooks to hang it on:eek: ). TCAS has been a significant help in the last few years.

Speed limit enquiries abound because it is far from unusual for them to be 'lifted', so we ask to check. We ARE, I hope, intelligent enough to know that when it is busy it is pointless, although I have been 'surprised' by our excellent LGW approachers nipping us in quickly in a busy spell when they see a gap. If they upset/irritate you, I'm afraid I cannot see what can be done about it.

Bring on those famils!

[van der vart]

any radar controllers use mach number at high levels to control aircraft's descent speed? or just use indicated airspeed?

isn't mach nummber varies at different altitudes?

is aircraft able to maintain a certain mach number during descent?

roughly, each M0.05 = 30 kts indicated?

[BOAC]

vdv -

1) A few
2) yes
3) yes for a while till IAS takes over
4) yes

Sorry to be 'short' but am busy!

[BlueEagle]

The 'normal' mach no. in cruise for a 744 is a factor of the particular airlines fuel policy and the fuel index figure put into the FMC. Whilst, for Intruder, .86 would be 'pushing it', for other operators it is their normal cruise Mach no. at the optimum flight level, coming from the far east and approaching TOD for, say, LHR you can expect to see quite a lot of B744 doing .86 mach.

[VectorLine]

Thanks again for all your discussion on this subject.

I have found that Mach numbers imposed on transfer from the French are very useful in helping me create a gap, in conjucntion with some vectors. A good Mach difference seems to work much better than IAS - e.g front a/c M0.78 second a/c 0.75 - Gap happens! Of course, this does work better for the long run in from ABB - GUBAR - MAY than the short trip into BEXIL. However, there is more scope for parallel headings for EGKK and EGLL.

Now, for your SS and GW? - they just HAVE to be in trail (10 miles if poss) by MAY (or AVANT on the other side).

-- So, for pilots into those destinations - How do you feel about the occaisional doglegs of right 340 degrees then left onto 290 degrees (via MAY) for spacing. *And I must add, that it is very rare that pilots inbound SS/GW grumble or moan about these big vectors. A credit to your understanding and patience - sometimes :p

-- And like BOAC, would you prefer us to knock your speed right back rather than those doglegs. Bearing in mind that if you are at the back of the queue, this might mean min mach/min clean from about GUBAR?

-- Roughly what would be the difference in kts of say M0.77 at FL300 and FL310? (of course ignoring temp/wind etc) For some reason, I have 6 kts in my head but I can't think where I have got that from....:confused:

-- One last Q for the moment. We are always taught that a/c can't 'Slow Down and Go Down'. However - I am sure you folks have some clever aviation tricks up your sleeves these days. What are the chances, if asked of you being able to reduce your ground speed but increase your rate of descent?? Of course this is normally only required when we have screwed it - although don't expect us to admit that!! :) - we will still blame you he he he :E

Thanks for all your help
VL

[BOAC]

VL - in the example you quote 0.78 to 0.75 is around 18kts less Ground speed.

The only info on different IAS I can give is from the Boeing 737 table which shows a !YES! 6 kt increase in GS at M0.745 between 310 and 300 (good head you have there!)

Ref going down and slowing down - as Monica discovered, it ain't easy:p You will always lose out either on descent or deceleration capability, but yes it can be done. The expression 'you cannot' is an over-simplification to avoid expectation of miracle working!