At FL400 M.85 is close to 270KIAS. FL200-250 is generally where the Mach to IAS conversion will take place when doing a normal conversion from M.85 to 310 KIAS or M.82 to 290 KIAS or similar. You need to get your head around the Mach/IAS relation at high altitude.

You also need to learn more about the airspace outside your own little box. If you don't understand your adjacent sectors, how can you provide good ATC "services"?

Switchover between 200 and 250? That is mighty low. We do standard switchover at 295 and FMC switchover is usually higher up to FL360.

From PSR's post he has a grasp of the Mach/IAS concept and of switchovers. The instruction seems quite clear to me, fly whatever mach number you want until you reach an IAS of 270kts and then fly that IAS. Not that hard to grasp and even easier to program into an FMC. Speed control is there to squeeze more of us into a given constant airspace and of course prevents slot restrictions and holdings, it is in our best interest to follow it as we follow every other instruction by ATC. And if we can't "unable" seems to work quite well.

Switchover altitude depends on the high-altitude Mach number and the low-altitude IAS setting you program into the FMS. Those also vary with the Cost Index you set.

With a low CI, the switchover altitude may be very close to cruise altitude, because the FMS is going to revert to a low IAS (as low as 250 KIAS in the 744) very quickly. With a higher CI, the switchover altitude will be lower. If you manually program a low Mach number and higher IAS (e.g., .80/310), the switchover will be much lower. With a high CI (>250) and left to its own desires, the 744 FMS will program a low-altitude descent of >335KIAS, and will reach it closer to FL 200.

Also, without further direction, it is implied that you will retain your present cruise Mach in the descent until reaching the switchover IAS. Last I knew, the default low-altitude descent speed is 290-310KIAS above 10,000'. There is seldom a "do whatever you want" aspect to it.

On the contrary I do understand my adjacent sectors very well. And you're right, that is absolutely essential in providing a good service.

Perhaps my understanding of the mach/IAS is not quite correct then. My understanding was higher level cruise (depends on a/c type but usually above FL300) was done in mach. Maintain a constant mach during descent (e.g. 0.795), during descent on a constant mach groundspeed will increase slightly as you get lower. IAS, even though not flying with reference to it at higher level, will be fairly low at FL400/M0.795, something like 220kts or maybe less. This will also increase gradually as you descend on a constant mach. Then on reaching a specified IAS (say 290kts) you switch from flying mach to flying IAS. Descent on IAS will then cause groundspeed to decrease as you descend. You maintain that IAS (provided no other factors such as turburlence etc require you to alter it) until reaching lower level where any other procedural speed restrictions may kick in (e.g. 250kts at FL100).

That is my understanding provided there is no intervention from ATC. If it is incorrect then please do say as this is what I use on a daily basis to separated a/c and I would really appreciate further understanding. As Denti explained, my instruction was intentionally issued at high level, and in good time for the pilot to plan a descent at not above 270kts. Since at high level he is highly unlikely to be flying faster than 270kts, the speed restriction wouldn't really kick in until he is at say FL350(ish) (if flying at M0.85 that is approximately where I'd expect it to be). Bare in mind I never asked the pilot his mach number to begin with as I couldn't care less. I just needed him slower below FL300 as he descends and meets other traffic.

Of the airport, but to make easier for you to comprehend, that will occur when being handled by a TRACON, versus an eroute facility, and the closer I get to the airport, the quicker my response will be.

PSR

Your understanding is pretty good and the earlier posts are either showing remarkable lack of basic knowledge, or trying (as your case illustrated) "to cheat the system".

"Switchover" altitude partly depends on atmospheric conditions, but more on what the mach / IAS specified are. In an A320 series, at FL350, if you ask me to slow to min/holding speed, I am effectively flying IAS at FL350.

My (and I am unusual) main concern with ATC speeds are unrealistically high speeds required at lower FLs, particularly "increase speed" instructions. These cost a lot of fuel, and given we still fly in random directions / hold, achieve nothing!

You were the one who said (Post 41) you didn't necessarily understand operations in other sectors...

Your understanding is essentially correct, but with a major bad assumption: IAS can well be above 270 at higher altitudes (the 747 frequently will cruise at 310 at M.85), so you SHOULD care about the cruise speed/Mach. If you want a specific speed at a specific altitude, then make it perfectly clear by stating "270 KIAS by FL300"! Otherwise, you will be at the mercy of the current FMS setting and a possibly lower switchover altitude.

Indeed I was and this is completely correct. There are at least 60 sectors in the UK, of which I work 7, and I do have a detailed knowledge of sectors surrounding mine, and I have a basic knowledge of the sectors beyond them. But that probably accounts for maybe 30% of the UK. There is simply no requirement for me to have any kind of knowledge about the rest, as nothing I can do will affect them. Therefore it is true to say I don't necessarily understand, that would depend on which sector is being referred to. The post I was responding to didn't make clear which sector or route he was talking about.

And it's no problem if it is. I assigned him 270kts or less. If he was flying faster than 270kts then I would expect him to slow down to 270kts (or less) and comply with the restriction. If he can't then I would expect him to tell me he is unable (with a genuine reason) and I would change the plan.

Fair enough, I can understand how annoying that must be. I can think of a few reasons why that would happen, though none of them excuse it:

1. The controller gets caught off guard when the aircraft descends at anything less than say around 270kts from high level. Of course we don't know what speed you'll be flying when you convert, and the vast majority of airlines fly at or faster than this (in an A320 series a/c). We usually only apply speed control if we have less than approx. 12 miles in trail, and we may have a lot more than this (20+ miles) at high level. 20 or even 30 miles will very quickly erode if you fly at say 250kts and you are in front, and that may prompt an increase speed instruction (but would normally be more last minute when a large catch up is apparent). If the first a/c is doing 250kts for economy, and the second is doing 300kts for speed, we may give both 275kts as a compromise. The problem at this point is the first a/c is low on profile and the second is high on profile, therefore both a/c have wasted a lot of fuel compared to if they had known the speeds before commencing descent and were able to plan accordingly.
2. The other main scenario would be when you are cruising at say FL240 and there is higher descending traffic for the same destination/STAR fairly close behind who has no chance of overtaking you without some speed manipulation. Either we slow down (to facilitate an overtake), or we speed up the aircraft at FL240. The problem here is that applying the same speed will still not provide enough separation because of the level difference. The closer the a/c are, the bigger the difference will need to be (If we had 15 miles we could probably give both a/c 270kts, because we could allow a catch up to begin while there is a large level difference - the speed difference will become much less as the 2nd a/c becomes much closer vertically to the front a/c). If we only had 7 miles we'd need similar groundspeeds immediately, so something more like 300kts for the front a/c and 270kts for the back a/c would be needed to account for the large level difference. The situation is made far worse when the back a/c are transferred already high on profile and therefore need the higher speed to make the level restriction. I hope that makes sense, quite difficult to try to put into words.

There is absolutely no requirement to do so and you shouldn't really need to, but you are most welcome to tell us what speed you are planning on when you call us in the cruise. This saves anyone getting caught out and will obviously save you fuel. Until controllers/the airspace design changes to accomodate the more economical speeds being flown, this is the only short term solution I can think of at the moment to this problem.

I don't mean to add fuel to the fire but.........
 

I like the discussion about FMC cost index & crossover speeds but.......

You have a 737 at 17,000' indicating 320 KIAS.

You (ATC) tell the flight to slow to 300 KIAS. The flight does not slow to 300 KIAS. 300 seconds pass and you see no change in groundspeed.

Put an end to the nonsense and file a violation against the crew; just make sure that your phraseology was textbook.

By the time the crew gets through all the related tension, pressure, work and expense involved with defending themselves against an alleged violation of aviation regulations they will have wished they had simply complied.

Guaranteed the "news" will spread that XYZ Center/approach darn well means 300 knots when they say 300 knots. Likely one will see company bulletins, online notices and "emphasis items" during line checks (here we go again).

No, you are not being an A**, you are trying to do your job safely with limited airspace. If you do not get cooperation then YOU get to make the decision on initiating certificate action. Was the non-compliance out of misunderstanding or arrogance? If it was arrogance; then file the violation.

I think the vast overwhelming majority of us try and comply with every clearance every time recognizing we work with fellow professionals.

PSR ...keep in mind that B747 will burn less fuel being vectored extra distance for separation at zero flap speed, rather than having to slow down "way out" and dragging flaps. Zero flap speed upon arrival could typically be around 220kts. :ok:

Yeah! Let's drag 'em into court!
Good point Northbeach :ugh:

Got to agree with Penko's post. If you've planned on a certain speed for decent and then get asked to slow down it will put you high on profile and some pilots hate using speed brakes. Personally I just advise that I'm unable to make the restriction, however as NOD says it can sometimes take a while to get a word in edgeways. Secondly as someone else pointed out, jet aircraft don't go and slow down. If you allow it to do a little bit of both it will take a while to reduce speed.

The biggest time that I'm guilty of not adhering to ATC assigned speeds is the 160 to 4d request. The aircraft simply will not slow down without dropping the gear. Very often it ends up being 170ish in which case I'll drop the gear at 6 miles and be reducing through 150 by 4 miles. (basically too fast for the first couple of miles and too slow for the last couple of miles). Not strictly correctly but I believe it evens out correctly. The alternative is drop the gear when requested, p**sing away loads of fuel and making lots of noise. Of course if the speed is ridiculous I will drop the gear early.

Standing by for abuse :)

How does ATC "justify" it? How can anyone who flies not know the answer is "traffic"?

If your "extremely tight margins" don't allow you to "hope for the best, plan for the worst", it calls into question the viability of the operation itself. If saving 50 lbs of fuel is going to make or break the company then the real concern is how such a shoe-string operation can afford far more costly and more critical things like maintaining the aircraft to a safe standard or hiring experienced pilots who understand the difference between theory and reality when it comes to flight planning.

In the future, you should apply real-world aviation experience to your operation's fuel-figuring and (that's right) uplift more fuel. It sounds like the policy (or the person driving that policy) itself is flawed or not well-versed enough if it/they ignore the world of ATC delays/changes etc. They are as much a fact of aviation life as dynamic/unforecast weather situations, blocked runways, State aircraft showing up, aircraft in distress, etc, etc that can force delays that require more fuel than what your planning program includes. A couple bingo-fuel diversions in a year will blow any teacup-fuel savings you think you're realizing by flying around in a potential pan-state all the time.

In the meantime, unless you've stated to the controller you can't accept a speed/altitude assignment or its been given at pilot's discretion, you're obligated to comply promptly, not "as tardily as you think you can get away with". Your faulty planning or erroneously thinking yourself unique in the cause to save fuel/money for your operation does not justify unprofessional behaviour in the cockpit.

My experience is that when flying to the UK the workload is always higher than when flying anywhere else in Europe. This is mainly due to ATC. As a relatively new FO it's hard to know what to "usually" expect from ATC the first couple of times when flying to a new destination. Never flown the whole STAR into any UK aerodrome.. ATC trying to get the most out of the airspace increases the workload on the flight crew making them more prone to mistakes.

Sometimes winds, weights and icing conditions make it pretty hard to lose speed/altitude.

Re. the speed issue; nobody has mentioned the fact that on the Airbus in managed descent mode, the speed will sometimes vary a bit (if the winds are different to those forecast/entered) in order to stay on the correct descent profile. Our SOPs state (and common sense suggests) that selected speed should be used if ATC have assigned a descent speed but that doesn't stop some pilots (who think they know better) to leave it in managed rather than use the speed brakes.

That happened to me a couple of time. After I received and speed reduction instruction , I followed that as required, but wind was from calm to tail or front to calm, during descent, so my IAS was 20 kts less as required by ATC control, but my GS was still the same one. Sorry, as ATC Control please asked for another reduction for another 20 or more kts. This could be what happened to you.