time at an ATC directed way point

[hung start]

If you are told by ATC to be at waypoint YYYYY at XXXX time what are your tolerances ? I have searched the AIP and Jepps and can only find reference to flight plan time keeping and leaving the hold .

[Pontius]

As far as I know there are no tolerances. If you're told to be at a waypoint at a certain time then you comply with that. However, that type of restriction is rare and much more normal is a, "cross XXXXX at YYYY or later"/"YYYY or before". They are obviously sequencing traffic so being +/- on a restriction is not going to help their plans, nor endear yourself to them :)

[hung start]

They must have some tolerance i.e. you couldn't cross that point 5 mins earlier than asked because they have given a time to be at YYYYY waypoint . They are obviously doing it for traffic management . Recently we have been given some major speed reduction which make the time impossible so have had vectors to make the time . If there was a plus or minus 2 mins that would be ok but in the terminal area or approaching it , 4 mins maybe too much . I am happy to comply as i don't have the big picture . Just a point to ponder .

[SHRAGS]

Recent AIP supplement refers. Up to 30 seconds early is ok at 250 kts.

http://www.airservicesaustralia.com/aip/current/sup/s14-h72.pdf

[hung start]

Perfect thanks SHRAGS . I had money on 1 min early but can't be late .

[apache]

I have yet to be issued a crossing time for a waypoint where I had to speed up!!!
Generally it is a slow down, and most times almost unachievable

[Going Nowhere]

Used to be...

[Capt. On Heat]

ATC AU 611.

[le Pingouin]

Better to be early that late - we can always slow you further or vector to lose a few miles, but it can be rather hard to make up time once it's been lost. Depending on the traffic ahead we might be able to let it run (or not).

Maestro works by calculating what everyone's untouched landing time is (based on descent profiles, winds and typical times from fix to the threshold for type and airline). That gives the landing order. It shows us the untouched time at the fix and we can accept it or adjust it (possibly shuffling the sequence) by putting in a time you tell us, a time we think you'll do (based on observation of previous traffic) or we might fiddle it a little for ease of traffic processing and overall efficiency or to fit medical traffic and such.

An acceptance rate is also set - so many per hour or so many seconds between arrivals (depending on wind affecting time spacing on final, weather, arrivals or departure mode and such). Maestro simply takes the first aircraft, adds the time spacing between subsequent arrivals and back calculates a time for you to cross the fix at.

As you can see if you miss your fix time you'll end up out of position relative to the aircraft around you in the sequence. You're 60 seconds late and the next guy is 60 seconds early and left untouched you'd be maybe 30 seconds apart at the threshold. Sometimes Maestro does a less than stellar job (actual winds different to those used for the calculations for instance) and it generates dead heats from different directions, but that's what approach gets paid the big bucks for!

You'll note we only give fix times in minutes while Maestro calculates in seconds, meaning there's already 30 seconds of play in the system so even if you're on time to the second you may get tweaked for optimal spacing on final. For better or worse it's a dynamic environment with different aircraft flying different profiles, so despite everyone's best efforts tweaking will be required. We're not doing it just for fun believe me!

[Kharon]

LP - Honest question – why not issue a simple speed reduction, save time on the radio and tweeking about in the cockpit, power for TAS to ground speed/ distance calculation etc. Always wanted to know as it is one of my 'pet' though unspoken, very minor dislikes; especially on short-ish sectors or at short notice, or, on descent, through the lumps. It's doable, and no sweat but – I've always wanted to ask....?

[tyler_durden_80]

Apparently the 'industry preferred' method (quoting the docs available in my aisle at work) is to allocate the time crossing the feeder fix to the pilot, with 250kts after...in practise though, If running a sequence I'd want more control over it, so i issue a descent speed, give an indication of the feeder fix time we're after, and finesse through vectors if needed. That way I avoid having a leading aircraft reducing to 230kts to meet the time at the feeder fix, only to have number 2 in the sequence at 280kts yet still meeting their feeder fix time.

[hung start]

Thanks for the replies , i understand you ATC guys don't do this for controller amusement , we just had several ideas of the answer in the cockpit and sometimes its nice to know the correct answer .

[topdrop]

I have yet to be issued a crossing time for a waypoint where I had to speed up!!!At least once a week, we will get a response from a pilot that he can't get there til 2 or 3 mins after the time given. This is even when using a prayer wheel and allowing for slow up on descent shows that the time given is a slowup. To be diplomatic, I say cross not before the original time and funnily enough they always cross at that time - must be an FMS issue.

[Capn Bloggs]

That way I avoid having a leading aircraft reducing to 230kts to meet the time at the feeder fix, only to have number 2 in the sequence at 280kts yet still meeting their feeder fix time.
Please explain? If the first aeroplane is at 230KIAS, he's ahead of where he should be on the railway line. If the following aeroplane is at 280KIAS, he's obviously behind where he should be on the railway line. If they both go through the station at 250, they will at the very least be only at the correct spacing, and probably wider than desired. What's the problem?

At least once a week, we will get a response from a pilot that he can't get there til 2 or 3 mins after the time given.
Is that after you have asked for the original estimate? I have had "system estimates" that haven't been within a bull's roar of what we see in the cockpit.

[topdrop]

Is that after you have asked for the original estimate?We (non maestro) work out untouched landing time. Gives us the landing sequence. Work out landing time to make sequence work, then work out time at feeder fix to make good the landing time. Only give a fix time that requires a slow up, we don't rely on max giving us anything, except in a very few cases.
Our original estimate for fix A is 43. Needs to lose 2 mins, so instruct to cross at time 45. Pilot says he can't get there til 47, despite our estimates showing he should be there at 43, so we issue cross not before 45 and they always seem to turn up at time 45 despite their original estimate of time 47.

[Nautilus Blue]

Is that after you have asked for the original estimate? I have had "system estimates" that haven't been within a bull's roar of what we see in the cockpit.

TAATS/MAESTRO estimates for the descent phase are very unreliable. They use the TAS in the flight plan, forecast winds and a defined profile based on aircraft type, none of which necessarily reflect reality. Therefore for PH we ask aircraft for their estimate and then put that in MAESTRO. I'll be diplomatic and say some aircraft/airlines are more precise than others. "Unforecast headwinds" often mean aircraft can't make the estimate they gave us at 200 miles out.


Please explain? If the first aeroplane is at 230KIAS, he's ahead of where he should be on the railway line. If the following aeroplane is at 280KIAS, he's obviously behind where he should be on the railway line. If they both go through the station at 250, they will at the very least be only at the correct spacing, and probably wider than desired. What's the problem?

You would think that, but no. Sometimes the second is ahead of where that should be, and planning on slowing down later.

Honest question – why not issue a simple speed reduction, save time on the radio and tweeking about in the cockpit, power for TAS to ground speed/ distance calculation etc. Always wanted to know as it is one of my 'pet' though unspoken, very minor dislikes; especially on short-ish sectors or at short notice, or, on descent, through the lumps. It's doable, and no sweat but – I've always wanted to ask....?
Thats what was done 20+years ago in ML when I started, there is just too much traffic now. We need you over a point at a time, the thinking is your computer has all the information to calculate the required speed and descent profile, we don't. To use a ridiculous extension, we would never give you a power setting to achieve a speed, we just give you the speed and let you work it out. Carry that forward, we give you a time and let you work everything out. Plus we were told that one airline has boasted they could make fix times the the second (they can't).

I do sometimes wonder though when we are told "Industry wants..." if our people who'd don't actually control are talking to airline people who don't actually fly.

With regard to the original question, think in terms of spacing. On a good day we will run a two minute sequence. 30 seconds early puts you only 3/4 of the required distance behind. If you are a minute early and the preceding is a minute late, you are going to be sharing a piece of sky.

[Kharon]

Thanks NB – itch scratched.

Solid gold. Me too...

I do sometimes wonder though when we are told "Industry wants..." if our people who'd don't actually control are talking to airline people who don't actually fly.

http://thumb7.shutterstock.com/photos/thumb_large/498865/119471821.jpg

[le Pingouin]

If we give you a speed rather than a time we're just guessing - it might be an educated guess based on observation of past performance but it's still a guess and we might miss the fix time by a considerable margin.

The idea is we try to let you fly the profile you want to achieve the time for the sake of efficiency. As has been said, that's supposedly what your accountants want. However if you're in trail I'll ask the lead aircraft their speed and give a speed for the following to keep the desired spacing.

The problem with speed differences come from different crews/aircraft choosing to lose time in different ways - if the front one reefs it right back early the back can close up remarkably quickly.

[hung start]

Can we program Maestro to give us another 15, 20 mins to slow down .We always try hard to make the time however sometimes we need to come back from cruise speed quickly to be back at min speed then accelerate to 250 kts which is not efficient flying

[Oakape]

which is not efficient flying

When you get a lot of traffic wanting to be at the same place at the same time, efficiency needs go out the window. It can't be done efficiently with current technology. Trying to fly efficiently in these scenarios just leads to everyone getting stuffed around.

The only thing that may help is getting the required time at the fix much earlier. It is easier to get there at the time required while in the cruise than on the descent. RTA works better in the cruise & you generally don't get the big wind changes that you get during the descent.

For me, having a required time at a pre-descent point & then a fixed speed schedule for descent would be the easiest way of ensuring everyone gets where they are needed at the correct time. Any corrections required due to wind changes on descent could then be dealt with by ATC speed control or vectoring.

Plus we were told that one airline has boasted they could make fix times to the second (they can't).

The 737NG displays the time at the next fix to the tenth of a minute. However that moves around a lot as things change on the descent, so it can be a tiresome exercise to keep the arrival at the fix to a specified time that is that accurate. It also depends a lot on the person doing the flying. Some are better at it than others.

I don't know about Airbus, but the Boeing FMC doesn't do well in the descent planning stage when there in an increase in speed in the descent. Reducing speeds at various points throughout the descent is fine, but descending at 230 kts & then having an increase to 250 kts further on means that the descent is practically manually calculated. Again, some quite good at it & some not so good.

Time to leave the hold can also be an issue. You will see the ETA at the fix moving around as you fly the hold. The Boeing FMC is not to great at adjusting the pattern to meet a time at the fix, so once again you end up adjusting it manually.

Having a defined requirement in the AIP for all these scenarios would be helpful. As said earlier, there is only the time to leave the hold - 1 minute earlier & no later, or the +/- 2 mins at a en-route waypoint. (I stand to be corrected if It has changed & I have it wrong). If we had the required time tolerances for all these scenarios it would be easier to get it right. In the absence of any other information, I have been using the 'time to leave the hold' tolerances when assigned a time at a waypoint during the descent. It would seem that more accuracy than that is actually required.

[Capn Bloggs]

The problem with speed differences come from different crews/aircraft choosing to lose time in different ways - if the front one reefs it right back early the back can close up remarkably quickly.
Seen that a few times. There are a couple of ways to skin the cat; one of them results in exactly that. Tigermoth cruise speed and Concorde shuttle descent speed...

Can we program Maestro to give us another 15, 20 mins to slow down
Agree. In the "old" days when humans did it, we got our FF time nice and early (even in the climb!) and didn't need to climb to the moon, only to descend back down almost straight away. Now the machine's doing it, we get our FF time much later which requires more drastic action. That said, over here the ATCs are getting adept at dropping "hints" about how much time we'll have to lose instead of just saying "reduce by 0.05", which in the big scheme of things has very little effect at all.

[Nautilus Blue]

Oakape interesting, thanks foe that. I had assumed (i know) that you just fed in the time and the machine took care of everything.

For me, having a required time at a pre-descent point & then a fixed speed schedule for descent would be the easiest way of ensuring everyone gets where they are needed at the correct time. Any corrections required due to wind changes on descent could then be dealt with by ATC speed control or vectoring.

A very similar idea has been floated. Simply move the fix much further out, i.e. prior to descent point. The descent phase then becomes mush easier for us as everyone is doing the same speed on descent (just like the old days pre FF's). The downside is that it gives you a lot less time to achieve the requires time.

Capn Bloggs and hung start when we went to MAESTRO in PH a conscious decision was taken not to issue FF times until with the inner sectors (134.2/133.8 from the north, 135.8/118.2 from the east).

MAESTRO will recalculate the entire sequence every time a single piece of information is changed (aircraft estimates, RWY config, acceptance rate). For a RWY change this is a massive advantage over the manual system, but other time its can be a disadvantage.

MAESTRO starts sequencing (i think) at least two hours out, but it will be in a state of flux as estimates change and short leg flights depart etc. The closer you get to arriving the more stable the times become. It was decided that prior to the inner sectors, rather than issuing a time that will in all likelihood change more than once, we would give speed reduction based on the current MAESTRO time. I can't remember the exact values but it's something like up to 5 minutes - no reduction, 5-10 reduce by M0.05, grater than 10 reduce by M0.05 or greater, expect an X minutes delay.

One side advantage of this is that two aircraft in trail given a delay will both reduce by the same amount, reducing the different profile/speed issue. None of these procedures are set in stone however, and can be altered if that's what industry wants.

Its the same reason we changed to giving a hold departure time rather than 'depart the hold to make the fix time'. That way between the holding point and the fix everybody will be doing the same speed.

With regard to the speeding up to 250 after the fix, I was very surprised that jets would do less than 250 prior. I was expecting a 250 knot descent and than if that was not enough a request for vectors/holding. I didn't even know a jet could do 210 on descent, I was always taught 250 was a minimum we could assign, if we had really messed up we could ask nicely for 240 or maybe 230.

[Oakape]

With regard to the speeding up to 250 after the fix, I was very surprised that jets would do less than 250 prior. I was expecting a 250 knot descent and than if that was not enough a request for vectors/holding. I didn't even know a jet could do 210 on descent, I was always taught 250 was a minimum we could assign, if we had really messed up we could ask nicely for 240 or maybe 230.

The minimum speed depends on altitude & weight, along with type to a lesser extent. Generally, the lower you get the slower you can go - down to minimum clean speed, which is around 200-210 kts at typical landing weights on the NG. (taking flaps will get you slower, but can't be used above FL200 & that is the last thing we want to do anyway!) Generally you are light when arriving & in an NG, light enough to get back to 210-220 kts somewhere around FL300. Above FL300 would require a bit more speed. It depends a little on the particular pilot's (read captain's) pucker factor or simply personal preference, and also on turbulence, etc. The B777 wasn't too bad either if I remember correctly, with slightly higher speeds, maybe 5 or 10 kts more.

Constant speed descents are easier for the FMC to calculate & fly in VNAV, which is the way the boss likes us to fly it. However, it can handle one or two speed reductions, preferably over waypoints (easier to program). Once you get speed increases or vectoring, you are off the 'path' & then it is back to manual calculations. It can be done & is really no bother to most, it just offends our sense of economy & passenger comfort.

Once everyone gets their heads around what has to be done to move large volumes of traffic, then you will get a better response to any restrictions & less complaints. Guys are still trying to hang onto economy. Perhaps a couple of months in Dubai would educate them & they would see that it is still pretty good in this neck of the woods.

[le Pingouin]

In the good ol' days the flow was the only one doing the times - he'd do all the fiddling of figures and we'd just pass it on. Things were set in concrete early. Now there are many fingers in the pie and the times aren't set in concrete until much later - the dilemma for controllers is give a time too early and it's likely to change several times, frustrating and time wasting for everyone.

Maestro is another thing for us to monitor so if we're busy separating traffic we might not look at it for a bit and not notice you've gone from no delay to 6 minutes - we don't get directed warnings, it just happens.

We know you like times early and I'll try to slow you early when I can but that relies on me even noticing you before you're handed off to me - when it's busy that often gets missed because it's low priority.

Re. efficiency of constant profile descents and such - I cack myself laughing every time I read of the latest scheme proposed to save fuel. You can achieve whatever you want when you're the only aircraft in the sky but as you say, the more aircraft the less likely that is to happen.