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time at an ATC directed way point

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time at an ATC directed way point

Old 30th Oct 2014, 08:19
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time at an ATC directed way point

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 .
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Old 30th Oct 2014, 08:41
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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
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Old 30th Oct 2014, 08:53
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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 .
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Old 30th Oct 2014, 08:53
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Recent AIP supplement refers. Up to 30 seconds early is ok at 250 kts.

http://www.airservicesaustralia.com/aip/current/sup/s14-h72.pdf
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Old 30th Oct 2014, 09:01
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Perfect thanks SHRAGS . I had money on 1 min early but can't be late .
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Old 30th Oct 2014, 11:15
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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
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Old 30th Oct 2014, 11:21
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Used to be...
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Old 30th Oct 2014, 11:58
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ATC AU 611.
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Old 30th Oct 2014, 15:02
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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!
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Old 30th Oct 2014, 19:16
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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....?
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Old 30th Oct 2014, 22:56
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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.
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Old 30th Oct 2014, 23:27
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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 .
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Old 31st Oct 2014, 01:16
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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.
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Old 31st Oct 2014, 04:32
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Originally Posted by tyler_durden_80
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.
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Old 31st Oct 2014, 06:18
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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.
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Old 31st Oct 2014, 07:02
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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.
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Old 31st Oct 2014, 19:54
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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.
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Old 31st Oct 2014, 20:13
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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.
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Old 31st Oct 2014, 21:46
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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
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Old 31st Oct 2014, 23:02
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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.
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