Query to RPT pilots

[umop3pisdn]

When instructed by ATC to cross a point at a certain time for sequencing, what is the process that is followed to achieve this? e.g. Is it just a matter of telling the FMS that you want to hit the point at a certain time and the autopilot handles the rest?

Subsequent question; where does the FMS obtain the grib wind data from for each flight?

[Keg]

On some FMS you can telling an RTA (required time of arrival) for a waypoint and it will achieve that.

However, rubbishy in equals rubbish out. If the winds aren't accurate you can get some silly solutions with massive speed up and subsequent slow downs as the conditions change. Every RTA I've seen used for a waypoints not on descent has been a complete and utter cluster.

For my money I prefer to fly a 250 knot descent as that is invariably the speed ATC want after the waypoint and then adjust the cruise speed accordingly to meet the time. If that doesn't work then I'll simply insert the slower descent speed and see how close that gets me to the time. Most often I simply eyeball the profile to match.

One of the weird clearances I've experienced recently into PER is 'cross BEVLY at time 40 or earlier then normal speed'. I can't work out why the speed after BEVLY is important if I can get there at anytime prior to 40. Eg if ATC doesn't care if I get there at 35 or 39 then surely my speed after BEVLY is irrelevant? Im open to thoughts from that one. AWOL57?

The winds in the FMS depend a bit on its capability. For QF 330 ops we uplink the wind forecasts and then the FMS extrapolates between current wind and forecast wind over a disgrace before going to completely forecast wind. The 767 was a bit more generic and sometimes you needed to use a bit of rat cunning as to what the wind was likely to do in order to generate accurate estimates.

[Awol57]

My guess for normal speed would be someone behind you from somewhere. Bit of an odd one but I guess they are saying do what you need to to make that time then profile speed. But that's a bit out of my league as a procedural approach guy :)

[Density]

On some FMS you can telling an RTA (required time of arrival) for a waypoint and it will achieve that.

However, rubbishy in equals rubbish out. If the winds aren't accurate you can get some silly solutions with massive speed up and subsequent slow downs as the conditions change. Every RTA I've seen used for a waypoints not on descent has been a complete and utter cluster.

For my money I prefer to fly a 250 knot descent as that is invariably the speed ATC want after the waypoint and then adjust the cruise speed accordingly to meet the time. If that doesn't work then I'll simply insert the slower descent speed and see how close that gets me to the time. Most often I simply eyeball the profile to match.

One of the weird clearances I've experienced recently into PER is 'cross BEVLY at time 40 or earlier then normal speed'. I can't work out why the speed after BEVLY is important if I can get there at anytime prior to 40. Eg if ATC doesn't care if I get there at 35 or 39 then surely my speed after BEVLY is irrelevant? Im open to thoughts from that one. AWOL57?

The winds in the FMS depend a bit on its capability. For QF 330 ops we uplink the wind forecasts and then the FMS extrapolates between current wind and forecast wind over a disgrace before going to completely forecast wind. The 767 was a bit more generic and sometimes you needed to use a bit of rat cunning as to what the wind was likely to do in order to generate accurate estimates.

Yep the FMS in the 738 works the same way. Funny you mention about RTA Keg. I don't know anyone that uses the RTA function reliably it is a horses ass. In the dash not completely fms driven so back to the old way of adjusting the power or altitude or both and watch the time change on the screens respectively.

After going through all these creative routes, more so with holding crossing times which we would get pretty close to, atc often didn't trust us so would instruct us to turn inbound before the fms was to turn us. 😀

[RENURPP]

I'm with KEG, RTA works well as long as it is used correctly and understood. It won't work if you stay at cruise altitude and min speed won't achieve the time, how can it? Adjusting your descent speed before entering the RTA will make a difference.
When I'm given a time to achieve I reduce the cost index to achieve a descent speed of 250. I enter an RTA. It should be obvious whether you will be able to achieve the RTA at your cruise altitude. As a rough rule in the aircraft I fly with 150nm to run to the fix for every min I need to loose I will need to descend 2000ft. I.e.. If I'm at 37000 feet and need to loose 8 mins I will descend to 21000 feet. The descent need to be a normal (flight idle) descent, none of this mucking around at 500 vs. the faster you get to the lower level the sooner your TAS / GS reduces. Then RTA will do the job very nicely thank you. (Within 5 seconds is the norm)
You other choice is to remain at cruise level and hold, burning more fuel.
The difference in fuel burn between 37000 and 20000 is around 300kgs/hr. Not much but over several flights it adds up. It also seems to help ATC as they regularly voice their appreciation for our efforts.

[gordonfvckingramsay]

What? No wizz wheel? :8

The difference in fuel burn between 37000 and 20000 is around 300kgs/hr. Not much but over several flights it adds up.

Isn't the only reason pilots worry about fuel is when the don't have enough?

[caneworm]

What? No wizz wheel?

Won't fit into top pocket of new uniform ��

[umop3pisdn]

On some FMS you can telling an RTA (required time of arrival) for a waypoint and it will achieve that.

However, rubbishy in equals rubbish out. If the winds aren't accurate you can get some silly solutions with massive speed up and subsequent slow downs as the conditions change. Every RTA I've seen used for a waypoints not on descent has been a complete and utter cluster.

For my money I prefer to fly a 250 knot descent as that is invariably the speed ATC want after the waypoint and then adjust the cruise speed accordingly to meet the time. If that doesn't work then I'll simply insert the slower descent speed and see how close that gets me to the time. Most often I simply eyeball the profile to match.

One of the weird clearances I've experienced recently into PER is 'cross BEVLY at time 40 or earlier then normal speed'. I can't work out why the speed after BEVLY is important if I can get there at anytime prior to 40. Eg if ATC doesn't care if I get there at 35 or 39 then surely my speed after BEVLY is irrelevant? Im open to thoughts from that one. AWOL57?

The winds in the FMS depend a bit on its capability. For QF 330 ops we uplink the wind forecasts and then the FMS extrapolates between current wind and forecast wind over a disgrace before going to completely forecast wind. The 767 was a bit more generic and sometimes you needed to use a bit of rat cunning as to what the wind was likely to do in order to generate accurate estimates.

Thanks for the reply!

I've noticed a disparity between aircraft types regarding achieving a fix time and couldn't figure out why it was occurring. From responses I'm still unable to figure it out and can only put it down to winds.
As it appears as though the best way to achieve a time is through adjusting the speed until the box says that the ETA is congruent with what's being asked for, a certain airframe consistently misses the mark. If there are any F100 pilots lurking I'd be interested to hear how you do it?

As for the BEVLY query; the phraseology is born from a number of factors and the intent is the following.

"There is a gap in front of you in the sequence in which I don't believe you can catch the aircraft in front if you maintain a normal descent speed. If you get there early then you free up space behind you but I've done the figures and if I give you a high speed descent from the feeder fix point, you're most likely to catch whoever is in front of you."

By not locking you into the time, it's a bonus if you're early.

[Capt Chambo]

Keg, has largely answered the OP's question.

The RTA can be used, but if it starts chasing ridiculous speeds then I tend to note the speeds it was going to use and program the FMC with those speeds and see how we go.

As a rule of thumb, if the time to be lost is <3mins then I generally try to do it via reducing my descent speed only. When the time to be lost is between 3-5mins then I will also reduce my cruise speed and well as my descent speed.

RENURPP highlights an interesting conundrum, which is, is it better to descend to be able to fly at a reduced IAS>TAS>G/S, but with a higher fuel burn, or should you continue at altitude maybe changing your cost index to zero, and accepting the delay in the holding pattern, but "probably" burning less fuel overall?

As I see it, if you want to fly most efficiently (in this case burn the least amount of fuel), given that the time at the fix is not ours to modify anymore, then we are effectively flying for maximum endurance.

Given the relationship between speed, drag, altitude and jet engine efficiency, I would be curious to know what altitude, speed and thrust setting would be best for your typical passenger jet.

Any thoughts?

[travelator]

RENURPP highlights an interesting conundrum, which is, is it better to descend to be able to fly at a reduced IAS>TAS>G/S, but with a higher fuel burn, or should you continue at altitude maybe changing your cost index to zero, and accepting the delay in the holding pattern, but "probably" burning less fuel overall?



Don't confuse endurance with range/efficiency.

If you have to lose time, the best way is go low and slow.

If you stay high and hold, you will burn more fuel than if you go low. Staying high keeps the TAS high and therefore parasite drag high and a high power setting to maintain speed resulting in high fuel flow.

Go low, much lower TAS equals lower drag resulting in a lower thrust setting and fuel flow.

On my type, best endurance is found in the very low 20s ie minimum drag.

[topdrop]

Most pilots manage to hit the times pretty close at the waypoints into Cairns. This includes GA through to RPT jets - well done ladies and gentlemen.

[Capn Bloggs]

If you have to lose time, the best way is go low and slow.
Correct. Losing time is, in effect, holding. Check out the best holding altitude and speed in your FCOM...

In my machine, RTA is a WOFTAM. The descent speed will remain the FMS speed for the Cost Index. If you don't have a lot of time in the cruise to fly at a low or high speed to adjust the time, it won't work. Adjusting the Cost Index to achieve the Feeder Fix time is king. It will also reduce the times when someone is cruising at Vmin then suddenly accelerates to warp speed as they start descending. Us: "Err, Centre, isn't that 737 ahead supposed to be behind us?" ATC: "XXX, will you be able to make your time?" Them: "Yessir, we'll be descending at 550KIAS!!", which obviously doesn't work. I exaggerate, but this has happened to me on a few occasions.

On my jallopy, FMS winds are loaded by the crew from the computerised flight plan (or NAIPS SPFIB).

The biggest pain is that the ETAs on my box aren't shown in decimal minutes (until on the last segment to the Feeder Fix). It would make life so much easier it they were.

[Square Bear]

As a mathematical solution:

Required GS to achieve new waypoint arrival time = (time to loose/current time to run) multiplied by current GS.

At new speed an occassional check with Distance to Run/Time to Go, adjust speed accordingly. Accurate time piece needed.

RTA works well enough for me though. :)

[Kooka]

When instructed by ATC to cross a point at a certain time for sequencing, what is the process that is followed to achieve this? e.g. Is it just a matter of telling the FMS that you want to hit the point at a certain time and the autopilot handles the rest?

Crossing times usually come close to TOD. RTA only works in cruise (in the Airbus and if my recollection is correct the Boeing).

1. Adjust the FMS descent speed to meet the crossing time.
2. Insert a time marker for the crossing time.
3. Manually adjust speed to keep the time marker circle over the crossing point. (Airbus; "Selected Speed". Boeing; "Speed Intervene"?)
4. Make small and frequent adjustments
5. Cross within plus/minus 5 seconds every time

So to answer the question, setting the time in the FMS will not get the desired result. The descent speed needs to be adjusted manually to meet the requirement. In cruise an RTA will work but cruise crossing time requirements are rare in Australia.

[Snakecharma]

Thales 330 FMGC latest update does RTA on descent. Don't know about the Honeywell version but assume so

[RENURPP]

. In cruise an RTA will work but cruise crossing time requirements are rare in Australia
Into BRISBANE it is rare not to be notified with at least 150nm to run to the fix. Enough to lose around 9 minutes.
I rarely get a time into Sydney, they seem to control via speed.

[Square Bear]

....with at least 150nm to run to the fix. Enough to lose around 9 minutes.

Wouldn't want too many 9 minute losses in 150 nm, typical jet would be GS 200 +/- knots on descent...be fun from the north in winter with a 100 kt jet stream.

[porch monkey]

Don't have much problem with using the RTA function in the NG. Works pretty well I find, usually within 5 secs, provided you don't expect it to work miracles, descent or cruise.

[RENURPP]

Wouldn't want too many 9 minute losses in 150 nm, typical jet would be GS 200 +/- knots on descent...be fun from the north in winter with a 100 kt jet stream
Let's look at the numbers then?
150nm / Tas 470kts = 19 mins average speeds not accounting for descent.
To save 9 mins would require a G/S of 320kts.

You could probably reduce speed at cruise alt and save a 1 min maybe 2 and reduce descent speed to save a couple more BUT a I would descend immediately to 20,000 at flight idle normal speed to get down ASAP and at min speed have a TAS of around 320kts or slightly slower. That with a slower descent and you have saved your 9 mins. Unless your heading west you could assume a reduction in g/s due wind which als helps. It doesn't always work but 95% of the time it will with savings in fuel. I.e. More left in tanks for other reasons (weather etc)

[Square Bear]

RENURPP

I totally agree with your maths, but 470 to 320kts is not instantaneous, and with a 100kt tailwind 570 to 320 less so) hence my numbers. With the given proviso that once at speed, recalculate the speed required to make the waypoint.

But in reality, the FMC RTA function works fine for me (Level forecast winds entered) . And considering the speed reduction requests are normally made when it all starts to get busy, the FMC option gives me more time for other things.

But I am first to admit that there are many ways to pluck a duck and respect each to their own. :)

Cheers

[le Pingouin]

As a controller I have to agree that most of the time you guys do an excellent job at meeting the times (I handle ML arrivals) - we're often amazed at just how much you can lose, particularly in to wind.

We have to laugh sometimes though - we get pilots (from a select few airlines) who say "we can't possibly do that" when asked at 200 miles out to lose, say, four minutes in a heavy, when I'm only giving them the time to make sure they won't be barrelling in at 330kts, and 280 will probably do it nicely.

Or the ones who are two minutes early with five minutes to run to the fix, "report speed", "300kts". Really?

And then there are the A330s who are maintaining F250 at 140 miles and only have a minute left to lose - "track direct and increase to 300kts" is the usual response.

As I said, it's only the select few.

[Transition Layer]

Bloggs,

The biggest pain is that the ETAs on my box aren't shown in decimal minutes (until on the last segment to the Feeder Fix). It would make life so much easier it they were

Likewise on the 737, however we just enter the desired waypoint on the fix page and then select abeam (which is actually not abeam but overhead), and it gives you the ETA with a decimal point. Does the 717 have a pretty similar FMC?

[Snakecharma]

Le ping my pet hate is being given a time, which you are going to make within about 10 seconds, being given an indeterminate vector for reasons best known to the controller because they don't give you a reason and then assume you will continue to make the given time. "You" being a generic term not you specifically :)

In my view, it is one or the other!

If I am set up to cross a waypoint at a given time and speed, giving me extra track miles, particularly when you don't tell me how many extra track miles you are giving me, means I MUST speed up!

I do everything I can to meet the times, descending up to 100-150 miles before I would normally just get the speed off, continually adjusting the speed to make the time, and to have someone at the end jump in and give me a vector and not tell me why just gives me the ****s.

And yes I drive an a330.

[Derfred]

... and then assume you will continue to make the given time.

I suspect the problem here is you.

If an ATC clearance is not clear to you, clarify.

[Snakecharma]

Thanks for pointing that out captain obvious! And as a point of clarification, the assumption is on their part not mine.

Yes indeed I do ask the question and the answer back is almost always to meet the same time.

Now how do you do that when you are on a vector and pointing away from the fix - what speed do you do to make good the time?

The speed you had, when pointing at the fix, had you crossing within ten or so seconds in most cases, so exactly what speed are yo supposed to do when you don't know how long the vector is or what the vector is for?

If you can answer that can you give me next weeks Lotto numbers?

[le Pingouin]

How close to the fix are you talking? Quite often the problem is the one ahead not you. Continuing with your current speed would my suggestion. One problem for us is Maestro only does times to the minute so if someone is 30 seconds late on the time given they make actually be a minute late on where they need to be and vice versa with early.

Shall we say the problem children I mentioned aren't domestic A330s.

[pcx]

I'm curious.
Once the controller issued a vector does that not mean that he is accepting responsibility for your navigation and any previous clearance is cancelled unless specifically stated.
By the way, I am not going to try to find AIP references so if you guys that are more up to date then me want to sort me out, I am happy to be educated.

[RENURPP]

I've noticed if my FMS planned descent speed is considerably different to my normal profile, either faster or slower, the controller may interfere with my cruise speed and/or vector or ask for a speed increase in an attempt to meet what he/she believes is the fix time. I will add that often if they are attempting to get us there on time it doesn't work and once on approach they do the opposite, I.e. Slow us down or speed us up.
Having said that us pilots are our own worst enemy in that a fair percentage think rough enough is close enough. If the box says 27 and that's what they asked for then continue with the coffee. 27 in my machine is from 27 until a sec before 28. Maybe 7 miles behind where your expected to be.

[Capt Claret]

Advice from a few ATCO jump seaters in the last year or so suggests that ATC would rather we're at the feeder fix 30 secs early, than 30 secs late. Based on this my preference is to juggle CI & FL to get the right time, then fine tune the CI so that the displayed time shows the previous minute and a CI decrease Bloggs knew what I meant] of 1 shows the correct minute.

So far it seems to work ok.

[Capn Bloggs]

You are supposed to be at the fix "on time" or up to 30 seconds early. ENR 1.9, 4.2.3 refers.

[le Pingouin]

RENURPP, the problem is that it's a dynamic environment and getting you to the fix on time sometimes doesn't achieve the correct spacing in the sequence. Works well enough when everyone is flying through the same fix but not necessarily so well when slotting aircraft in from different directions - the time from the fix to the threshold that Maestro calculates is only as good as the wind and performance model it uses and some days it doesn't work so well.

I've had aircraft exactly on time through WENDY and ARBEY onto R16 at ML and they end up being a dead heat at Bolinda - everyone flying 250kts from the fix and the star speed restrictions.

[Capn Bloggs]

fix page and then select abeam (which is actually not abeam but overhead), and it gives you the ETA with a decimal point.
No such luck here, TL. Whole minutes only... :{

[fl610]

Ah Australia, the only third world country where you can drink the water! :{:ooh:

[umop apisdn]

nice name copy cat