airfly
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The Accurate ETA

Mon Jan 16, 2006 2:45 am

Hello there,

How do airlines have a quite accurate significant ETA? I mean what are the involving calculations?
In simulation I usually divide the TA/Distance in NM, then add ( I think ) from E-W= -20 minutes( including descend ), and W-E= +20 minutes ( including descend ) of wind, which is not very accurate. Say I was flying an A342 from; FAO-GRU. 4220NM distance, and a speed of around 498 TA. That would conclude on 8:47389...(average of 8:50) as it is a E-W flight SO; -20 minutes would have a total of 8:30 hours.
I tried that myself, and well I got there 25 minutes early. Of course tail wind might of happened, but How are airlines so accurate with their times?
The example I gave is for FS, but I thought that real flight would not have a big difference.
What is the actual calculation?

Cheers
Lucca M.
L - I - B - E - R - A - L
 
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Starlionblue
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RE: The Accurate ETA

Mon Jan 16, 2006 2:56 am

Just taking the distance and factoring in airspeed is a start, but there are many factors, including wind, traffic, holds and so forth.

I guess airlines have been doing this for a while so they have very good baseline data. Add some not extremely sophisticated software to the database and you can recalculate on the fly and with great accuracy.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
jamesbuk
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RE: The Accurate ETA

Mon Jan 16, 2006 2:57 am

I think its from experience of the flight and then with winds they add and minus a bit

rgds --James--
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Goldenshield
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RE: The Accurate ETA

Mon Jan 16, 2006 2:57 am

In real-world flight planning, the wind for each turn in the flight plan is accounted for, and each will have different relative wind values. This is how we get almost accurate enroute times. Usually, for climbout and descent, plans are given straight line values to the first fix, and from the last fix, so the the actual time may vary by a minute or two.

[Edited 2006-01-15 18:58:51]
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airfly
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RE: The Accurate ETA

Mon Jan 16, 2006 3:16 am

Quoting Starlionblue (Reply 1):
guess airlines have been doing this for a while so they have very good baseline data. Add some not extremely sophisticated software to the database and you can recalculate on the fly and with great accuracy.

I guess that's the best secret!

Cheers
Lucca M.
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bond007
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RE: The Accurate ETA

Mon Jan 16, 2006 12:27 pm

Well, don't forget also, that the last 50-100 miles or so is going to be 200-250 kts or less, depending on restrictions. I work for a company that calculates rough ETAs, and we use average groundspeed for everything up to the last 100 miles, then average 200 kts for last 100.


Jimbo
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3201
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RE: The Accurate ETA

Mon Jan 16, 2006 12:50 pm

Detailed four-dimensional wind/temperature forecasts come out from 4 to 24 times a day, depending on where in the world you're talking about. Detailed aircraft climb/cruise/descent data exist and those who buy/lease aircraft, and the third-party vendors who provide software or services to them, have access to these data. Thus, both the flight planning system (on the ground) and the FMS (on the airplane) can do detailed and accurate calculations for that day.

For schedule planning, historical data about the winds and temperatures are available from several sources, and historical taxi time data at specific airports at specific times of day also exist.

(Also don't forget that TAS isn't fixed for a flight -- if you're flying constant-Mach, it changes with temperature, and at cost-index it changes with both temperature and wind.)
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Grbld
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RE: The Accurate ETA

Mon Jan 16, 2006 7:04 pm

3201, good description. With cost-index it also changes according to weight.

Basically, before the flight, dispatchers print out the flight plan which takes into account the planned weight and forecasted winds at all positions and altitudes with standard calculations for departure and arrival segments.

In flight, the FMC will continually calculate these things as your actual weight may differ from the planned (more/less passengers/bags/cargo) which may result in a different cruise altitude and/or speed (cost index), which influences your ground speed (different winds and true airspeed than at the planned altitude). The arrival procedure can be entered in more detail by the pilots based on their expectancy and experience (if they foresee a delay, or ATC arrival methods that are not on the arrival charts, like a visual approach) so you get an even more accurate ETA. The approach phase is usually what can screw up the ETA that was announced in the early part of the flight. Depending on how busy it is, you can easily get 10 minutes of extra flying time.

Grbld
 
SlamClick
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RE: The Accurate ETA

Tue Jan 17, 2006 1:11 am

When dealing with average accuracies, experience is worth all the computers and equations out there. I know several people who can flight plan within maybe 5% on time and fuel with nothing more than a glance at a Rand McNally map of the US.

I ran a flight department for a company that logged its airplane time in hours and tenths, not minutes. My schedule was based on my working the first flight plan manually. I factored what I knew about prevailing winds and traffic and any other thing that could affect the flight. Over 90% of the flights we then flew over those routes came in to the tenth as I had scheduled them. Occasionally one would arrive way early, sometimes very late.

I was in my boss' office one day and he took a charter call. He listened to the guy's travel needs then walked over and spread his hand across a National Geographic wall map of the US and gave the guy a quote on the spot.

Now, with major airlines with big fleets fuel burn is a huge financial factor and a company can be fined if their on-time performance gets bad enough, so the sophisticated segment-by-segment analyses like the posts above talk about are used all the time. They are quite accurate!
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Grbld
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RE: The Accurate ETA

Tue Jan 17, 2006 10:43 pm

Quoting SlamClick (Reply 8):
...experience is worth all the computers and equations out there.

That is very true. Unfortunately, there's a trend where people increasingly rely on just the computers and don't actually work their brain anymore, which is the downside of very accurate computer systems. That's why computers should still be considered as a tool or aid, not an entity that makes decisions.
 
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Starlionblue
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RE: The Accurate ETA

Tue Jan 17, 2006 11:05 pm

Quoting Grbld (Reply 9):
That's why computers should still be considered as a tool or aid, not an entity that makes decisions.

Well, they are just a tool. But you are correct in your statement. I work in the sofware industry and I see it all the time. People actually believe (consciously or subconsciously) that computers think for themselves.

I think it's very much a consequence of how vendors want their software to have a thinking look and feel. By mimicking human-like responses the illusion of thought can be created (if you squint). Just look at all those online plain language helpers at banks and so forth.

While the "thinking computer" interface makes life easier, it doesn't make the "thinking computer" a reality.

What worries me is when people who should know better are fooled. Garbage In Garbage Out still applies people!!!
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Tristarsteve
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RE: The Accurate ETA

Wed Jan 18, 2006 3:05 am

THe computor generated flight plan takes into account winds and aircraft weight. They are uncannily accurate. ACARS gives the lift off time, and the flight planning system gives the flight plan flight time and I find that the computor generated arrival time is accurate to the minute even after an eight hour flight. Where it goes wrong is the arrival delays. I work at a relatively uncongested airport ARN so we can trust the arrival times in the system.
 
airfly
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RE: The Accurate ETA

Wed Jan 18, 2006 5:40 am

Yes,

Nowadays, Computers take over our own way of thinking transforming knowledge to "Over-Knowledge", which the computer creates. Is important having an accurate ETA, as it is dependent on the Aircraft arrival and aiport movement.
But, are there any quick thinking maths which could be used on any emergency of computer fail?

Cheers
Lucca M.
L - I - B - E - R - A - L
 
Grbld
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RE: The Accurate ETA

Wed Jan 18, 2006 8:59 pm

Sure, but this is usually an exponent of experience as well. After a while, you'll know how much fuel you'd need to get to an alternate in case of a diversion, what your minimums are and how much fuel you use per hour in a certain scenario.

At the end of a trip, it's almost always fuel, and not time, which is restricting. If anything, your amount of fuel determines how much time you have. It's a constant consideration that should be monitored and calculated by the pilots during the entire flight.


Grbld
 
airfly
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RE: The Accurate ETA

Thu Jan 19, 2006 5:54 am

Ok, say delays happen... What is the process involving the actual delay time to reach the airport?
What sort of radio transmission would be used?

Cheers
Lucca M.
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Grbld
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RE: The Accurate ETA

Fri Jan 20, 2006 2:18 am

Usually none at all. If it's caused by congestion or adverse weather and the delay is not too bad (say less than 30 minutes), there's no real need to contact ground ops, they'll know when you're on the ground and the pilots might be busy anyway.

If there's a longer delay or it's a non-standard cause (eg. mechanical probs), usually diversion scenarios will come into play and contact will be made via data link (ACARS) or VHF radio.


Grbld

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