Sign in to follow this  
hasse29

Thoughts about weather and FMC

Recommended Posts

Besides flying myself, I occationally enjoy watching videos of other skilled simmers out there. It's interesting and sometimes I pick up something new I haven't thought about before. I have been flying FS since FS95, but still have a lot to learn. One thing I have noticed is that people load weather into FMC during the preflight, and just fly. But that will give incorrect values. I don't know if ASNext gives the wind data at that exact moment or if it try to predict (anyone here knows? I guess it only gives at the actual moment), but if you fly a longhaul, values will become very inactual. I am loading all wind data during preflight, but I am also updating it during the flight. How do you do it? How do they do it in "real life"? I made a reference flight with 772 EGLL-YSSY without updating until moments before TOD to check this out, and values did really differ.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hi Mikael,

 

It depends what you consider "incorrect" values.

 

Yes, the wind and weather data you get during the preflight is just a forecast. But then that is also the forecast that your flight planning is based on, and that is the case in real life as well. Sometimes the forecast is accurate, sometimes it's not: that's why you load contingency fuel, and even then it's possible for the actual wind or weather conditions to be so different to the forecast that a diversion becomes necessary.

 

In real life, you can't just pull up ASN and enter the "exact" data at any given moment during the flight. Depending on the equipment fit, it may be possible to obtain an updated wind forecast via ACARS at a later point in the flight/prior to TOD, but even that data will still be just a forecast. The FMC blends the actual sensed values with the forecast values, with the actual sensed values taking more priority near the aircraft and a gradually increasing weight to the FMC-entered values further down the route.

Share this post


Link to post
Share on other sites

Hi Mikael,

 

It depends what you consider "incorrect" values.

 

Yes, the wind and weather data you get during the preflight is just a forecast. But then that is also the forecast that your flight planning is based on, and that is the case in real life as well. Sometimes the forecast is accurate, sometimes it's not: that's why you load contingency fuel, and even then it's possible for the actual wind or weather conditions to be so different to the forecast that a diversion becomes necessary.

 

In real life, you can't just pull up ASN and enter the "exact" data at any given moment during the flight. Depending on the equipment fit, it may be possible to obtain an updated wind forecast via ACARS at a later point in the flight/prior to TOD, but even that data will still be just a forecast. The FMC blends the actual sensed values with the forecast values, with the actual sensed values taking more priority near the aircraft and a gradually increasing weight to the FMC-entered values further down the route.

 

Thank you for you answer. Of course I know they don't have ASNext in a real airplane. But it would be more accurate if you filled in correct wind during flight also. If used correct, a company could save a lot om fuel money in a long haul, by maybe flying at FL300 instead of FL360 at the end of flight during wind condition. But I am not a pilot, so I don't know, that's why I wondering :).

Share this post


Link to post
Share on other sites

All the winds are just a forecast, but forecasts are usually fairly accurate. On larger modern planes with an ACARS system, up to date winds can be transmitted to the aircraft to give a better idea of what is available. On the majority of shorter haul flights though (2-3 hours or less), a difference of 10-15 knots in winds is negligible on saving time and fuel and changing altitude may not be available due to ATC. The only accurate way to get the winds is to be actually flying through it and seeing it in real time.

Flying in the RW I have seen winds that are 30 knots and 90 degrees off of what was forecasted. Sometimes it just happens.

Share this post


Link to post
Share on other sites

All the winds are just a forecast, but forecasts are usually fairly accurate. On larger modern planes with an ACARS system, up to date winds can be transmitted to the aircraft to give a better idea of what is available. On the majority of shorter haul flights though (2-3 hours or less), a difference of 10-15 knots in winds is negligible on saving time and fuel and changing altitude may not be available due to ATC. The only accurate way to get the winds is to be actually flying through it and seeing it in real time.

 

Flying in the RW I have seen winds that are 30 knots and 90 degrees off of what was forecasted. Sometimes it just happens.

 

Thank you for your answer :). On long haul, if the airplane have ACARS and wind is way off, is it common that pilots interact on it, or do they let it be as it is?

Share this post


Link to post
Share on other sites

Thank you for your answer :). On long haul, if the airplane have ACARS and wind is way off, is it common that pilots interact on it, or do they let it be as it is?

 

Firstly, ASN uses somewhat of a forecast, from what I can tell, so the wind should be as accurate a possible. Still, I'll pull a new wind update down every few hours or so (usually coincident with step climbs), just to be sure. Remember that forecasts have increasing variability as time goes on. This is why predicted hurricane tracks are conical: as the time between actual and forecast increases, it's increasingly difficult to determine something as clearly. Wind forecasts can be viewed the same way: the farther ahead of the actual time you go, the more prone to error that forecast is (this is also why weather forecasts only usually go out 7-10 days - beyond that, the quality of the forecast is so poor, it's not really worth the effort).

 

irene_2005_08_09_11pm.jpg

 

One thing a lot of people don't realize, as Nick has alluded to, when you go to look at wind aloft data, you'll often find it under the forecast section of the products. There are varying amounts of forecast added in, but there is no actual, in-the-moment data as you'd get with something like a METAR. Additionally, have a look at a wind map and note the vast spaces between sampling locations. Here in the states, we have only two locations for the entire state of Utah. While wind follows fluid dynamics, and interpolation yields decent results, knowing what's between these two locations would be helpful, and only strengthen the data.

 

fball.gif

 

 

As for changing the data if you see it different from how the forecast has it, it won't do you much good. The FMC is already aware of the wind, and is using that data in conjunction with the forecast data. While you could make inferences as to how different the forecast data is versus the actual data, at best, it would be a guess, which really doesn't help your situation much. Pull another weather update and see what it looks like. If that still doesn't resolve it, keep an eye on it, and if it might have a significant effect on your fuel planning, then you may want to take a look at your options.

Share this post


Link to post
Share on other sites

Firstly, ASN uses somewhat of a forecast, from what I can tell, so the wind should be as accurate a possible. Still, I'll pull a new wind update down every few hours or so (usually coincident with step climbs), just to be sure. Remember that forecasts have increasing variability as time goes on. This is why predicted hurricane tracks are conical: as the time between actual and forecast increases, it's increasingly difficult to determine something as clearly. Wind forecasts can be viewed the same way: the farther ahead of the actual time you go, the more prone to error that forecast is (this is also why weather forecasts only usually go out 7-10 days - beyond that, the quality of the forecast is so poor, it's not really worth the effort).

 

irene_2005_08_09_11pm.jpg

 

One thing a lot of people don't realize, as Nick has alluded to, when you go to look at wind aloft data, you'll often find it under the forecast section of the products. There are varying amounts of forecast added in, but there is no actual, in-the-moment data as you'd get with something like a METAR. Additionally, have a look at a wind map and note the vast spaces between sampling locations. Here in the states, we have only two locations for the entire state of Utah. While wind follows fluid dynamics, and interpolation yields decent results, knowing what's between these two locations would be helpful, and only strengthen the data.

 

fball.gif

 

 

As for changing the data if you see it different from how the forecast has it, it won't do you much good. The FMC is already aware of the wind, and is using that data in conjunction with the forecast data. While you could make inferences as to how different the forecast data is versus the actual data, at best, it would be a guess, which really doesn't help your situation much. Pull another weather update and see what it looks like. If that still doesn't resolve it, keep an eye on it, and if it might have a significant effect on your fuel planning, then you may want to take a look at your options.

 

Thank you for your answer, very insightful. By mathematical standard, they can only predict weather within a few hours with accurasy, three days within an acceptable tolerance of error. When you see a 10 day forecast on the news, their guesses are barely better than your own. But I got some real good and valuable information from all writers here. Thank you.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this