It appears the altitude is calculated above ground.

TOD in A​SN reads FL 310 winds 245/061/-43C

NIPZO 4000 winds 160/6/+7C

SUNST 4300 winds 160/6/+8.7C

KIMME 10000 winds 205/12/+8.7

CHIPZ 14300 234/25/-1.2C

 

... and so on back up... then back down again. The TOD is fine, but then its like I'm descending into the mountains, then I cross the mountains and I'm back in the air. I remember this profile is what it was like last time I flew into Vegas from LA, but I didn't have ASN and instead used the default weather engine and picked off stations.

How are you supposed to enter this into the descent forecast in the FMC?​

 

 

 

How are you supposed to enter this into the descent forecast in the FMC?​

 

Why not use a STAR with a pre-defined descent profile? KEPEC3 is an RNAV arrival into Vegas that you can take from LA via DAG. Fly it correctly and you'll be lined up and established for an approach to either runway 25R or 25L in Vegas.

Why not use a STAR with a pre-defined descent profile? KEPEC3 is an RNAV arrival into Vegas that you can take from LA via DAG. Fly it correctly and you'll be lined up and established for an approach to either runway 25R or 25L in Vegas.

 

But if you're using the STAR with a full VNAV profile, the FMC still needs accurate descent wind forecasts to meet the altitude/speed profile.  If not, you'll either find yourself off profile or need to manage manually (defeating the purpose of the STAR).

Yes I understand all of that. I am not making it clear why I am confused.

 

I load the flight plan for LAX to LAS, which includes KEPEC3. Tonight

ASN's flight plan looks like below.

 

Whats confusing me is notice the Altitude after TOD.. it goes from 31,000, to 4,000, to 4,3000, back up to 10,000, and so on..

 

In the plane, obviously on barometric pressure, the descent goes down (as it should), not up. Although, if you've flown the route before, you know the ground gets closer to you before you clear the mountains. Its as if the altitude in active sky is giving me the above ground altitude...

 

I guess maybe its doing what its supposed to do, but when I type in the descent forecast in the FMC it is the altitude of the plane, not the above ground level of the plane.

 

It does a similar oddity with the temperature. AT TOD OAT is ​-43, and just two minutes later according to the briefing, the OAT is +7.

What am I doing wrong or misunderstanding?​

 

 

​

 

How are you supposed to enter this into the descent forecast in the FMC?​

 

all of those waypoints are within a few miles of the airport so the winds are pretty much what they are if you look at the conditions page for KLAS.

 

they are part of the sunst3 star too, you would normally fly them at an altitude of 9000-8000 according to the charts.

 

so just look at the weather conditions for KLAS and grab the number from 9000ft and you're good to go.

also keep in mind that those winds are only 6-12 knots which makes almost no practical difference in your descent.

 

so it would be something like 

31000 245/61

18000 241/44

9000 205/12

 

and then add in the oat differnce from 15 and the pressure into the spot on the right..

 

really in most cases if you just take the winds for the flight levels from your destination the practical variation will be well within the autopilot abilities to follow the vnav path. maybe you'll need a little speedbrake but maybe not

 

cheers

-andy crosby

Thank you for the info Andy, and that may be the way to go, but its not how the tutorial taught me how to do it. I must be misunderstanding something, or not using ASN correctly... or trying to get too much info out of ASN, I just want to try and understand what I'm seeing.

 

I think the oddities (If they are even oddities) in this forecast throw off the reported average winds & temp by ASN. Also, (by the tutorial) calculating the iSA deviation temp at 18,000 might be weird if im to do it off of interpolating data from that flight plan (picking off the temps for say CHIPZ (14300) and the TOD (31000) and interpolating the temp for 18000 and calculating ISA DEV from that.

Not sure if this is relevant... that flight plan came from SimBrief. Not sure if SImBrief gave ASN those elevations or ASN got those elevations...

ah, yeah i understand your confusion, that AGL stuff from the levels is a little weird and might even be kind of a bug in how they are reporting.. as you noticed, the info it shows for NIPZO and SUNST are kind of weird, but also all of those altitudes are very wrong for what you would normally be flying at for the star . CHIPZ should be 8000 and POKRR AND PRINO might be lower than that as they are the final fixes for the runway.

 

maybe there is an option to have it not use AGL?

 

besides that, i guess what i'm saying is you shouldn't put too much stock in what ASN is saying from waypoint to waypoint anyway.  all those waypoints have nearly identical conditions and especially if they are only a few miles apart and a few miles from the airport like in this case. with such small distances the variation is tiny!

 

in the big picture if you are concerned about nailing the descent profile you want to be thinking about the flight levels and altitudes where the wind is changing, usally from quite some distance to the airport, not so much going backwards from a few waypoints that are essentially on the downwind leg of the approach..

 

the places where you will really notice a difference from the descent forecast start to happen if you have a STAR with very long legs and high winds. and even then it should be pretty obvious where the different wind layers start and end, i think in those cases the numbers you are getting from simbrief might make more sense too.

also i just loaded the route into simbrief for tonight and the OFP that simbrief generates has numbers that look correct.. it may just really be some anomalies from ASN's way of showing the data that make it difficult to interpret in practice. simbrief shows something like:

 

NIPZO  19400 235/047

SUNST 17500 233/042

KIMME 13700 228/032

CHIPZ  10200 229/022

POKRR 8600 224/018

PRINO 6700 212/013

 

in the end i would still be entering forecast numbers similar to the ones i listed in the previous post, as you can see the numbers are a smooth transition down to the 212/12 at the end. when you only have 3 points to pick for wind values it makes sense to pick ones that are far apart to capture the important changes in the wind rather than worrying about the last ones.

 

cheers!

-andy crosby

These questions regarding ASN keep coming up in various forms, and it's easy to see where some of the confusion lies. As has been pointed out previously, ASN creates an atmospheric model by interpolating widely spaced real world atmospheric measurements of temperature and wind speed. See the following link for the locations within the US at which daily radiosonde measurements are made

 

http://www.ua.nws.noaa.gov/nws_upper.htm

 

This is a hugely complex scientific and mathematical problem. especially when one takes into account that the measurements are only made every 12 or 24 hours. So one shouldn't take too seriously the physical details of the model that ASN creates. It's doing a great job in a PC that really requires a large supercomputer to do more realistically. Basically, it is what it is and we should be thankful for it.

 

Also understand that ASN does not take into account the crossing altitudes that are specified in a STAR or SID. It just creates an altitude profile that gets it to the cruise altitude and back down to the destination airport elevation. It doesn't know about mountains so if you follow that profile, you may be in for a surprise. So the altitudes displayed in the table are calculated based on the climb and descent rates you input along with the airspeed for those phases of flight. In that regard, the calculation is somewhat bogus because we don't fly our climbs and descents at constant KTAS but rather (more or less) constant KIAS (or mach number at higher altitudes).

 

As has been suggested earlier the most realistic simulation might be to take the Simbrief preflight as the basis for forecast winds, because that's all that real world pilots have. Keep comparing that forecast with the inflight winds as created by ASN and as observed on the ND during your flight, just as a reality check and be prepared to intervene prior to or during descent if the observed wind differs significantly from predicted. As noted in these forums previously, there will almost certainly be differences, but most times they're not terribly significant. And when they are significant, that's when things get most interesting.

 

Hope this helps.

 

John

Keep it simple. The aircraft will do just fine in most cases without entering descent forecast winds. I do exactly as suggested by Andy, put KLAS in the station field on the Conditions page and enter the winds for FL340 FL240 and 12000. Works pretty good.  The TAI entry on the descent forecast page is probably the most important piece of information for being able to predict a vertical path.

We don't stress over the descent forecast page in the real airplane.  If you have good information go ahead and enter it.  If you don't make a reasonable guess, or do nothing.  Every descent is probably going to require some level of pilot intervention anyway so I wouldn't get stressed over it.  Stay ahead of the airplane and if it's not doing what you want in VNAV take whatever action is required to fix it.  In short, fly the airplane.

 

This is one area where simmers get too obsessed with the details.

Great information from all. Thank you. One of my goals with this hobby is to learn what goes on in the real world, and again great insight from you all.

 

I just wanted to verify ​that I wasn't doing anything wrong. It seems that the anomalies I was seeing is just a limitation of the sim, and if it happens again I have been given suggestions to rectify the situation.

Thanks again.​

I wish I had seen this post before I made mine as it's just about the same question I just asked the other day. Really helpful stuff. Thanks!

Setting up the flight for me is half the fun. A year ago I was jumping in the plane, hitting Ctrl+E and taking off (that's all I knew). 

Last night and most nights is now at least 30-40 minutes of getting the plane ready to go before taking off. I have the plane setup to be in a state where the passengers have just deboarded, and start every flight as if I were walking into the captain's seat to pickup the turnaround. Some nights is a scramble to get everything done before passengers are boarded (I have it set so that it takes them about 20 mins to board). Such an incredible learning experience this plane and the other programs that work along side it, so rewarding.

 

Fuel planning alone is awesome, to think a year ago I never once considered fuel when playing the game.

 

The only missing component for me is ATC. I don't know if I want to make that jump into VATSIM. Seems awkward for me to be talking ATC to my computer as my wife is laying in bed in the same room trying to sleep. IM sure someday though I'll make the jump. 

The only missing component for me is ATC. I don't know if I want to make that jump into VATSIM. Seems awkward for me to be talking ATC to my computer as my wife is laying in bed in the same room trying to sleep. IM sure someday though I'll make the jump. 

 

Yeah, I'm in the same boat. Most of my time flying is after the wife goes to bed and we're in a small place as well. I'm very confident she won't be onboard with me chatting it up.

 

I've been enjoying simbrief.com for planning but I too have gone from jumping into the cockpit and actually planning stuff out. It's probably the reason I'll never graduate to anything larger than the 737. I don't have that much time.  :smile: