Descent Forecast

[pcubine 143]

Using the search option for the PMDG FMC Guide I find nothing on how to enter destination airport winds if they are variable. Just pick a direction and hope you guessed right?

 

Thank you

Michael Cubine

[PHXMD-11 86]

Just pick a direction and hope you guessed right?

 

Choose the direction in the middle, i.e. direction between 000 and 090, choose 045?

[scandinavian13 5,192]

There's no real reason to add them in. As discussed in a thread in the NGX subforum, and a lengthy discussion in the 744 subforum, the only real concern is your VREF adjustments and that's a quick mental calculation.

 

What information would you gain from adding this into the FMC?

Autothrust compensates based on actual winds, so reported wind is of no use.

The only other thing I could think of it your runway assignment, but that's something you handle by yourself when you add the STAR in.

 

The descent forecast really only needs to know the wind at various altitudes in order to adjust for drastic differences as you descend, pertaining to the T/D calculation. As an example, you have a 100 knot tailwind at cruise, but a 20 knot headwind at 10,000. That's something the FMC is not going to expect when it runs its T/D calcs, so you tell it what to expect at the various FLs. Too often, simmers just randomly choose a couple evenly distributed altitudes, whereas you're really looking for the altitudes that are drastically different from the previous assumption. Destination anything is not a consideration with one exception: ISA DEV and BARO. Why?

 

ISA DEV and BARO are in there because the first controller to descend you out of the FLs is going to give you the altimeter - you guessed it - at the destination field. The other controllers downline will as well. So, because there's going to be a jump in the altitude when you change from STD to the local altimeter setting, the T/D needs to take this into account. Additionally, ISA DEV affects this as well, and also helps in the descent calc.

 

 

 

TL;DR:

The descent forecast page is there to help the systems determine a more accurate T/D. That's it. Appropriate runways, approach speeds, or anything of that nature, is either on another page, or not a concern of the automation.

[WR269 367]

Using the search option for the PMDG FMC Guide I find nothing on how to enter destination airport winds if they are variable. Just pick a direction and hope you guessed right?

 

Thank you

Michael Cubine

 

Go to the PERF page, there should be 3 pages, or 2 if you are in the cruise. Move to the Descent Page and you will see an entry for "Forecast", press it and you are in the descent weather forecast page.

 

You can enter up to 3 different altitude winds plus destination winds and temp, the FMC will then adjust your top of descent accordingly.

[pcubine 143]

I can't imagine McDonnell Douglas engineers or their subconractors that built the FMC just saying "Hey, we got extra space on this page. What are we going to do with it. I know, lets's stick the destination airport winds there." You can't even enter the descent forecast into the FMC unless those blanks are completed. There in no INSERT by the left LSL6 until the airport blanks are completed. It makes no difference now, KMIA just went 350/9.

 

Michael Cubine

[scandinavian13 5,192]

I apologize - I'd forgotten which subforum I was in. The MD-11 does indeed have a prompt for the information. As far as entering the information, if it's VRB, I'd honestly go with the heading of the runway in use at half of whatever speed is reported (just my own judgment call there). Depending on the speed, I'd almost leave it blank (the ATC world - Stateside - considers anything less than 3 knots to be calm, just as a reference).

 

I can't imagine what it would do with the information, however. Wind is conspicuously absent on the APPROACH page, and the affect of the wind at your destination affects maybe the last couple minutes of the flight, which are negligible in the descent forecast. The MD-11 FCOM I have makes no mention of what it uses the data for, either.

[3-2-1-Now 126]

I've never used it, and know many RW pilots who never use it either. It just adds to the workload, and doesn't change things that much anyway. Some RW pilots (particularly Training Captains) are OTT about entering them, but why add to the stress and worry about the fuel savings when ATC then gives you an arbitrary 10 mile vector for spacing?

 

Unless you're experiencing a 150 kts tail wind, I'd say it was not worth it.

 

The FMS should account for actual conditions anyway, so if you have a tail wind, the ToD should be based on what the aircraft is doing right now. I know the MD-11 does this - it will vary cruise speed based on the winds to target a ground speed, and take advantage of fuel savings from tail wind. It's available in ECON SPD mode, but can't remember what it is called specifically (I want to say Ground Speed Mini but that's an Airbus function on approach).

 

Best regards,

Robin.

[scandinavian13 5,192]

That's not entirely true. If you have a tailwind now because you're riding the jetstream, you'll certainly need an earlier T/D. If it drops from 100 knots of tailwind to 50 or 20, or even a headwind while you're still higher up, it's going to affect the profile.

 

The FMC does take the current situation into account, but it cannot predict changes unless you give it the forecast. In most of the cases, you're not going to see disparate enough winds to really make a huge difference, but if you see winds in the descent changing drastically, you should toss them in.

 

Further, ATC won't give you arbitrary vectors. In fact, they're required to give you a reason for the vector any time one is issued. I see your point - that the traffic picture can affect your fuel burn and negate the fuel saving you would've picked up - but as someone who works with controllers all the time, I figured I'd just make sure that was clear. There are already too many pilots who think controllers are just there to mess with them.

[3-2-1-Now 126]

There are already too many pilots who think controllers are just there to mess with them.

I don't. ATC can see the "bigger picture". A higher percentage of fatalities occur on approach/landing than anywhere else.

 

If you have a tailwind now because you're riding the jetstream, you'll certainly need an earlier T/D. If it drops from 100 knots of tailwind to 50 or 20, or even a headwind while you're still higher up, it's going to affect the profile.

 

The FMC does take the current situation into account, but it cannot predict changes unless you give it the forecast.

No, but the time spent inputting the winds detracts from flying the aircraft in a busy phase of flight, and again, many RW don't bother as it makes little difference anyway.

 

Best regards,

Robin.

[scandinavian13 5,192]

No, but the time spent inputting the winds detracts from flying the aircraft in a busy phase of flight

 

Cruise is busy? Man...I must be screwing up when I fly.

The whole idea of the descent forecast is to adjust the T/D to better fit the wind changes in the descent. There's no point in entering them during the descent because you've already left cruise.

 

I don't.

 

Never said you did. I was just pointing out that your use of "arbitrary" (defined as "capricious; unreasonable; unsupported") doesn't exactly paint a nice picture of them.

[3-2-1-Now 126]

Ahh I see. Sorry about that. I meant arbitrary as in "anything you like".

 

"Shooting yourself in head will kill you. The type of gun is arbitrary".

 

ar·bi·trar·y/ˈärbiˌtrerē/

Adjective:

• Based on random choice or personal whim, rather than any reason or system.
  

 

So it goes for ATC vectors. What's the point saving 300 lbs of fuel by an slightly earlier ToD when ATC will assign arbitrary vectors?

 

Nothing personal - just clarifying my meaning. :smile:

 

As for being busy - that's not my opinion but that of people who did it 4 to 6 times a day on short-haul routes. Worrying about inputting winds was very low of the list of priorities.

 

Best regards,

Robin.

[scandinavian13 5,192]

Nothing personal - just clarifying my meaning.

 

As for being busy - that's not my opinion but that of people who did it 4 to 6 times a day on short-haul routes. Worrying about inputting winds was very low of the list of priorities.

 

Gotcha. Still, it isn't based on personal whim. There's a reason behind every vector. The Prime Directive of ATC is the "safe and expeditious" flow of traffic. Heck, it's less effort if I push everyone through my sector faster anyway, so vectors (unless in the case of making sure I don't put two together) are going to be a way of saving time/fuel if anything. The sooner everyone gets through my sector, the sooner traffic dies down, the sooner I can combine my sector with Jonny over there, and the sooner I can go mainline more coffee and play XBOX in the break room. Haha...

 

Yeah, on a short route, I can see that being the case. If you think about it, saved is saved. If ATC is going to give the vector anyway, you're going to burn that fuel anyway, so if you save 300 pounds by delaying the descent and offset that with vectors, you're at a net of zero. If you didn't, you're at a net of -300. Just playing Devil's Advocate at this point...

[3-2-1-Now 126]

Fair enough!

 

Best regards,

Robin.