FMC reduces flaps up speed during STAR

[Driver170 42]

Hi guys not sure this is down to human error but i can't see why this is doing this. I'm on my STAR at F120 in LNAV and VNAV instead of the system reducing my speed to 240 as you can see in the pict below in dropbox that it continues to 210 flaps up speed.

 

why is it doing this and braking my hard alt and speed rest?

 

 

 

 

https://www.dropbox.com/s/8n81f7y13dg72zt/2014-9-1_17-43-1-887.BMP?dl=0

[rsvit 16]

As you can see you are above FMC calculated path and it cannot descend as you have 12000 set on MCP and VNAV ALT mode engaged. So FMC just gives you some additional margin by lowering the speed so you can "catch" the path from above while your vertical speed would be higher due to your acceleration to 240kt. Also slower flying means less distance travelled, the closer calculated descend path would be to you.

[Driver170 42]

Right i see but that was a crossing rest i had on my STAR that i entered for RC 40NM out from dest and was only 630ft high. so if i lowered my alt to 7000 it would of been fine then? i was seconds away from doing that infact

how can i avoid this happening again?

Possibly spd intv?

[downscc 1,327]

I usually set my MCP ALT to something just below GS intercept. When I need to intervene with VNAV, which is rare in the NGX, I prefer to use MCP SPD and V/S with the green banana providing guidance to the next constraint (of course in this case MCP ALT is set to the constraint).

[scandinavian13 5,192]

 

 

how can i avoid this happening again?

 

Quite honestly, the first big step forward in flying the plane like it should be flown is to drop Radar Contact.  It's mostly better than FSX default ATC, but it's absolute trash when it comes to realistic arrival handling.  I know I'll probably get flamed by the RC crowd, but it's true.

 

Sure, "be at X altitude, Y nm from the airfield" is a valid instruction from ATC, but it's a poor one at best.  Controllers know that their job is to ensure safety and expeditiousness in the handling of air traffic.  Their job is not to increase pilot workload to decrease theirs.  Furthermore, controllers have SOPs where traffic should be at X altitude by the border of their airspace, or by Y fix (or, in the case of larger airports, "via the STAR"), and they can easily do the mental math to give a simple "descend and maintain" instruction.

 

Radar Contact is asking you to do something that is not realistic, and you're straddling the coder's compromise: make it worldwide at the cost of being realistic.  In other words, instead of coding the program to be able to understand STARs and their associated descents (which would be a bear for the developer), the developer put in a blanket descent tactic for any airport to be at X altitude, Y miles from the field.  This is a nice compromise, but it will not work if you're trying to simulate realistic procedures (real STARs are included in that), in a plane that handles those realistic procedures realistically.

 

TL;DR:

What you're asking the plane to do is fly a realistic STAR, all while adding a layer of unrealistic to the mix.

 

Also, this might help some (particularly when paired with the video I posted in your other thread):

[Driver170 42]

Thanks kyle i will check that youtube video out again, sure i watched it few months back but prob understand it much better this time round lol

 

Yeh its a pain because i have to make a PBD waypoint at the 40nm fix ring and once i level off at 12000 or 11000 my AC gets high on the path and that causes my speed to drop to the minimum manoeuvre speed FLAPS UP SPEED!

 

So the way i overcome this is 1 use SPD INTV or 2 use LVL CHG

[scandinavian13 5,192]

Thanks kyle i will check that youtube video out again, sure i watched it few months back but prob understand it much better this time round lol

 

One thing I've found with aviation (and my own brain), is repetition and time.

 

When I was training for my PPL, I used to try and cram the FARs into my brain.  Now I can pretty much them recite them without a hassle, simply as a matter of working with them so much over the years.  Repetition, and time to let it sink in, is crucial.

 

 

 

Yeh its a pain because i have to make a PBD waypoint at the 40nm fix ring and once i level off at 12000 or 11000 my AC gets high on the path and that causes my speed to drop to the minimum manoeuvre speed FLAPS UP SPEED!

 

This is what it's supposed to do, actually.  It's called energy compensation:

"The FMC uses a special program called energy compensation at certain times during an ACT PATH DES. This program goes into effect when the MCP has been temporarily set to an altitude above the planned descent path. The airspeed cursor will slowly move toward a slower airspeed while the "TARGET" speed on the FMC remains constant. The airspeed reduction improves the capability of recapturing the descent path. When the airplane is cleared to resume the descent, the airspeed will slowly build up to the FMC target speed as the airplane recaptures the planned descent path."

 

So the way i overcome this is 1 use SPD INTV or 2 use LVL CHG

 

That's one way to compensate, yes, though again, I'd stress ditching RC.  I know that sounds callous, and tough (since it's payware), but you're trying to work two add-ons that are competing for levels of realism: RC on a very basic level, and the NGX on a very high level.  Those two mix about as well as oil and water.

[bjratchf 80]

Two things came to mind...

 

1) Thank god the 767 doesn't do this!

 

2) Spoilers are your friend!

[Driver170 42]

I figured that out my head is going to hurt for years! I'm reading over and over everyday lol

 

Time is key!

 

Or i could just leave it to reduce speed till i reset the MCP ALT once i get further descent clearance from RC

 

OR ditch RC

[scandinavian13 5,192]

 

 

OR ditch RC

 

This would be my suggestion.  Or at the very least, get a taste for VATSIM/IVAO whenever you can.  Those procedures mimic the real world a lot better.

 

Example flight from ORD to IAD:

Human: "United 488, descend via the GIBBZ2 arrival."

Follow the VNAV path, because it follows that instruction.  Done.

RC: "United 488, descend and maintain 12,000 by 40 DME of Dulles Airport."

Create some weird P/B/D waypoint to get it to fit into the realistic STAR you're trying to use, but the altitudes probably won't match up too nicely.

 

 

 

Or i could just leave it to reduce speed till i reset the MCP ALT once i get further descent clearance from RC

 

This would be my next suggestion.  Heck, even on VATSIM, the I've had the controller forget to drop me down, and energy compensation has gotten me mostly back on track (he forgot about me for a while, and I wanted to test techniques for getting back on track).  At first, it really got me on edge, because I thought that it might screw up any aircraft behind me, and then I realized it would get the controller's attention pretty quickly, and the speed will get right back up in the "recovery" (descent).

[Driver170 42]

Tried vatsim for a while but it lacked controllers only a few down at gatwick anywhere else was your own control !

 

I loved it, like you say, it was realistic!

 

Energy compensation? Thats something new i've learn't. What a neat trick!!!

[scandinavian13 5,192]

 

 

Energy compensation? Thats something new i've learn't. What a neat trick!!!

 

Yep.  If you're not expecting it, it seems like a bug in the NGX, but it's not.

[Driver170 42]

I can enter a level flight segment aswell? Two waypoints about 5nm in width at 40nm out that will give me time to level off and receive my new descent instructions and without getting high on profile!

[NM5K 1]

 

2) Spoilers are your friend!

 

They can be, but sometimes they are a bandaid for poor energy managment.

I try to use the spoilers as least as possible. I'd say... uhhhmmm... 91.46 % of the

time, I don't use the spoilers at all. Only when I get oddball routing from ATC and

they leave me high and fast might I use the speed brakes.

Most times not, but every once in a while you get the perfect storm..  :(

 

 

 

 

[NM5K 1]

 

 

2) Spoilers are your friend!

 

They can be, but sometimes they are a bandaid for poor energy managment.

I try to use the spoilers as least as possible. I'd say... uhhhmmm... 91.46 % of the

time, I don't use the spoilers at all. Only when I get oddball routing from ATC and

they leave me high and fast might I use the speed brakes.

Most times not, but every once in a while you get the perfect storm..  :(

 

A slight add to this.. In my opinion, which is artificial when it comes to aircraft,

if you use the speed brakes, you have wasted some energy. In the perfect world,

the planning would be as to not require any speed brakes, and thus not waste any

energy that should be propelling the airplane to the runway, and not being wasted with

a high drag manuever.

So to me, it kind of a game to use the least speed brake as possible, and on most flight,

not use the speed brake at all.

If I fly a "normal path" VNAV wise, if I have to use the speed brakes, I feel like I've flown

a sloppy flight. Only if ATC gives me a scruffy approach which requires the drag, do

I feel I'm flying right when pulling the handle back. Sometimes you do have to use it

if you want to land when they want you to.  :|       @#^& happens..

But when flying normal, I don't use the speed brakes.  At all.