smartt1982
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Lnav Overshoot?

Thu Jul 12, 2012 6:02 pm

How does it work with LNAV track predictions? It is just on certain SIDS with some turns, we end up having to put rather slow speeds on some of the NAV points to tidy up the magenta line and stop it turning into a big squiggly line. For example on one particular turn I put in 195Knots which tidy’s up the track very nicely. This would equate to about Flaps 1 speed on the 737. When I actually fly this turn in LNAV I can actually be cleaned up and flying at 220 Knots and the aircraft has no problem at all following it. IN our FCOM it says that the aircraft is capable of flying all turns up to 250 Knots without overshoot. Is there a discrepancy then between what the FMC thinks the aircraft can do and what the aircraft is ACTUALLY capable of doing?
 
tdscanuck
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RE: Lnav Overshoot?

Thu Jul 12, 2012 9:43 pm

Quoting smartt1982 (Thread starter):
IN our FCOM it says that the aircraft is capable of flying all turns up to 250 Knots without overshoot. Is there a discrepancy then between what the FMC thinks the aircraft can do and what the aircraft is ACTUALLY capable of doing?

Are you playing with the bank angle limits? That will alter how tight a turn the autopilot will actually do.

Tom.
 
IAHFLYR
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RE: Lnav Overshoot?

Thu Jul 12, 2012 10:29 pm

Quoting smartt1982 (Thread starter):
IN our FCOM it says that the aircraft is capable of flying all turns up to 250 Knots without overshoot. Is there a discrepancy then between what the FMC thinks the aircraft can do and what the aircraft is ACTUALLY capable of doing?

It could be the fixes on the route are not coded as fly-by waypoints so, you won't the get turn inside the fix to intercept the track to the next leg on the route. If they are coded fly-over you'll never have anything but overshoots to the next leg on the route.

There are some SIDS in the U.S. which have specific speeds coded in the database for route containment due to terrian or airspace constraints, but that is
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barney captain
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RE: Lnav Overshoot?

Fri Jul 13, 2012 9:08 am

Quoting tdscanuck (Reply 1):
Are you playing with the bank angle limits? That will alter how tight a turn the autopilot will actually do.

Tom.

Im fairly certain the bank angle limits on the heading selector are ignored by the A/P while in VNAV.
Southeast Of Disorder
 
barney captain
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RE: Lnav Overshoot?

Sat Jul 14, 2012 9:48 am

....that should read .....while in LNAV...
Southeast Of Disorder
 
CosmicCruiser
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RE: Lnav Overshoot?

Sat Jul 14, 2012 3:15 pm

If I understand exactly what you're saying I'll add a comment or two. I've seen situations where you've created a situation that the FMS can't really do and you get circles or in some cases a direct to bypasses a fix because you're too close abeam but never squiggly lines. We edit the speed often if the SID has an early turn that's 90 deg or more because it sure won't do it at 220 or higher. Once max bank is achieved that's it. Considering that a headwind/tailwind in the turn can also make a difference. We're told the FMS is "looking" for a speed around 190-200 for these types of turns. Some SIDs we see are noise sensitive so staying on the magenta line is very important.
 
smartt1982
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RE: Lnav Overshoot?

Sun Jul 15, 2012 1:12 pm

Quoting CosmicCruiser (Reply 5):
We're told the FMS is "looking" for a speed around 190-200 for these types of turns. Some SIDs we see are noise sensitive so staying on the magenta line is very important.

With that then, as you say to get rid of the by-pass you change it down to a slower speed. What I am confused about is that even though I put the speed in as 190knots, when we actually fly that SID (I have seen this whilst flying and also checked it when I had a spare few mins in our company sims) I can have the speed much higher ie aircraft cleaned up etc and the aircraft will fly that track no problem even though it is much higher than the speed I had entered in the FMC.

Is the LNAV criteria much more sensitive in terms of overshoots and is its criteria much lesss than what the aircraft can actually fly without overshooting the track?

Quoting tdscanuck (Reply 1):
Are you playing with the bank angle limits? That will alter how tight a turn the autopilot will actually do.

No, it is all in the 25AOB
 
CosmicCruiser
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RE: Lnav Overshoot?

Sun Jul 15, 2012 3:02 pm

Like I said winds can certainly be a factor but other than that I can't say. I do know at 250kts our MD-11 will not stay on the magenta line if the turn is close to 90 deg or greater. We've had a few guys prove it. You might look to see what is the max bank angle with and without flaps which may be a factor too. And in some cases perhaps the jet can make at a higher speed but the co. prefers a more conservative speed for winds etc.
Slightly off subject someone mentioned fly over points which will cause you to over shoot. True and in SOME cases, e.g.. a SID out of CDG with an over 90 deg turn, you are not expected to come back to the magenta line but go direct after the waypoint. I saw one crew let the jet start coming back to the course line and CDG dept. asked "what are you doing, proceed direct xxxxx".
 
atct
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RE: Lnav Overshoot?

Mon Jul 16, 2012 6:35 am

Quoting IAHFLYR (Reply 2):
There are some SIDS in the U.S. which have specific speeds coded in the database for route containment due to terrian or airspace constraints, but that is

Up here we have some SID's / STAR's with speed restrictions for lateral separation with terrain. Mostly the Alaska Airlines RNP specific stuff.

atct
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IAHFLYR
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RE: Lnav Overshoot?

Mon Jul 16, 2012 11:56 am

Quoting CosmicCruiser (Reply 7):
True and in SOME cases, e.g.. a SID out of CDG with an over 90 deg turn, you are not expected to come back to the magenta line but go direct after the waypoint. I saw one crew let the jet start coming back to the course line and CDG dept. asked "what are you doing, proceed direct xxxxx".



Wonder what leg type is coded, if TF then wouldn't it come back to the track rather than direct? I'd think a DF leg would be used from waypoint XXXXX to waypoint YYYYY if that is what the controllers expects.   
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CosmicCruiser
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RE: Lnav Overshoot?

Mon Jul 16, 2012 1:54 pm

A good example is the OPALE SID off 09L in CDG. The diagram shows the mandatory flyover point, PG092, then a left turn to OPALE. The procedural text says PG092-OPALE and in the FMS you'll see a sharp magenta line that goes from PG092 to OPALE that's about a 120 deg turn. Since it's not noise sensitive we let the jet accelerate and as it passes PG092 it WILL try to go back to the magenta line but we then just do a Direct To for OPALE. rare perhaps but interesting.
Now take the BUZAD 7R off 22 in STN and you had better be on the line all the way hence a speed edit.
 
IAHFLYR
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RE: Lnav Overshoot?

Mon Jul 16, 2012 2:50 pm

Quoting CosmicCruiser (Reply 10):
A good example is the OPALE SID off 09L in CDG. The diagram shows the mandatory flyover point, PG092, then a left turn to OPALE. The procedural text says PG092-OPALE and in the FMS you'll see a sharp magenta line that goes from PG092 to OPALE that's about a 120 deg turn. Since it's not noise sensitive we let the jet accelerate and as it passes PG092 it WILL try to go back to the magenta line but we then just do a Direct To for OPALE. rare perhaps but interesting.



As you mention the chart does show "DF" which should indicate direct to the fix as you wrote, but am now curious just how it is coded in the database if the FMC is attempting to return to the magenta line as it most likely would if it was coded as "TF". MMMMM
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CosmicCruiser
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RE: Lnav Overshoot?

Mon Jul 16, 2012 3:19 pm

I'll have to check next time and see but it looks to me that it knows it's a DF but as in any "overshoot" it's trying to get back "on course " for that direct route. I've never seen our FMS create another direct to route after a theoretical overshoot, it just tries to recover and in this case the turn is so sharp there's no way it can do it anyway and stay on the magenta line. The last time I did it the NAV display doesn't show it like the SID diagram shows that nice curving left turn after PG092, it shows a hard angle from PG092 to OPALE and the jet can't do it.

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