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Lnav Overshoot?  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (2 years 5 months 1 week 2 days 21 hours ago) and read 3461 times:

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?

12 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (2 years 5 months 1 week 2 days 17 hours ago) and read 3404 times:

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.


User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 2, posted (2 years 5 months 1 week 2 days 16 hours ago) and read 3390 times:

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



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlinebarney captain From United States of America, joined Nov 2001, 997 posts, RR: 12
Reply 3, posted (2 years 5 months 1 week 2 days 5 hours ago) and read 3279 times:

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.



...from the Banana Republic....
User currently offlinebarney captain From United States of America, joined Nov 2001, 997 posts, RR: 12
Reply 4, posted (2 years 5 months 1 week 1 day 5 hours ago) and read 3113 times:

....that should read .....while in LNAV...


...from the Banana Republic....
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 5, posted (2 years 5 months 1 week 23 hours ago) and read 3070 times:

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.

User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Reply 6, posted (2 years 5 months 1 week 1 hour ago) and read 2929 times:

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


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 7, posted (2 years 5 months 1 week ago) and read 2907 times:

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".


User currently offlineatct From United States of America, joined Mar 2001, 2345 posts, RR: 38
Reply 8, posted (2 years 5 months 6 days 8 hours ago) and read 2835 times:

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



"The way to get started is to quit talking and begin doing." - Walt Disney
User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 9, posted (2 years 5 months 6 days 3 hours ago) and read 2806 times:

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.   



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 10, posted (2 years 5 months 6 days 1 hour ago) and read 2776 times:

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.


User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 11, posted (2 years 5 months 6 days ago) and read 2761 times:

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



Any views shared are strictly my own and do not a represent those of any former employer.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 12, posted (2 years 5 months 5 days 23 hours ago) and read 2755 times:

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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