FMS "Direct To" too brutal

[alex98 13]

Hi PMDG team,

A question regarding the "Direct To" function of the FMS. I noticed that the autopilot tends to behave quite abruptly, ie large bank angle to capture the new track. IIRC the NGX or the 777 do not behave so quickly and limit the bank angle to 15° or so. Is this consistent with the real airplane behaviour? Is there a smoother way to use the "Direct To" option?

Cheers

Alex

[serviceceiling 135]

Yup. The same applies to larger heading changes, LNAV course changes or radial/loc intercepts. The AP in the 747 is quite harsh when it comes to turns. This seems to be a limitation of the fsx/p3d flight model. 

[downscc 1,327]

8 hours ago, serviceceiling said:

This seems to be a limitation of the fsx/p3d flight model. 

Were that true the same would hold for the NGX and 777, right?

12 hours ago, alex98 said:

Is there a smoother way to use the "Direct To" option?

Sure, it's so obvious you'll kick yourself for not thinking of it.  Line select the fix without hitting EXEC and note the bearing, then use HDG to turn to the approximate bearing.  You have full control over how much bank you want in HDG mode. Once aircraft is pointed towards the fix then EXEC the change and go back to LNAV.

[Tabs 2,790]

LNAV has a whole series of rules that determine the bank limit (similar to what HDG SEL does in AUTO mode) - in most flight regimes it will bank up to 25-30 degrees as required by the course change. This is realistic - these systems are not perfect in real life either, and there's a whole host of "tricks" real pilots use to make certain things the autopilot system does more comfortable for the passengers. This is one of them, as Dan alluded to above. On a large course change direct-to, use HDG SEL to get the plane more or less heading in the right direction using whatever bank angle limit you want. At that point you can then execute the direct to in the FMC and then put it back in LNAV.

Another similar real-life trick at top-of-descent - the system can be quite abrupt in chopping the thrust to idle and starting down at the T/D marker, so what many real pilots do is start down slightly early in V/S mode using small gradual increases in the descent rate with the wheel on the MCP until intercepting the VNAV path and reengaging it. This way the passengers barely notice the onset of the descent.

Always worth remembering that you are the pilot, not the computer systems - if you want it to do something differently, take over with a different mode on the MCP and make it happen.

[alex98 13]

Thanks Ryan for your help! Always appreciated!

Alex

[serviceceiling 135]

14 hours ago, downscc said:

Were that true the same would hold for the NGX and 777, right?

It is also the same in the 737 and 777, just a bit less apparent/violent.

 

12 hours ago, Tabs said:

LNAV has a whole series of rules that determine the bank limit (similar to what HDG SEL does in AUTO mode) - in most flight regimes it will bank up to 25-30 degrees as required by the course change. This is realistic - these systems are not perfect in real life either, and there's a whole host of "tricks" real pilots use to make certain things the autopilot system does more comfortable for the passengers. This is one of them, as Dan alluded to above. On a large course change direct-to, use HDG SEL to get the plane more or less heading in the right direction using whatever bank angle limit you want. At that point you can then execute the direct to in the FMC and then put it back in LNAV.

Another similar real-life trick at top-of-descent - the system can be quite abrupt in chopping the thrust to idle and starting down at the T/D marker, so what many real pilots do is start down slightly early in V/S mode using small gradual increases in the descent rate with the wheel on the MCP until intercepting the VNAV path and reengaging it. This way the passengers barely notice the onset of the descent.

Always worth remembering that you are the pilot, not the computer systems - if you want it to do something differently, take over with a different mode on the MCP and make it happen.

I don't the author of this topic was referring to the bank limit per se but to the actual initial roll rate when said changes to the lateral modes are made. The initial roll rate is a bit violent and excesive. The only addons I know of that behave smooth in that regard are the ones that have an external flight model (A2A, Mejastic). 

[downscc 1,327]

6 hours ago, serviceceiling said:

The initial roll rate is a bit violent and excesive.

According to?  The OP clearly stated bank angle, not roll rate.  I've never seen a B744 do a "violent" roll rate... I guess this is pretty subjective.  Regardless, I put my faith in the tech team that evaluated such things before it moved to beta.

[serviceceiling 135]

18 minutes ago, downscc said:

According to?  The OP clearly stated bank angle, not roll rate.  I've never seen a B744 do a "violent" roll rate... I guess this is pretty subjective.  Regardless, I put my faith in the tech team that evaluated such things before it moved to beta.

He also stated it was "quite abruptly" which describes the bahaviour as well. 

[Tabs 2,790]

8 hours ago, serviceceiling said:

The only addons I know of that behave smooth in that regard are the ones that have an external flight model (A2A, Mejastic). 

The autopilot roll rate has nothing to do with whether the flight model is "external" or not. We have full control over our roll rate commands.

(as an aside, A2A is not doing the same thing Majestic does with respect to this - they are still using the FSX/P3D flight dynamics engine with modifications on top of it, just as we do. Majestic uses the open-source JSBSim flight dynamics library to create what are essentially slew commands to the sim, which (as we've explained numerous times) we feel creates a number of unacceptable compromises in other areas of the simulation such as the lack of reaction to weather and turbulence, issues with the view and sound systems, etc that are not a road we wish to go down with our products. There is more than one way to skin a cat and we've been at the "doing things outside of FS" game longer than almost anyone in this market)

[alex98 13]

Hi,

I did mean bank angle and not roll rate!

What surprised me is that in the specific "direct to" configuration the autopilot LNAV mode orders high bank angle turns (around 25°) - even with small course corrections - which seems to me excessive at high altitudes/high speeds. I dont recall anything similar in the 737 or in the 777. But I do understand the autopilot systems are different... Again just a little surprised ;)

Put it differently, it looks like the new track is an immediate turn without anticipation or consideration for aircraft speed thus the high bank angle turn to stay on track.

Voilà. :)

I have great confidence in the PMDG team to clear all the (few) little bugs. But sometimes some fly under radar coverage :))

Best regards

[Captain_Al 66]

On 4/1/2017 at 6:07 PM, Tabs said:

LNAV has a whole series of rules that determine the bank limit (similar to what HDG SEL does in AUTO mode) - in most flight regimes it will bank up to 25-30 degrees as required by the course change. This is realistic - these systems are not perfect in real life either, and there's a whole host of "tricks" real pilots use to make certain things the autopilot system does more comfortable for the passengers. This is one of them, as Dan alluded to above. On a large course change direct-to, use HDG SEL to get the plane more or less heading in the right direction using whatever bank angle limit you want. At that point you can then execute the direct to in the FMC and then put it back in LNAV.

Another similar real-life trick at top-of-descent - the system can be quite abrupt in chopping the thrust to idle and starting down at the T/D marker, so what many real pilots do is start down slightly early in V/S mode using small gradual increases in the descent rate with the wheel on the MCP until intercepting the VNAV path and reengaging it. This way the passengers barely notice the onset of the descent.

Always worth remembering that you are the pilot, not the computer systems - if you want it to do something differently, take over with a different mode on the MCP and make it happen.

For real, I would use DES NOW before using V/S starting down before the T/D point that way you are still remaining in VNAV with a 1250 fpm gradual descent rate till picking up the path. If you get distracted using V/S, which we know can happen at the wrong time, you run the risk of missing the path and then you defeat the whole purpose of what you were trying to do as the thrust slams back to idle and the jet increases the descent to get back on the now high path.

Along the same lines, what we commonly did on a lateral offset was to go into heading select, put the offset in and execute, pick up a 45 degree heading for the intercept, and then re-arm LNAV. Same thing returning to normal navigation. Up high, this was much nicer for passenger comfort then the jet aggressively banking during an OFFSET in LNAV.

[BlueStar 323]

5 hours ago, Tabs said:

The autopilot roll rate has nothing to do with whether the flight model is "external" or not. We have full control over our roll rate commands.
 

Ryan,

I would expect the roll rate used by the autopilot to be based on a "rate of turn" of 3 degrees/second not to exceed a bank angle of 25 degrees.  This is what ATC expects when a turn is given to an aircraft or when an aircraft is in holding.  A simple formula is TAS/10 + 7 not to exceed 25.  As near as I can tell this is what the PMDG B744 autopilot is doing.  

blaustern 

 

[PerWel 23]

Just wonder how FSLab done this in their A320. That AP follows the LNAV smooth and without any brutal rolls even in 90+ degree turns, starts banking easy increasing roll rate slowly to 25 degree. Then levels right in time for new heading, no compensating in the other direction.

Is their AP unrealistic or is it the Airbus systems that work different?

Just a question, I am very happy with Queen 2 and have nothing bad to say about her.

[PerWel 23]

Forgot to say that FSLabs A320 does not nail the FL like PMDGs, A320 always 50-80 feet above or below set FL. Fslab says that a real Airbus does this, so going up/down around set FL is normal.
One good thing is that the Queen takes around 3-400 Mb less VAS then FSLabs A320 and 747 have higher FPS. Same flight same scenery and airports. Tested that.

I have no idea how a real one does, never have and will never ever fly a real Boeing or Airbus. Maybe a real sim someday if I have the money for it.

 

[skelsey 765]

3 hours ago, PerWel said:

Just wonder how FSLab done this in their A320. That AP follows the LNAV smooth and without any brutal rolls even in 90+ degree turns, starts banking easy increasing roll rate slowly to 25 degree. Then levels right in time for new heading, no compensating in the other direction.

Is their AP unrealistic or is it the Airbus systems that work different?

An A320 is not a Boeing .

In real life, the two aircraft have very different autoflight gains etc and these are recreated pretty faithfully in both products, it would appear.